Microcontroller and Embedded Systems 2023

Module - 4
Syllabus
Embedded System Components: Embedded Vs General computing system, History of embedded
systems, Classification of Embedded systems, Major applications areas of embedded systems, purpose
of embedded systems.
Core of an Embedded System including all types of processor/controller, Memory, Sensors, Actuators,

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LED, 7 segment LED display, stepper motor, Keyboard, Push button switch, Communication Interface
(on-board and external types), Embedded firmware, Other system components..
Laboratory Component:
1. Interface and Control a DC Motor.
2. Interface a Stepper motor and rotate it in clockwise and anti-clockwise direction.

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3. Determine Digital output for a given Analog input using Internal ADC of ARM controller.
4. Interface a DAC and generate Triangular and Square waveforms.
5. Interface a 4x4 keyboard and display the key code on an LCD.
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6. Demonstrate the use of an external interrupt to toggle an LED On/Off.
7. Display the Hex digits 0 to F on a 7-segment LED interface, with an appropriate delay in
between.
Introduction
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An embedded system is an electronic/electro-mechanical system designed to perform a specific

function and is a combination of both hardware and firmware (software). Every embedded
system is unique, and the hardware as well as the firmware is highly specialised to the
application domain. Embedded systems are becoming an inevitable part of any product or
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equipment in all fields including household appliances, telecommunications, medical equipment,

industrial control, consumer products, etc
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Embedded Vs General computing system

General Purpose Computing System Embedded System
A system which is a combination of a A system which is a combination of special
generic hardware and a General Purpose purpose hardware and embedded OS for
Operating System for executing a variety of executing a specific set of applications.
applications.
Contains a General Purpose Operating May or may not contain an operating system
System (GPOS) for functioning
Applications are alterable (programmable) The firmware of the embedded system is pre-
by the user (It is possible for the end user to programmed and it is non-alterable by the end-

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re-install the operating system, and also add user (There may be exceptions for systems
or remove user applications) supporting OS kernel image flashing through
special hardware settings)
Performance is the key deciding factor in Application-specific requirements (like
the selection of the system. Always, ‘Faster performance, power requirements, memory
is Better’ usage, etc.) are the key deciding factors.
Less/not at all tailored towards reduced Highly tailored to take advantage of the power

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operating power requirements, options for saving modes supported by the hardware and
different levels of power management the operating system
Response requirements are not time-critical For certain category of embedded systems like
mission critical systems, the response time
requirement is highly critical
Need not be deterministic in execution Execution behaviour is deterministic for certain

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behaviour types of embedded systems like ‘Hard Real
Time’ systems

History of Embedded Systems

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Embedded systems were present even before the Information Technology revolution. Initially,
embedded systems were built around vacuum tube and transistor technologies; and embedded
algorithm was developed by using low level programming languages.
 Advances in semiconductor and nano-technology and IT revolution gave way to
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development of miniature embedded systems.

 Apollo Guidance Computer (AGC) developed (during 1960) by MIT Instrumentation
Laboratory for the lunar expedition is the first recognized modern embedded system.
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 AGC included both Command Module (CM-to encircle the moon) and Lunar Excursion
Module (LEM-to go down to the moon surface and land there safely).
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 There were 16 reaction control thrusters, a descent engine (designed to provide thrust to
the lunar model out of the lunar orbit and land it safely on moon) and an ascent engine.
 Original design was based on 4K words of fixed memory (ROM) and 256 words of
erasable memory (RAM); which has been enhanced (during 1963) to 10K fixed and 1K
erasable memory. The clock frequency was 1.024 MHz.
 The computing unit of AGC consisted of approximately 11 instructions on 16-bit word
logic.
 A calculator type user interface was given and is known as DSKY (display/ keyboard).
The first mass-produced embedded system was the guidance computer, Autonetics D-17, for the
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Minuteman-I missile in 1961; built using discrete transistor logic and a hard-disk for main
memory.
The first microprocessor, the Intel 4004, was designed for calculators and other small systems;
but still required many external memory and support chips.
First microcontroller, TMS 1000, developed in 1974 by Texas Instruments. It had ROM, RAM,

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and clock circuitry on the chip along with the processing chip.
In 1980, Intel introduced 8051 MCU and called it MCS-51 architecture.
Laser and Inkjet printers emerged during 1980s; and early 1990, cell phones having five or six
DSPs and CPUs emerged.
Classification of Embedded Systems

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Classification Based on Generation
Generation with Example Description
First Generation (1G) 8-bit microprocessor and 4-bit microcontroller like 8085
- Digital telephone keypads and Z80 was used in 1G.
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- Stepper motor Hardware circuit was simple.
Assembly code is used for developing firmware.
Second Generation (2G) Uses 16-bit microprocessor and 8-bit microcontroller.
- Data acquisition systems like They are more complex and powerful than 1G
ADC, SCADA system microprocessor and microcontroller.
Third Generation (3G) Uses 32-bit microprocessor and 16-bit microcontroller.
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- Robotics Domain specific processor and controllers are used.

Fourth Generation (4G) Uses 64-bit microprocessor and 32-bit microcontroller.
- Smart phones The concept of system on chips, multi-core processors
evolved.
Highly complex and very powerful.
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Classification Based on Complexity and Performance

(i) Small-Scale Embedded Systems: Small-scale embedded systems are usually built around
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low performance and low cost 8 or 16 bit microprocessors/ microcontrollers. It may or may not
contain an operating system for its functioning.
(ii) Medium-Scale Embedded Systems: They are usually built around medium performance,
low cost 16 or 32 bit microprocessors/ microcontrollers or digital signal processors & usually
contain an embedded operating system (general purpose/ real-time).
(iii) Large-Scale Embedded Systems/ Complex Systems: They are employed in mission
critical applications demanding high performance. These systems are commonly built around
high performance 32 or 64 bit RISC processors/ controllers or Reconfigurable System on Chip

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(RSoC) or multi-core processors and programmable logic devices. They usually contain a high
performance Real Time Operating System (RTOS) for task scheduling, prioritization, and
management.
Major Application Areas of Embedded Systems
Embedded systems play a vital role in our day-to-day life, starting from home to computer

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industry. Embedded technology has acquired a new dimension from its first generation model,
the Apollo Guidance Computer, to the latest radio navigation system combined with in-car
entertainment technology and wearable computing devices (Apple watch, Microsoft band, Fitbit
fitness trackers, etc.). A few of the important domains and products are listed below:
i. Consumer Electronics: Camcorders, cameras, etc.

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ii. Household Appliances: Television, DVD players, washing machine, fridge, microwave
oven, etc.
iii. Home Automation and Security Systems: Air conditioners, sprinklers, intruder detection
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alarms, closed circuit television cameras, fire alarms, etc.
iv. Automotive Industry: Anti-lock breaking systems (ABS), engine control, ignition
systems, automatic navigation systems, etc.
v. Telecom: Cellular telephones, telephone switches, handset multimedia applications, etc.
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vi. Computer Peripherals: Printers, scanners, fax machines, etc.

vii. Computer Networking Systems: Network routers, switches, hubs, firewalls, etc.
viii. Healthcare: Different kinds of scanners, EEG, ECG machines, etc.
ix. Measurement & Instrumentation: Digital multi meters, digital CROs, logic analyzers
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PLC systems, etc.

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x. Banking & Retail: Automatic teller machines (ATM) and currency counters, point of
sales (POS).
xi. Card Readers: Barcode, smart card readers, hand held devices, etc.
xii. Wearable Devices: Health and fitness trackers, Smartphone screen extension for
notifications, etc.
xiii. Cloud Computing and Internet of Things (IoT).
Purpose of Embedded Systems
Embedded systems are used in various domains like consumer electronics, home automation,
telecommunications, automotive industry, healthcare, control & instrumentation, retail and

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banking applications, etc. Each embedded system is designed to serve the purpose of any one or
a combination of the following tasks:
(i) Data Collection, Storage & Representation: Purpose of embedded system design is data
collection such as values or measurements. It can be numbers, words, measurements,
observations, or even just description of things. It performs acquisition of data from the external

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world. Data collection is usually done for storage, analysis, manipulation, and transmission.
Data can be analog or digital. Embedded systems with analog data capturing techniques collect
data directly in the form of analog signal; whereas embedded systems with digital data
collection mechanism convert the analog signal to corresponding digital signal using analog to
digital (A/D) converters. If the data is digital, it can be directly captured by digital embedded

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system. A digital camera is a typical example of an embedded system with data collection,
storage, and representation of data. Images are captured and captured image may be stored
within the memory of the camera. The captured image can also be presented to the user through
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a liquid crystal display unit.
(ii) Data Communication: Embedded data communication systems are deployed in
applications ranging from simple home networking systems to complex satellite communication
systems. Network hubs, routers, switches are examples of dedicated data transmission embedded
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systems. Data transmission is in the form of wire medium or wireless medium. Initially wired
medium is used by embedded systems; and as technology changes, wireless medium becomes
de-facto standard in embedded systems. USB, TCP/ IP are examples of wired communication;
and BlueTooth, ZigBee and Wi-Fi are examples for wireless communication. Data can be
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transmitted by analog or digital means.

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(iii) Data (Signal) Processing: Embedded systems with signal processing functionalities are
employed in applications demanding signal processing like speech coding, synthesis, audio
video codec, transmission applications, etc. A digital hearing aid is a typical example of an
embedded system employing data processing.
(iv) Monitoring: Almost all embedded products coming under the medical domain are with
monitoring functions. Patient heart beat is monitored by Electro cardiogram (ECG) machine.
Digital CRO, digital multi-meters, and logic analysers are examples of monitoring embedded
systems.
(v) Control: Sensors and actuators are used for controlling the system. Sensors are connected to

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the input port for capturing the changes in environmental variable or measuring variable.
Actuators connected to output port are controlled according to the changes in input variable. Air
conditioner system used to control the room temperature to a specified limit is a typical example
for embedded system for control purpose. The air conditioner’s compressor unit (actuator) is
controlled according to the current room temperature (sensor) and the desired room temperature

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set by the user.
(vi) Application Specific User Interface: These are embedded systems with application-
specific user interfaces like buttons, switches, keypad, lights, bells, display units, etc. Mobile
phone is an example for this. In mobile phone, the user interface is provided through the keypad,
graphic LCD module, system speaker, vibration alert, etc.

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A typical embedded system contains a single chip controller, which acts as the master brain
of the system. The controller can be a
Microprocessor (Intel 8085) or a
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Microcontroller(Atmel AT89C51) or a Field
Programmable Gate Array (FPGA) device
(Xilinx Spartan) or a Digital Signal Processor
(DSP) (Blackfin® Processors from Analog
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Devices) or an Application Specific Integrated

Circuit (ASIC) or an Application Specific
Standard Product (ASSP) (ADE7760 Single
Phase Energy Metreing IC from Analog Devices for energy metering applications).
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Embedded hardware/ software systems are basically designed to regulate a physical variable or
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to manipulate the state of some devices by sending some control signals to the Actuators or
devices connected to the O/P ports of the system, in response to the input signals provided by
the end users or Sensors which are connected to the input ports. Hence an embedded system can
be viewed as a reactive system. Key boards, push button switches, etc. are examples for
common user interface input devices whereas LEDs, liquid crystal displays, piezoelectric
buzzers, etc. are examples for common user interface output devices for a typical embedded
system. The Memory of the system is responsible for holding the control algorithm and other
important configuration details.

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Core of the Embedded System

Embedded systems are domain and application specific and are built around a central core. The
core of the embedded system falls into any one of the following categories:
(i) General purpose and domain specific processors
a. Microprocessors

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b. Microcontrollers
c. Digital Signal Processors
(ii) Application Specific Integrated Circuits (ASICs)
(iii) Programmable Logic Devices (PLDs)
iv) Commercial off-the-shelf Components (COTS)
General purpose and domain specific processors: Almost 80% of the embedded systems are

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processor/ controller based. The processor may be a microprocessor or a microcontroller or a
digital signal processor, depending on the domain and application.
Microprocessors: A Microprocessor is a silicon chip representing a central processing unit
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(CPU), capable of performing arithmetic as well as logical operations. In general, the CPU
contains the arithmetic and logic unit (ALU), control unit and working registers. A
microprocessor is a dependent unit and it requires the combination of' other hardware like
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memory, timer unit, and interrupt controller, etc., for proper functioning. Intel claims the credit
for developing the first microprocessor unit, Intel 4004, a 4-bit processor released in November
1971. In April 1974, Intel launched the first 8-bit processor, the Intel 8080, with 16-bit address
bus and program counter and seven 8-bit registers. Intel 8080 was the most commonly used
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processors for industrial control and other embedded applications in the 1975s. Immediately
after the release of Intel 8080, Motorola also entered the market with their processor, Motorola
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6800 with a different architecture and instruction set compared to 8080. In 1976 Intel came up
with the upgraded version of 8080 Intel 8085, with two newly added instructions, three interrupt
pins and serial I/0. Clock generator and bus controller circuits were built-in and the power
supply part was modified to a single +5 V supply. In July 1976 Zilog entered the microprocessor
market with its Z80 processor as competitor to Intel. Intel, AMD, Freescale,
GLOBALFOUNDRIES, TI, Cyrix, NVIDIA, Qualcomm, MediaTek, etc. are the key players in
the processor market.
Microcontrollers: A Microcontroller is a highly integrated chip contains a CPU, scratch pad
RAM, special and general purpose register arrays, on chip ROM/ FLASH memory for program

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storage, timer and interrupt control units and dedicated I/O ports. Since a microcontroller
contains all the necessary functional blocks for independent working, they found greater place in
the embedded domain in place of microprocessors. Apart from this, they are cheap, cost
effective and are readily available in the market. Texas Instrument's TMS 1000 (1974) is
considered as the world's first microcontroller. TI followed Intel's 4004, 4-bit processor

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design and added some amount of RAM, program storage memory (ROM) and 1/O support on a
single chip, there by eliminated the requirement of multiple hardware chips for self-functioning.
In 1977 Intel entered the microcontroller market with a family of controllers coming under one
umbrella named MCS-48™ family. Intel came out with design in the 8-bit microcontroller
domain-the 8051 family and its derivatives. It was developed in the 1980s and was put under the

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family MCS-51. Almost 75% of the microcontrollers used in the embedded domain were 8051
family based controllers during the 1980-90s. 8051 processor cores are used in more than 100
devices by more than 20 independent manufacturers like Maxim, Philips, Atmel, etc. under the
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license from Intel. Due to the low cost, wide availability, memory efficient instruction set,
mature development tools and Boolean processing (bit manipulation operation) capability, 8051
family derivative microcontrollers are much used in high-volume consumer electronic devices,
entertainment industry and other gadgets where cost-cutting is essential.
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Digital Signal Processors (DSPs): Digital Signal Processors are powerful special purpose 8/
16/ 32 bit microprocessors designed specifically to meet the computational demands and power
constraints of today’s embedded audio, video, and communications applications. Digital signal
processors are 2 to 3 times faster than the general purpose microprocessors in signal processing
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applications. DSPs implement algorithms in hardware speeds up the execution, whereas general
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purpose processors implement the algorithm in firmware and the speed of execution depends
primarily on the clock for the processors. In general, DSP can be viewed as a microchip
designed for performing high speed computational operations for 'addition', 'subtraction',
'multiplication' and 'division'. A typical digital signal processor incorporates the following
four key units:
(i) Program Memory: Memory for storing the program required by DSP to process the data.
(ii) Data Memory: Working memory for storing temporary variables and data/ signal to be
processed.
(iii) Computational Engine: Performs the signal processing in accordance with the stored

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program memory. Computational Engine incorporates many specialized arithmetic units and
each of them operates simultaneously to increase the execution speed. It also incorporates
multiple hardware shifters for shifting operands and thereby saves execution time.
(iv) I/O Unit: Acts as an interface between the outside world and DSP. It is responsible for
capturing signals to be processed and delivering the processed signals.

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Audio video signal processing, telecommunication and multimedia applications are typical
examples where DSP is employed. Digital signal processing employs a large amount of real-
time calculations. Sum of Products (SOP) calculation, Convolution, Fast Fourier Transform
(FFT), Discrete Fourier Transform (DFT), etc, are some of the operations performed by digital
signal processors. Blackfin® processors from Analog Devices is an example of DSP which

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delivers breakthrough signal processing performance and power efficiency while also offering a
full 32-bit RlSC MCU programming model.
RISC versus CISC Processors/ Controllers: The term RISC stands for Reduced Instruction
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Set Computing. All RISC processors/ controllers possess lesser number of instructions, typically
in the range of 30 to 40. CISC stands for Complex instruction Set Computing. The instruction
set is complex and instructions are high in number. Atmel AVR microcontroller is an example
for a RISC processor and its instruction set contains only 32 instructions. The original version of
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8051 microcontroller (AT89C51) is a CISC controller and its instruction set contains 255
instructions.
Harvard versus Von-Neumann Processor/ Controller Architecture: The terms Harvard and
Von-Neumann refers to the processor architecture design. Microprocessors/ Controllers based
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on the Von-Neumann architecture shares a single common bus for fetching both instructions and
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data. Program instructions and data are stored in a common main memory. Von-Neumann
architecture based processors/ controllers first fetch an instruction and then fetch the data to
support the instruction from code memory. The two separate fetches slows down the controller's
operation. Von-Neumann architecture is also referred as Princeton architecture, since it was
developed by the Princeton University. Microprocessors/ Controllers based on the Harvard
architecture will have separate data bus and instruction bus. This allows the data transfer and
program fetching to occur simultaneously on both buses. With Harvard architecture, the data
memory can be read and written while program memory is being accessed. These separated data
memory and code memory buses allow one instruction to execute while the next instruction is

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fetched ("pre-fetching"). The pre-fetch theoretically allows much faster execution than Von-
Neumann architecture.
Big-Endian versus Little-Endian Processors/ Controllers: Endianness specifies the order in
which the data is stored in the memory by processor operations in a multi-byte system. Suppose
the word length is two byte; then data can be stored in memory in two different ways:

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Little-Endian: Higher order of data byte at the higher memory and lower order of data byte at
location just below the higher memory. For example a 4 byte long integer Byte3 Byte2 Byte1
Byte0 will be stored in the memory as follows:

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Big-Endian: Lower order of data byte at the higher memory and higher order of data byte at
location just below the higher memory. For example a 4 byte long integer Byte3 Byte2 Byte1
Byte0 will be stored in the memory as follows:
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Load Store Operation and Instruction Pipelining: For example x, y and z are memory
locations and we want to add the contents of x and y and store the result in location z.
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The first instruction load

R1, x loads the register R1
with the content of memory
location x, the second
instruction load R2, y loads
the register R2 with the
content of memory location y. The instruction add R3, R1, R2 adds the content of register R1
and R2 and store the result in register R3. The next instruction store R3, z stores the content of

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register R3 in memory location z.

In conventional program execution, the fetch and decode operations are performed in sequence.
Whenever the current instruction is executing the program counter will be loaded with the
address of the next instruction.
In case of jump or branch instruction, the new location is known only after completion of the

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jump or branch instruction. Depending on the stages involved in an instruction (fetch, read
register and decode, execute instruction, access an operand in data memory, write back the result
to register, etc.), there can be multiple levels of instruction pipelining. Below figure illustrates
the concept of Instruction pipelining for single stage pipelining.

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Application Specific Integrated Circuits (ASICs): Application specific integrated circuit is a
microchip designed to perform a specific or unique application. It is used as replacement to
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conventional general purpose logic chips. It integrates several functions into a single chip and
there by reduces the system development cost. As a single chip, ASIC consumes a very small
area in the total system. ASICs can be pre-fabricated for a special application or it can be custom
fabricated by using the components from a reusable building block library of components for a
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particular customer application. ASIC based systems are profitable only for large volume
commercial productions. Fabrication of ASICs requires a non-refundable initial investment
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(known as Non-Recurring Engineering Charges (NRE), a one-time expense) for the process
technology and configuration expenses.
Features of ASICs Drawbacks of ASICs
NRE cost Inflexible design
Less complex Updates require a re-design
High Performance Deployed systems cannot be upgraded
Low power consumption Complex and expensive development tool.
If Non-Recurring Engineering Charges (NRE) is accepted by a third party and the Application
Specific Integrated Circuit (ASIC) is made openly available in the market, the ASIC is referred
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as Application Specific Standard Product (ASSP).

ASICs ASSPs
Microchip designed to perform If third party is ready to pay NRE cost and ASIC is made
specific application available into the market, the ASIC referred as ASSP
Most of the ASICs are proprietary Openly available in the market
product
Programmable Logic Devices: Logic devices provide specific functions, including device-to-

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device interfacing, data communication, signal processing, data display, timing and control
operations, and almost every other function a system must perform. Logic devices can be
classified into two broad categories-fixed and programmable. As the name indicates, the circuits
in a fixed logic device are permanent, they perform one function or set of functions-once

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manufactured, they cannot be changed.
Programmable Logic Devices (PLDs) offer customers a wide range of logic capacity, features,
speed, and voltage characteristics; and these devices can be re-configured to perform any
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number of functions at any time.
Advantages of PLDs:
 PLDs offer customers much more flexibility during the design cycle because design
iterations are simply a matter of changing the programming file, and results of design
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changes can be seen immediately in working parts.

 PLDs do not require long lead times for prototypes or production parts-the PLDs are
already on a distributor's shelf and ready for shipment.
 PLDs do not require customers to pay for large NRE costs and purchase expensive mask
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sets-PLD suppliers incur those costs when they design their programmable devices.
 PLDs allow customers to order just the number of parts they need, when they need them,
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allowing them to control inventory.

 PLDs can be reprogrammed even after a piece of equipment is shipped to a customer.
Programmable Gate Arrays (FPGAs) and Complex Programmable Logic Devices
(CPLDs): FPGAs offer the highest amount of logic density, the most features, and the highest
performance. At present the largest FPGA shipping part of the Xilinx Virtex™.
CPLDs FPGAs
PLD is used for construction of CPLD Logic blocks are used for construction of FPGA
CPLD is non-volatile & less costly FPGA is volatile & costly
Delays are much more predictable in Prediction of delay is difficult in FPGA
CPLDs
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Operating speed is low & is suitable for Operating speed is high & is suitable for timing
control circuit circuit
CPLD has less flexibility and design FPGA has more flexibility as well as design
capacity capacity
CPLD could work immediately after power FPGA could not work until the configuration is
up done
CPLDs are considered as ‘coarse-grain’ FPGAs are considered as ‘fine-grain’ devices

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devices

FPGAs ASICs
FPGA is a reprogrammable integrated ASIC is a unique type of integrated circuit
circuit meant for a specific application
FPGA is not efficient in terms of use of ASIC wastes very little material, recurring cost
materials is low
FPGA is better than ASIC when building Cost of ASIC is low only when it is produced in

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low volume production circuits large quality
FPGA is alterable Once created, ASIC can no longer be altered
FPGAs are useful for research and ASICs are not suitable for research and
de3velopment activities. Prototype development purpose, as they are reconfigurable
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fabrication using FPGA is affordable and
fast

Commercial Off-the-Shelf Components (COTS): COTS products are designed in such a way
to provide easy integration and interoperability with existing system components. The COTS
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component itself may be developed around a general purpose or domain specific processor or an
ASICs or a PLDs. Typical examples of COTS hardware unit are remote controlled toy car
control units including the RF circuitry part, high performance, high frequency microwave
electronics (2-200 GHz), high bandwidth analog-to-digital converters, devices and components
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for operation at very high temperatures,

electro-optic IR imaging arrays, UV/IR
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detectors, etc. The major advantage of using

COTS is they are readily available in the
market, cheap and a developer can cut down
development time too. This in turn reduces the
time to market the embedded systems. The TCP/IP plug-in module available from various
manufactures like WIZnet, Freescale, Dynalog, etc
Memory
The processors/ controllers contain built in memory and this memory is referred as on-chip

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memory. Others do not contain any memory inside the chip and requires external memory to be
connected with the controller/processor to store the control algorithm called as off-chip memory.
Also some working memory is required for holding data temporarily during certain operations.
Program Storage Memory (ROM): The program memory or code storage memory of an
embedded system stores the program instructions and it can be classified into different types.

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The code memory retains its contents even after the power to it is turned off. It is generally
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known as non-volatile storage memory. Depending on the fabrication, erasing and programming
techniques, they are classified into the following types:
Masked Memory (MROM): Masked ROM is a one-time programmable device. Masked ROM
makes use of the hardwired technology for storing data. The device is factory programmed by
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masking and metallization process at the time of production itself, as per the data provided by
the end user. The primary advantage of this is low cost for high volume production. Different
mechanisms are used for the masking process of the ROM, like (i) Creation of an enhancement
or depletion mode transistor through channel implant. (ii) By creating the memory cell either
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using a standard transistor or a high threshold transistor. Masked ROM is a good for storing the
embedded firmware for low cost embedded devices. Once the design is proven and the firmware
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requirements are tested and frozen, the binary data (The firmware cross compiled/assembled to
target processor specific machine code) corresponding to it can be given to the MROM
fabricator.
The limitation with MROM based firmware storage is the inability to modify the device
firmware against firmware upgrades. Since the MROM is permanent in bit storage, it is not
possible to alter the bit information.
Programmable Read Only Memory (PROM)/ One Time Programmable Memory (OTP):
PROM is not pre-programmed by the manufacturer. The end user is responsible for
programming these devices. This memory has nichrome or polysilicon wires arranged in a
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matrix. These wires can be functionally viewed as fuses. It is programmed by a PROM

programmer which selectively burns the fuses according to the bit pattern to be stored. Fuses
which are not blown/ burned, represents logic 1; whereas fuses which are blown/ burned
represents logic 0. The default state is logic "1".
OTP is widely used for commercial production of embedded systems whose prototyped versions

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are proven and the code is finalized. It is a low cost solution for commercial production. OTPs
cannot be reprogrammed.
OTPs are not useful and worth for development purpose. During the development phase, the
code is subject to continuous changes and using an OTP each time to load the code is not
economical.

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Erasable Programmable Read Only Memory (EPROM): EPROM gives the flexibility to
reprogram the same chip. It stores the bit information by charging the floating gate of an FET.
Bit information is stored by using an EPROM programmer, which applies high voltage to charge
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the floating gate. EPROM contains a quartz crystal window for erasing the stored information. If
the window is exposed to ultraviolet rays for a fixed duration, the entire memory will be erased.
Even though the EPROM chip is flexible in terms of re-programmability, it needs to be taken
out of the circuit board and put in a UV eraser device for 20 to 30 minutes. So it is a tedious and
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time-consuming process.
Electrically Erasable Programmable Read Only Memory (EEPROM): Electrically Erasable
Programmable Read Only Memory indicates; the information contained in the EEPROM
memory can be altered by using electrical signals at the register/Byte level. They can be erased
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and reprogrammed in-circuit. These chips include a chip erase mode; and in this mode, they can
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be erased in a few milliseconds. It provides greater flexibility for system design.

The only limitation is their capacity is limited (only few kilobytes) when compared with the
standard ROM.
FLASH: FLASH is the latest ROM technology and is the most popular ROM technology used
in today's embedded designs. FLASH memory is a variation of EEPROM technology. It
combines the re-programmability of EEPROM and the high capability of standard ROMs.
FLASH memory is organized as sectors (blocks) or pages. It stores information in an array of
floating gate MOSFET transistors. The erasing of memory can be one at sector level or page
level without affecting the other sectors or pages. Each sector/ page should be erased before re-

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 Microcontroller and Embedded Systems 2023

programming. The typical erasable capacity of FLASH is 1000 cycles.

NVRAM: Non-volatile RAM is a random access memory with battery backup. It contains static
RAM based memory and a minute battery for providing supply to the memory in the absence of
external power supply. The memory and battery are packed together in a single package.
The life span of NVRAM is expected to be around 10 years. DSJ644 from Maxim/ Dallas is an

om
example of 32KB NVRAM.
Read-Write Memory/ Random Access memory (RAM): RAM is the data memory or working
memory of the controller/ processor. Controller/ processor can read from it and write to it. RAM
is volatile, meaning when the power is turned off, all the
contents are destroyed. RAM is a direct access memory,

.c
meaning we can access the desired memory location
directly without the need for traversing through the
entire memory locations to reach the desired memory
de
position (i.e. random access of memory location).
This is in contrast to the Sequential Access Memory (SAM), where the desired memory location
is accessed by either traversing through the entire memory or through a 'seek' method. Magnetic
tapes, CD ROMs, etc. are examples of sequential access memories.
co

RAM generally falls into three categories: Static RAM (SRAM), dynamic RAM (DRAM) and
non-volatile RAM (NVRAM).
Static RAM (SRAM): Static RAM stores data in the form of voltage. They are made up of
flipflops. Static RAM is the fastest form of RAM available. In typical implementation, an
u

SRAM cell (bit) is realized using six transistors (or 6

vt

MOSFETs). Four of the transistors are used for building

the latch (flip-flop) part of the memory cell and two for
controlling the access. RAM is fast in operation due to its
resistive networking and switching capabilities.
This implementation in its simpler form can be visualized
as two-cross coupled inverters with read/ write control through transistors. The four transistors
in the middle form the cross-coupled inverters.
From the SRAM implementation diagram access to the memory cell is controlled by the line
Word Line, which controls the access transistors (MOSFETs) Q5 and Q6. The access

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 Microcontroller and Embedded Systems 2023

transistors control the connection to bit lines B & B\.

In order to write a value to the memory cell, apply the desired value to the bit control lines (For
writing 1, make B = 1 and B\ =0; For writing 0,
make B = 0 and B\ =1) and assert the Word
Line (Make Word line high). This operation

om
latches the bit written in the flip-flop.
o For reading the content of the memory cell,
assert both B and B\ bit lines to 1 and set the Word line to 1.
The major limitations of SRAM are low capacity and high cost. Since a minimum of six
transistors are required to build a single memory cell, imagine how many memory cells we can

.c
fabricate on a silicon wafer.
Dynamic RAM (DRAM): Dynamic RAM stores data in the form of charge. They are made up
of MOS transistor gates. The advantages of DRAM are its high density and low cost compared
de
to SRAM.
The disadvantage is that, since the information is stored as charge it gets leaked off with time;
and to prevent this, they need to be refreshed periodically.
Special circuits called DRAM controllers are used for the
co

refreshing operation. The refresh operation is done

periodically in milliseconds interval. The following Figure
illustrates the typical implementation of a DRAM cell.
The MOSFET acts as the gate for the incoming and outgoing
u

data, whereas the capacitor acts as the bit storage unit.

vt

SRAM Cell DRAM Cell

Made up of 6 CMOS transistors Made up of a MOSFET and a Capacitor
(MOSFET)
Doesn’t require refreshing Requires refreshing
More expensive Less expensive
Fast in operation, typical access time is 10 Slow in operation due to refresh requirement,
ns typical access time is 60 ns; write operation is
faster than read operation

NVRAM: Non-volatile RAM is a random access memory with battery backup. It contains static
RAM based memory and a minute battery for providing supply to the memory in the absence of
external power supply. The memory and battery are packed together in a single package. The

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 Microcontroller and Embedded Systems 2023

life span of NVRAM is expected to be around 10 years. DSJ644 from Maxim/ Dallas is an
example of 32KB NVRAM. Memory According to the Type of Interface: The interface
(connection) of memory with the processor/ controller can be of various types. It may be a
parallel interface (the parallel data lines (DO-D7) for an 8 bit processor/ controller will be
connected to DO-D7 of the memory) or a serial interface like I2C (Pronounced as I Square C - It

om
is a 2 line serial interface) or a SPI (Serial Peripheral Interface, 2+n line interface where n stands
for the total number of SPI bus devices in the system) a single wire interconnection (like Dallas
1-Wire interface).
Serial interface is commonly used for data storage memory like EEPROM. The memory density
of a serial memory is usually expressed in terms of kilobits, whereas that of a parallel interface

.c
memory is expressed in terms of kilobytes. Atmel Corporations AT24C512 is an example for
serial memory with capacity 512 kilobits and 2-wire interface. Memory Shadowing: Generally
the execution of a program or a configuration from a Read Only Memory (ROM) is very slow
de
(120 to 200 ns) compared to the execution from a random access memory (40 to 70 ns). From
the timing parameters, it is obvious that RAM access is about three times as fast as ROM access.
Shadowing of memory is a technique adopted to solve the execution speed problem in
processor-based systems. In computer systems and video systems, there will be a configuration
co

holding ROM called Basic Input Output Configuration ROM or simply BIOS.
In personal computer system, BIOS stores the hardware configuration information like the
address assigned for various serial ports and other non-plug 'n' play devices, etc. Usually it is
u

read and the system is configured accordingly to it during system boot up and it is time
consuming. Now, the manufactures included a RAM behind the logical layer of BIOS at its
vt

same address as a shadow to the BIOS; and the following steps happens:
 During the boot up, BIOS is copied to the shadowed RAM
 RAM is write protected
 BIOS’ reading is disabled
RAM is volatile and it cannot hold the configuration data which is copied from the BIOS when
the power supply is switched off. Only a ROM can hold it permanently. But for high system
performance, it should be accessed from a RAM instead of accessing from a ROM.
Memory Selection for Embedded Systems require
- A program memory for holding the control algorithm or embedded OS and applications,
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 Microcontroller and Embedded Systems 2023

- Data memory for holding variables and temporary data during task execution, and
- Memory for holding nonvolatile data likes configuration data, look up table, etc. which are
modifiable by the application.
The memory requirement for an embedded system in terms of RAM and ROM
(EEPROM/FLASH/NVRAM) is solely dependent on the type of the embedded system and the

om
applications for which it is designed.
The factors need to be considered when selecting the type and size of memory for embedded
system. For example, if the embedded system is designed using SoC or a microcontroller with
on-chip RAM and ROM (FLASH/EEPROM), depending on the application need the on-chip
memory may be sufficient for designing the total system.

.c
Consider an example of simple electronic toy design. As the complexity of requirements are less
and data memory requirement are minimal, we can think of a microcontroller with a few bytes
of internal RAM, a few bytes or kilobytes (depending on the number of tasks and the complexity
de
of tasks) of FLASH memory and a few bytes of EEPROM (if required) for designing the system.
Hence there is no need for externa1 memory at all. A PIC microcontroller device which satisfies
the I/O and memory requirements can be used in this case.
If the embedded design is based on an RTOS, the RTOS requires certain amount of RAM for its
co

execution and ROM for storing the RTOS image. Normally the binary code for RTOS kernel
containing all the services is stored in a non-volatile memory (like FLASH) as either
compressed or non-compressed data. During boot-up of the device, the RTOS files are copied
u

from the program storage memory, decompressed if required and then loaded to the RAM for
execution. The supplier of the RTOS usually gives a rough estimate on the run time RAM
vt

requirements and program memory requirements for the RTOS.

On a safer side, always buffer value to be added for the total estimated RAM and ROM size
requirements. A smart phone device with Windows mobile operating system is a typical
example for embedded device with OS. Say 64MB RAM and 128MB ROM is the minimum
requirements for running the Windows mobile device. So while building the system, count the
memory for that also and arrive at a value which is always at the safer side.
There are two parameters for representing a memory –
Size of the memory chip: There is no option to get a memory chip with the exact required
number of bytes. Memory chips come in standard sizes, like 512bytes, 1024bytes (1 kilobyte),

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 Microcontroller and Embedded Systems 2023

2048bytes (2 kilobytes), 4Kb, 8Kb, 16Kb, 32Kb, 64Kb, 128Kb, 256Kb, 512Kb, 1024Kb (1
megabytes), etc.
Suppose embedded application requires only 750 bytes of RAM, the option to choose the
memory chip with a size closer to the size is 1024 bytes.
Address range supported to the processor: A processor/ controller with 16-bit address bus can

om
addressed 216 = 65536 bytes = 64Kb. Hence, it is meaningless to select a 128Kb memory chip
for a processor with 16-bit wide address bus. Also, the entire memory range supported by the
processor/ controller may not be available to the memory chip alone. It may be shared between
I/O, other ICs and memory.
Word size of the memory: The word size refers to the number of memory bits that can be

.c
read/write together at a time. 4, 8, 12, 16, 24, 32 etc., are the word sizes supported by memory
chips. Ensure that the word size supported by the memory chip matches with the data bus width
of the processor/ controller.
de
FLASH memory is the popular choice for ROM (program storage memory) in embedded
applications. It is a powerful and cost-effective solid-state storage technology for mobile
electronics devices and other consumer applications.
FLASH memory comes in two major variants, namely, NAND and NOR FLASH. NAND
co

FLASH is a high-density low cost non-volatile storage memory; NOR FLASH is less dense and
slightly expensive. But NOR FLASH supports the Execute in Place (XIP) technique for program
execution. The XIP technology allows the execution of code memory from ROM itself without
u

the need for copying it to the RAM as in the case of conventional execution method.
The EEPROM data storage memory is available as either serial interface or parallel interface
vt

chip. If the processor/ controller of the device support serial interface and the amount of data to
write and read to and from the device are less, it is better to have a serial EEPR0M chip. The
serial EEPROM saves the address space of the total system. The memory capacity of the serial
EEPROM is usually expressed in bits or Kilobits: 512 bits, 1Kbits, 2Kbits, 4Kbits, etc. are
examples for serial EEPROM memory representation.
Sensors & Actuators
An embedded system is in constant interaction with the Real world, and the controlling/
monitoring functions, executed by the embedded system is achieved in accordance with the
changes happening to the Real world. The changes in system environment or variables are

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 Microcontroller and Embedded Systems 2023

detected by the sensors connected to the input port of the embedded system. A sensor is a
transducer device that converts energy from one form to another, for any measurement or
control purpose. Sensor which counts steps for pedometer functionality is an Accelerometer
sensor. Sensor used in smart watch devices to measure the high intensity is an Ambient Light
Sensor (ALS).

om
If the embedded system is designed for any controlling purpose, the system will produce some
changes in the controlling
variable to bring the controlled
variable to the desired value. It
is achieved through an actuator

.c
connected to the output port of
the embedded system.
Actuator is a form of transducer device (mechanical or electrical) which converts signals to
de
corresponding physical action (motion). Actuator acts as an output device. Smart watches use
Ambient Light Sensor to detect the surrounding light intensity and uses an electrical/ electronic
actuator circuit to adjust the screen brightness.
If the embedded system is designed for monitoring purpose only, then there is no need for
co

including an actuator in the system. ECG machine is designed to monitor the heart beat status
of a patient and it cannot impose a control over the patient's heart beat and its order. The sensors
used here are the different electrode sets connected to the body of the patient. The variations are
u

captured and presented to the user (may be a doctor) through a visual display or some printed
chart.
vt

Sensors Actuators
Sensor is an input device Actuator is an output device
Convert a physical parameter to an Convert an electrical signal to a physical
electrical output output
A device that detects events or changes in A component of a machine that is responsible
the environment and send the information for moving and controlling mechanisms
to another electronic device
Sensor help to monitor the changes in the Actuator helps to control the environment or
environment physical changes

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 Microcontroller and Embedded Systems 2023

The I/O Subsystem

The I/O subsystem of the embedded system facilitates the interaction of the embedded system
with the external world. As mentioned earlier the interaction happens through the sensors and
actuators connected to the input and output ports respectively of the embedded system.
Light Emitting Diode (LED): LED is an output device for visual indication in any embedded

om
system. LED can be used as an indicator for the status of various signals or situations. For
example indicating the presence of power conditions like Device ON,
Battery Low or Charging of Battery for a battery operated handheld
embedded devices.
Light Emitting Diode is a p-n junction diode and it contains an anode

.c
and a cathode. For proper functioning of the LED, the anode of it
should be connected to +ve terminal of the supply voltage and cathode to the –ve terminal of
supply voltage. A resister is used in series between the power supply and the LED to limit the
de
current through the LED. It can be interfaced to the port pin of a processor/ controller in two
ways.
In the first method, the anode is directly connected to the port pin and the port pin drives the
LED. In this the port pin sources current to the LED when the port pin is at logic High.
co

In the second method, the cathode of the LED is connected to the port pin of processor/
controller and the anode to the supply voltage through a current limiting resistor. LED is turned
on when the port pin is at logic Low.
u

7-Segment LED Display: The 7-segment LED display is an output device used for displaying
alphanumeric characters. It contains 8 LED segments arranged in a
vt

special form. Out of the 8 LED segments, 7 are used for displaying
alphanumeric characters and 1 is used for representing decimal
point.
The LED segments are named A to G and the decimal point LED segment is named as DP. For
displaying the number 4, the segments F, G, B and C are lit. For displaying 3, the segments A,
B, C, D, G are lit. All these 8 LED segments need to be connected to one port of the processor/
controller for displaying alpha-numeric digits. The 7-segment LED displays are available in two
configurations, namely; Common Anode and Common Cathode. In common anode
configuration, the anodes of the 8 segments is connected commonly & in common cathode

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 Microcontroller and Embedded Systems 2023

configuration, the 8 LED segments share a common cathode line.

om
Optocoupler: It is a solid state device to isolate two parts of a circuit. Optocoupler combines an
LED and a photo-transistor in a single housing (package). In
electronic circuits, an optocoupler is used for suppressing
interference in data communication, circuit isolation, high
voltage separation, simultaneous separation and signal

.c
intensification, etc. Optocoupler can be used in either input circuits or in output circuits.
de
co

Optocoupler is available as ICs from different semiconductor manufacturers. The MCT2M IC

from Fairchild semiconductor is an example for optocoupler IC.
Stepper Motor: A stepper motor is an electromechanical device which generates discrete
u

displacement (motion) in response, to de electrical signals. It differs from the normal DC motor
in its operation. The DC motor produces continuous rotation on applying DC voltage, whereas a
vt

stepper motor produces discrete rotation in response to the DC voltage applied to it. Stepper
motors are widely used in industrial embedded applications, consumer electronic products and
robotics control systems. The paper feed mechanism of a printer/ fax makes use of stepper
motors for its functioning. Based on the coil winding arrangements, a two-phase stepper motor
is classified into unipolar & bipolar.
Unipolar: A unipolar stepper motor contains two windings per phase. The direction of rotation
(clockwise or anticlockwise) of a stepper motor is controlled by changing the direction of
current flow. Current in one direction flows through one coil and in the opposite direction flows
through the other coil. It is easy to shift the direction of rotation by just switching the terminals
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 Microcontroller and Embedded Systems 2023

to which the coils are connected.

The coils are represented as A, B, C and D. Coils A and C carry
current in opposite directions for phase 1 (only one of them will be
carrying current at a time). Similarly, B and D carry current in
opposite directions for phase 2 (only one of them will be carrying

om
current at a time).
Bipolar: A bipolar stepper motor contains single winding per phase. For reversing the motor
rotation the current flow through the windings is reversed dynamically. It requires complex
circuitry for current flow reversal.
The stepping of stepper motor can be implemented in different ways by changing the sequence

.c
of activation of the stator windings. The different stepping modes supported by stepper motor
are wave, full step & half step. The rotation of the stepper motor can be reversed by reversing
the order in which the coil is energized.
de
Wave step: One coil is energised at a time & all four coils are energised one after another. It
produces less torque in compare with full step drive but power consumption is less.
Clockwise Step# Winding A Winding B Winding C Winding D Counter
clockwise
1 1 0 0 0
co

2 0 1 0 0
3 0 0 1 0
4 0 0 0 1
Full step: Two coils energised at a time. It produces high torque with high power consumption.
Clockwise Step# Winding A Winding B Winding C Winding D Counter
clockwise
u

1 1 1 0 0
2 0 1 1 0
3 0 0 1 1
vt

4 1 0 0 1
Half step: One & Two coil energised alternatively and used to increase the angular rotation of
the motor.
Clockwise Step# Winding A Winding B Winding C Winding D Counter
clockwise
1 1 0 0 0
2 1 1 0 0
3 0 1 0 0
4 0 1 1 0
5 0 0 1 0
6 0 0 1 1
7 0 0 0 1
8 1 0 0 1

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 Microcontroller and Embedded Systems 2023

Two-phase unipolar stepper motors are popular choice for embedded applications. The current
requirement for stepper motor is little high and hence the
port pins of a microcontroller/ processor may not be able
to drive the directly. Also the supply voltage required to
operate stepper motor varies normally in the range 5V to

om
24V. Depending on the current and voltage requirements,
special driving circuits are required to interface the
stepper motor with microcontroller/ processors.
ULN2803 is an octal peripheral driver
array available from Texas Instruments

.c
and ST microelectronics for driving a 5V
stepper motor. Simple driving circuit can
also be built using transistors. The circuit
de
diagram illustrates the interfacing of a
stepper motor through a driver circuit
connected to the port pins of a microcontroller/ processor.
Relay: Relay is an electro-mechanical device. In embedded application, the Relay unit acts as
co

dynamic path selectors for signals

and power. The Relay unit contains
a relay coil made up of insulated
u

wire on a metal core and a metal

armature with one or more contacts.
vt

Relay works on electromagnetic principle. When a voltage is applied to the relay coil, current
flows through the coil, which in turn generates a magnetic field. The magnetic field attracts the
armature core and moves the contact point. The movement of the contact point changes the
power/ signal flow path. They are available in different configurations.
The Single Pole Single Throw (SPST) configuration has only one path for information flow.
The path is either open or dosed in normal condition.
 For normally open Single Pole Single Throw relay, the circuit is normally open and it
becomes closed when the relay is energized.
 For normally closed Single Pole Single Throw configuration, the circuit is normally

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 Microcontroller and Embedded Systems 2023

closed and it becomes open when the relay is energized.

For Single Pole Double Throw Relay (SPDT), there are two paths for information flow and
they are selected by energizing or de-energizing the relay. The Relay is normally controlled
using a relay driver circuit connected to the port pin of the processor/ controller. A transistor is
used for building the relay driver circuit.

om
.c
A free-wheeling diode is used for free-wheeling the voltage produced in the opposite direction
de
when the relay coil is de-energized. The freewheeling diode is essential for protecting the relay
and the transistor.
Piezo Buzzer: It is a piezoelectric device for generating audio indications in embedded
application. A piezoelectric buzzer contains a piezoelectric diaphragm which produces audible
co

sound in response to the voltage applied to it. Piezoelectric buzzers are available in two types. (i)
Self-driving (ii) External driving
The Self-driving circuit contains all the necessary components to generate sound at a predefined
tone. It will generate a tone on applying the voltage.
u

External driving Piezo buzzers supports the generation of different tones. The tone can be varied
vt

by applying a variable pulse train to the piezoelectric buzzer.

A Piezo buzzer can be directly interfaced to the port pin of the processor/ control. Depending on
the driving current requirements, the Piezo buzzer can also be interfaced using a transistor based
driver circuit as in the case of a Relay.
Push Button Switch: Push button switch is an input device. This comes in two configurations,
namely Push to Make and Push to Break.
In the Push to Make configuration, the switch is normally in the open state and it makes a circuit
contact when it is pushed or pressed & in the Push to Break configuration, the switch is
normally in the closed state and it breaks the circuit contact when it is pushed or pressed.

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 Microcontroller and Embedded Systems 2023

The push button stays in the closed (for Push to Make type or open for Push to Break type) state
as long as it is kept in the pushed state and it breaks/
makes the circuit connection when it is released.
Push button is used for generating a momentary pulse.
In embedded application push button is generally used

om
as reset and start switch and pulse generator. It is
normally connected to the port pin of the host
processor/ controller.
Depending on the way in which the push button interfaced to the controller, it can generate
either a HIGH pulse or a LOW pulse.

.c
Keyboard: Keyboard is an input device HIGH Pulse generator for user interfacing. If the
number of keys required is very limited, push button switches can be used and they can be
directly interfaced to the port pins for reading. For larger number of keys it may not be possible
de
to interface each keys to a port pin due to the limitation in the number of general purpose port
pins available for the processor/ controller in use and moreover it is wastage of port pins. Matrix
keyboard is an optimum solution for handling large key requirement. It greatly reduces the
number of interface connections. For example, for interfacing 16 keys, in the direct interfacing
co

technique, 16 port pins are required,

whereas in the matrix keyboard only 8
lines are required. The 16 keys are
u

arranged in a 4 column x 4 Row matrix.

For detecting a key press, the keyboard
vt

uses the scanning technique, where

each row of the matrix is pulled low
and the columns are read. After reading
the status of each columns
corresponding to a row, the row is
pulled high and the next row is pulled
low and the status of the columns are
read. This process is repeated until the scanning for all rows are completed. When a row is
pulled low and if a key connected to the row is pressed, reading the column to which the key is

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 Microcontroller and Embedded Systems 2023

connected will give logic 0. Since keys are mechanical devices, proper key de-bouncing
technique should be applied.
Programmable Peripheral Interface (PPI): These devices are used for extending the I/O
capabilities of processors/ controllers. Most of the processors/ controllers provide very limited
number of I/O and data ports and at times it may require more number of I/O ports than the one

om
supported by the controller/ processor. A PPI device expands the I/O capabilities of the
processor/ controller. 8255A is a popular PPI device for 8-bit processors/ controllers. 8255A
supports 24 I/O pins, and these I/O pins can be grouped as either three 8-bit parallel ports (Port
A, Port B and Port C) or two 8-bit parallel ports (Port A and Port B) with Port C in any one of
the following configurations: (1) As 8 individual I/O pins (2) Two 4-bit ports; namely Port

.c
CUPPER (Cu) and Port CLOWER (CL). This is configured by manipulating the control register of
8255A. The control register holds the configuration for Port A, Port B and Port C.
Control register for mode selection of the 8255
de
u co

The generic interfacing of an 8255A device with an 8-bit processor/ controller with 16-bit
address bus (Lower order Address bus is multiplexed with data bus) is shown in the below
vt

figure.

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 Microcontroller and Embedded Systems 2023

Communication Interface
Communication interface is essential for communicating with various subsystems of the
embedded system with the external world. For an embedded product, the communication
interface can be viewed in two different perspectives.
(i) Device/ Board level communication interface (Onboard Communication Interface):

om
Embedded product is a combination of different types of components (chips/ devices) arranged
on a printed circuit board (PCB). The communication channel which interconnects the various
components within an embedded product is referred as Device/ Board Level Communication
Interface (Onboard Communication Interface). The examples of Serial interfaces are I2C, SPI,
UART, 1-Wire, etc., and parallel bus interface.

.c
(ii) Product level communication interface (External Communication Interface): Some
embedded systems are self-contained units and they don't require any interaction and data
transfer with other sub-systems or external world. On the other hand, certain embedded systems
de
may be a part of a large distributed system and they require interaction and data transfer between
various devices and sub-modules. The Product level communication interface is responsible for
data transfer between the embedded system and other devices or modules.
The external communication interface can be either a wired medium or a wireless media and it
co

can be a serial or a parallel interface.

Infrared (IR), Bluetooth (BT), Wireless LAN (Wi-Fi), Radio Frequency waves (RF), GPRS/ 3G/
4GLTE, etc. are examples for wireless communication interface.
u

RS-232C/ RS-422/ RS-485, USB, Ethernet IEEE 1394 port, Parallel port, CF-II interface, SDIO,
PCMCIA/ PCIex, etc., are examples for wired interfaces.
vt

Onboard Communication Interfaces: Onboard Communication Interface refers to the

different communication channels/ buses for interconnecting the various integrated circuits and
other peripherals within the embedded system.
Inter Integrated Circuit (I2C) Bus: It is a synchronous bidirectional half duplex (one-
directional communication at a given point of time) two wire serial interface bus. The concept of
I2C bus was developed by Philips Semiconductors in the early 1980s. The original intention of
I2C was to provide an easy way of connection between a microprocessor/ microcontroller
system and the peripheral chips in television sets.
The I2C bus comprise of two bus lines, Serial Clock-SCL and Serial Data-SDA. SCL line is
29 | P a g e
 Microcontroller and Embedded Systems 2023

responsible for generating synchronization clock pulses. SDA is responsible for transmitting the
serial data across devices.
I2C bus is a shared bus system to which many number of I2C devices can be connected. Devices
connected to the I2C bus can act as either Master device or Slave device. The Master device is
responsible for controlling the communication by initiating/ terminating data transfer, sending

om
data and generating necessary synchronization clock pulses. Slave devices wait for the
commands from the master and respond upon receiving the commands. The synchronization
clock signal is generated by the 'Master' device only & I2C supports multi-masters on the same
bus.

.c
de
co

The address to various I2C devices in an embedded device is assigned and hardwired at the time
u

of designing the embedded hardware. The sequence of operations for communicating with an
I2C slave device is listed below:
vt

i. The master device pulls the clock line (SCL) of the bus to HIGH.
ii. The master device pulls the data line (SDA) 'LOW', when the SCL line is at logic 'HIGH'
(This is the 'Start' condition for data transfer).
iii. The master device sends the address (7-bit or 10-bit wide) of the 'slave' device to which it
wants to communicate, over the SDA line. Clock pulses are generated at the SCL line for
synchronizing the bit reception by the slave device. The MSB of the data is always
transmitted first. The data in the bus is valid during the HIGH period of the clock signal
iv. The master device sends the Read or Write bit (Bit value = 1 Read operation; Bit value = 0
Write operation) according to the requirement.
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 Microcontroller and Embedded Systems 2023

v. The master device waits for the acknowledgement bit from the slave device whose address
is sent on the bus along with the Read/ Write operation command. Slave devices connected
to the bus compares the address received with the address assigned to them.
vi. The slave device with the address requested by the master device responds by sending an
acknowledge bit (Bit value = 1) over the SDA line.

om
vii. Upon receiving the acknowledge bit, the master device sends the 8-bit data to the slave
device over SDA line, if the requested operation is Write to device. If the requested
operation is 'Read from device', the slave device sends data to the master over the SDA
line.
viii. The master device waits for the acknowledgement bit from the device upon byte transfer

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complete for a write operation and sends an acknowledge bit to the Slave device for a read
operation.
ix. The master device terminates the transfer by pulling the SDA line HIGH when the clock
de
line SCL is at logic HIGH (Indicating the STOP condition).
Serial Peripheral Interface (SPI): SPI is asynchronous bi-directional full duplex four-wire
serial interface bus. The concept of SPI was introduced by Motorola. It is a single master multi-
slave system. It is possible to have a system where more than one SPI device can be master,
co

provided the condition only one master device is active at any given point of time, is satisfied.
SPI requires four signal lines for communication.
Master Out Slave In (MOSI): Signal line
u

carrying the data from master to slave device. It is

also known as Slave Input/Slave Data In (SI/SDI).
vt

Master In Slave Out (MISO): Signal line

carrying the data from slave to master device. It is
also known as Slave Output (SO/ SDO).
Serial Clock (SCL): Signal line carrying the
clock signals
Slave Select (SS): Signal line for slave device select. It is an active low signal.
The master device is responsible for generating the clock signal. It selects the required slave
device by asserting the corresponding slave device's slave select signal LOW. The data out line
(MISO) of all the slave devices when not selected floats at high impedance state. SPI works on

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 Microcontroller and Embedded Systems 2023

the principle of Shift Register. The master and slave devices contain a special shift register for
the data to transmit or receive. The size of the shift register is normally it is a multiple of 8.
During transmission from the master to slave, the data in the master's shift register is shifted out
to the MOSI pin and it enters the shift register of the slave device through the MOSI pin of the
slave device. At the same time, the shifted out data bit from the slave device's shift register

om
enters the shift register of the master device through MISO pin.
Universal Asynchronous Receiver Transmitter (UART): UART based data transmission is
an asynchronous form of serial data transmission. UART based serial data transmission doesn't
require a clock signal to synchronize the transmitting end arid receiving end for transmission.
Instead it relies upon the pre-defined agreement between the transmitting device and receiving

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device. The serial communication settings (Baud rate, number of bits per byte, parity, number of
start bits and stop bit and flow control) for both transmitter and receiver should be set as
identical. The start and stop of communication is indicated through inserting special bits in the
de
data stream. While sending a byte of data, a start bit is added first and a stop bit is added at the
end of the bit stream. The least significant bit of the data byte follows the start bit. The start bit
informs the receiver that a data byte is about to
arrive. The receiver device starts polling it’s
co

receive line as per the baud rate settings. The

baud rate is x bits per second, the time slot
available for one bit is 1/x seconds.
u

The receiver unit polls the receiver line at

exactly half of the time slot available for the bit.
vt

If parity is enabled for communication, the UART of the transmitting device adds a parity bit
(bit value is 1 for odd number of 1s in the transmitted bit stream and 0 for even number of 1s).
The UART of the receiving device calculates the parity of the bits received and compares it with
the received parity bit for error checking. The UART of the receiving device discards the Start,
Stop and Parity bit from the received bit stream and converts the received serial bit data to a
word. For proper communication, the Transmit line of the sending device should be connected
to the Receive line of the receiving device.
In addition to the serial data transmission function, UART provides hardware handshaking
signal support for controlling the serial data flow. UART chips are available from different

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 Microcontroller and Embedded Systems 2023

semiconductor manufacturers. National Semiconductor's 8250 UART chip is considered as the

standard setting UART. It was used in the original IBM PC.
1-Wire Interface: It is an asynchronous half-duplex communication protocol developed by
Maxim Dallas Semiconductor. It is also known as Dallas 1-Wire® protocol. It makes use of only
a single signal line (wire) called DQ for communication and follows the master-slave

om
communication model.

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de
The key feature of 1-wire bus is that it allows power to be sent along the signal wire as well. The
co

1-wire slave devices incorporate internal capacitor (typically of the order of 800 pF) to power
the device from the signal line. It supports a single master and one or more slave devices on the
bus.
Every 1-wire device contains a globally unique 64-bit identification number stored within it.
u

This unique identification number can be used for addressing individual devices present on the
bus in case there are multiple slave devices connected to the 1-wire bus. The identifier has three
vt

parts: an 8-bit family code, a 48-bit serial number and an 8-bit CRC computed from the first 56-
bits.
The sequence of operation for communicating with a 1-wire slave device is listed below:
i. The master device sends a 'Reset' pulse on the 1-wire bus.
ii. The slave device(s) present on the bus respond with a 'Presence' pulse.
iii. The master device sends a ROM command (Net Address Command followed by the 64-
bit address of the device). This addresses the slave device(s) to which it wants to initiate
a communication.
iv. The master device sends a read/write function command to read/ write the internal
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 Microcontroller and Embedded Systems 2023

memory or register of the slave device.

v. The master initiates a Read data/ Write data from the device or to the device.
All communication over the 1-wire bus is master initiated. The communication over the 1-wire
bus is divided into timeslots of 60 microseconds for regular speed mode of operation
(16.3Kbps).

om
Parallel Interface: The on-board parallel interface is normally used for communicating with
peripheral devices which are memory mapped to the host of the system. The host processor/
controller of the embedded system contain a parallel bus and the device which supports parallel
bus can directly connect to this bus system. The communication through the parallel bus is
controlled by the control signal interface between the device and the host. The Control Signals

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for communication includes Read/ Write signal and device select signal. The device normally
contains a device select line and the device becomes active only when this line is asserted by the
host processor. The direction of data transfer (Host to Device or Device to Host) can be
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controlled through the control signal lines for Read and Write. Only the host processor has
control over the Read and Write control signals. The device is normally memory mapped to the
host processor and a range of address is assigned to it. An address decoder circuit is used for
generating the chip select signal for the device. When the address selected by the processor is
co

within the range assigned for the

device, the decoder circuit
activates the chip select line and
u

thereby the device becomes

active. The processor then can
vt

read or write from or to the

device by asserting the
corresponding control line (RD\
and WR\ respectively). Strict
timing characteristics are followed for parallel communication.
The parallel communication is host processor initiated. If a device wants to initiate the
communication, it can inform the same to the processor through interrupts. For this, the interrupt
line of the device is connected to the interrupt line of the processor and the corresponding
interrupt is enabled in the host processor. The width of the parallel interface is determined by the

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 Microcontroller and Embedded Systems 2023

data bus width of the host processor. It can be 4-bit, 8-bit, 16-bit, 32-bit or 64-bit, etc. The bus
width supported by the device should be same as that of the host processor. Parallel data
communication offers the highest speed for data transfer.
External Communication Interfaces: The External communication interface refers to the
different communication channels/ buses used by the embedded system to communicate with the

om
external world. RS-232 C & RS-485: RS-232 C (Recommended Standard number 232, revision
C) from the Electronic Industry Association is a legacy, full duplex, wired, asynchronous serial
communication interface.
The RS-232 interface is developed by the Electronics Industries Association (EIA) during the
early 1960s. RS-232 extends the UART communication signals for external data

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communication. UART uses the standard TTL/ CMOS logic (Logic High corresponds to bit
value 1 and Logic Low corresponds to bit value 0) for bit transmission; whereas RS-232 follows
the EIA standard for bit transmission. As per the EIA standard, a logic 0 is represented with
de
voltage between +3 and +25V and a logic 1 is represented with voltage between -3 and -25V. In
EIA standard, logic 0 is known as Space and logic 1 as Mark.
The RS-232 interface defines various handshaking and control signals for communication apart
from the 'Transmit' and. 'Receive' signal lines for data communication.
co

RS-232 supports two different types of connectors:

u
vt

RS-232 is a point-to-point communication interface and the device involved in RS-232

communication is called 'Data Terminal Equipment (DTE)' and Data Communication
Equipment (DCE).
The Data Terminal Ready (DTR) signal is activated by DTE when it is ready to accept data.
The Data Set Ready (DSR) is activated by DCE when it is ready for establishing a
communication link. DTR should be in the activated state before the activation of DSR.
The Data Carrier Detect (DCD) control signal is used by the DCE to indicate the DTE that a
good signal is being received.

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 Microcontroller and Embedded Systems 2023

RS-232 supports only point-to-point communication and not suitable for multi-drop
communication. It uses single ended data transfer technique for signal transmission and thereby
more susceptible to noise and it greatly reduces the operating distance.
RS-422 is another serial interface standard from EIA for differential data communication. It
supports data rates up to l00Kbps and distance up to 400 ft. RS-422 supports multi-drop

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communication with one transmitter device and receiver devices up to 10.
RS-485 is the enhanced version of RS-422 and it supports multi-drop communication with up
to 32 transmitting devices (drivers) and 32 receiving devices on the bus. The communication
between devices in the bus uses the 'addressing' mechanism to identify slave devices.
Universal Serial Bus (USB): Universal Serial Bus is a wired high speed serial bus for data

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communication. The first version of USB (USB 1.0) was released in 1995 and was created by
the USB core group members consisting of Intel, Microsoft,
IBM, Compaq, Digital and Northern Telecom. The USB
de
communication system follows a star topology with a USB
host at the centre and one or more USB peripheral devices/
USB hosts connected to it. A USB 2.0 host can support
connections up to 127, including slave peripheral devices and
co

other USB hosts.

USB transmits data in packet format. Each data packet has a
standard format. The USB communication is a host initiated
u

one. The USB host contains a host controller which is

responsible for controlling the data communication, including establishing connectivity with
vt

USB slave devices, packetizing and formatting the data. There are different standards for
implementing the USB Host Control interface; namely Open Host Control Interface (OHCI) and
Universal Host Control Interface (UHCI). The physical connection between a USB peripheral
device and master device is established with a USB cable. The USB cable in USB 2.0 supports
communication distance of up to 5 meters. The USB 2.0 standard uses two different types of
connector at the ends of the USB cable for connecting the USB peripheral device and host
device.
Type A connector is used for upstream connection (connection with host) and Type B connector
is used for downstream connection (connection with slave device). The USB connectors present

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 Microcontroller and Embedded Systems 2023

in desktop PCs or laptops are examples for Type A USB connector. Both Type A and Type B
connectors contain 4 pins for communication.
Pin no Pin name Description
1 VBUS Carries power (5V)
2 D– Differential data carrier line
3 D+ Differential data carrier line

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4 GND Ground signal line
USB uses differential signals for data transmission. It improves the noise immunity. USB
interface has the ability to supply power to the connecting devices. Two connection lines
(Ground and Power) of the USB interface are dedicated for carrying power. It can supply power
up to 500 rnA at 5 V. USB supports four different types of data transfers, namely; Control, Bulk,
Isochronous and Interrupt. Control transfer is used by USB system software to query, configure

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and issue commands to the USB device. Bulk transfer is used for sending a block of data to a
device. Bulk transfer supports error checking and correction. Transferring data to a printer is an
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example for bulk transfer.
Isochronous data transfer is used for real-time data communication. In Isochronous transfer, data
is transmitted as streams in real-time. Isochronous transfer doesn't support error checking and
retransmission of data in case of any transmission loss. All streaming devices like audio devices
co

and medical equipment for data collection make use of the isochronous transfer.
Interrupt transfer is used for transferring small amount of data. Interrupt transfer mechanism
makes use of polling technique to see whether the USB device has any data to send. The
frequency of polling is determined by the USB device and it varies from 1 to 255 milliseconds.
u

Devices like Mouse and Keyboard, which transmits fewer amounts of data, uses interrupt
transfer.
vt

IEEE 1394 (Firewire): IEEE 1394 is a wired isochronous high speed serial communication bus.
It is also known as High Performance Serial Bus (HPSB). The research on 1394 was started by
Apple Inc. in 1985 and the standard for this was coined by IEEE. The implementation of it is
available from various players with different names. Apple Inc's implementation of 1394
protocol is popularly known as Firewire. i.LINK is the 1394 implementation from Sony
Corporation. Lynx is the implementation from Texas Instruments.
1394 supports peer-to-peer connection and point-to-multipoint communication allowing 63
devices to be connected on the bus in a tree topology. 1394 is a wired serial interface and it can
support a cable length of up to 15 feet for interconnection. There are two differential data
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 Microcontroller and Embedded Systems 2023

transfer lines A and B per connector. In a 1394 cable, normally the differential lines of A are
connected to B (TPA+ to TPB+ and TPA-to TPB-) and vice versa.
1394 is a popular communication interface for connecting embedded devices like Digital
Camera, Camcorder, and Scanners to desktop computers for data transfer and storage. Unlike
USB interface (except USB OTG), IEEE 1394 doesn't require a host for communicating

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between devices. For example, you can directly connect a scanner with a printer for printing.
The data- rate supported by 1394 is far higher than the one supported by USB2.0 interface. The
1394 hardware implementation is much costlier than USB implementation.
Infrared (IrDA): Infrared is a serial, half duplex, line of sight based wireless technology for
data communication between devices. IrDA is in use from the olden days of communication and

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you may be very familiar with it. The remote control of your TV, VCD player, etc., works on
Infrared data communication principle. Infrared communication technique uses infrared waves
of the electromagnetic spectrum for transmitting the data. IrDA supports point-point and point-
de
to-multipoint communication, provided all devices involved in the communication are within the
line of sight. The typical communication range for IrDA lies in the range l0 cm to 1 m. The
range can be increased by increasing the transmitting power of the IR device.
IR supports data rates ranging from 9600bits/second to 16Mbps. Depending on the speed of data
co

transmission IR is classified into Serial IR (SIR), Medium IR (MIR), Fast IR (FIR), Very Fast
IR (VFIR) and Ultra-Fast IR (UFIR). SIR supports transmission rates ranging from 9600bps to
115.2kbps. MIR supports data rates of 0.576Mbps and 1.152Mbps. FIR supports data rates up to
u

4Mbps. VFIR is designed to support high data rates up to l6Mbps. The UFIR supports up to
96Mbps.
vt

IrDA communication involves a transmitter unit for transmitting the data over IR and a receiver
for receiving the data. Infrared Light Emitting Diode (LED) is the IR source for transmitter and
at the receiving end a photodiode acts as the receiver. Both transmitter and receiver unit will be
present in each device supporting IrDA communication for bidirectional data transfer. Such IR
units are known as Transceiver. Certain devices like a TV require control always require
unidirectional communication and so they contain either the transmitter or receiver unit (The
remote control unit contains the transmitter unit and TV contains the receiver unit).
Bluetooth (BT): Bluetooth is a low cost, low power, short range wireless technology for data
and voice communication. Bluetooth was first proposed by 'Ericsson' in 1994. It operates at

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 Microcontroller and Embedded Systems 2023

2.4GHz of the Radio Frequency spectrum and uses the Frequency Hopping Spread Spectrum
(FHSS) technique for communication. Literally it supports a data rate of up to 1Mbps and a
range of approximately 30 to 100 feet (version dependent) for data communication. Like IrDA,
Bluetooth communication also has two essential parts; a physical link part and a protocol part.
The physical link is responsible for the physical transmission of data between devices supporting

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Bluetooth communication. The physical link works on the wireless principle making use of RF
waves for communication. Bluetooth enabled devices essentially contain a Bluetooth wireless
radio for the transmission and reception of data. The protocol part is responsible for defining the
rules of communication. The rules governing the Bluetooth communication is implemented in
the Bluetooth protocol stack.

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Each Bluetooth device will have a 48-bit unique identification number. Bluetooth
communication follows packet based data transfer. Bluetooth supports point-to-point (device to
device) and point-to-multipoint (device to multiple device broadcasting) wireless
de
communication. The point-to-point communication follows the master slave relationship. A
Bluetooth device can function as either master or slave. When a network is formed with one
Bluetooth device as master and more than one device as slaves, it is called a Piconet. A Piconet
supports a maximum of seven slave devices.
co

Wi-Fi: Wi-Fi or Wireless Fidelity is the popular wireless communication technique for
networked communication of devices. Wi-Fi follows the IEEE 802.11 standard. Wi-Fi is
intended for network communication and supports Internet Protocol (IP) based communication.
u

It is essential to have device identities in a multi-point communication to address specific

devices for data communication.
vt

In an IP based communication each device is identified by an IP address, which is unique to

each device on the network. Wi-Fi based communications
require an intermediate agent called Wi-Fi router/ Wireless
Access point to manage the communications. The Wi-Fi
router is responsible for restricting the access to a network,
assigning IP address to devices on the network, routing data
packets to the intended devices on the network.
Wi-Fi enabled devices contain a wireless adaptor for
transmitting and receiving data in the form of radio signals through an antenna. The hardware

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 Microcontroller and Embedded Systems 2023

part of it is known as Wi-Fi Radio. Wi-Fi operates at 2.4GHz or 5GHz of radio spectrum and
they co-exist with other ISM band devices like Bluetooth.
ZigBee: ZigBee is a low power, low cost, wireless network communication protocol based on
the IEEE 802.15.4-2006 standard. It is targeted for low power; low data rate and secure
applications for Wireless Personal Area Networking

om
(WPAN). The ZigBee specifications support a robust mesh
network containing multiple nodes. This networking
strategy makes the network reliable by permitting
messages to travel through a number of different paths to
get from one node to another. It operates worldwide at the

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unlicensed bands of Radio spectrum, mainly at 2.400 to
2.484 GHz, 902 to 928 MHz and 868.0 to 868.6 MHz.
ZigBee Supports an operating distance of up to 100 meters and a data rate of 20 to 250Kbps. In
de
the ZigBee terminology, each ZigBee device falls under any one of the following ZigBee device
category:
 ZigBee Coordinator (ZC)/ Network Coordinator: The ZigBee coordinator acts as the root
of the ZigBee network. The ZC is responsible for initiating the ZigBee network and it
co

has the capability to store information about the network.

 ZigBee Router (ZR)/ Full Function Device (FFD): Responsible for passing information
from device to another device or to another ZR.
u

 ZigBee End Device (ZED)/ Reduced Function Device (RFD): End device containing
ZigBee functionality for data communication. It can talk only with a ZR or ZC and
vt

doesn't have the capability to act as a mediator for transferring data from one device to
another.
General Packet Radio Service (GPRS), 3G, 4G, LTE: General Packet Radio Service is a
communication technique for transferring data over a mobile communication network like GSM.
Data is sent as packets in GPRS communication. The transmitting device splits the data into
several related packets. At the receiving end the data is re-constructed by combining the
received data packets. GPRS supports a theoretical maximum transfer rate of 171.2kbps. In
GPRS communication, the radio channel is concurrently shared between several users instead of
dedicating a radio channel to a cell phone user. The GPRS communication divides the channel
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 Microcontroller and Embedded Systems 2023

into 8 timeslots and transmits data over the available channel. GPRS supports Internet Protocol
(IP), Point to Point Protocol (PPP) and X.25 protocols for communication. GPRS is mainly used
by mobile enabled embedded devices for data communication. The device should support the
necessary GPRS hardware like GPRS modem and GPRS radio. To accomplish GPRS based
communication, the carrier network also should have support for GPRS communication. GPRS

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is an old technology and it is being replaced by new generation data communication techniques
like EDGE, High Speed Downlink Packet Access (HSDPA), etc. which offers higher
bandwidths for communication.
Embedded Firmware
Embedded firmware refers to the control algorithm (Program instructions) and or the

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configuration settings that an embedded system developer dumps into the code (Program)
memory of the embedded system. It is an un-avoidable part of an embedded system. There are
various methods available for developing the embedded firmware. They are listed below:
de
(i)Write the program in high level languages like Embedded C/ C++ using an Integrated
Development Environment
 The IDE will contain a editor, compiler, linker, debugger, simulator, etc.
 IDEs are different for different family of processors/ controllers.
co

 For example, Keil microvision3 IDE is used for all family member of 8051
microcontroller, since it contains the generic 8051 compiler C51.
(ii)Write the program in Assembly language using the instructions supported by your
u

application's target processor/ controller.

The instruction set for each family of processor/ controller is different and the program written
vt

in either of the methods given above should be converted into a processor understandable
machine code before loading it into the program memory. The process of converting the
program written in a high level language or processor/ controller specific Assembly code to
machine readable binary code is called HEX File Creation. The methods used for HEX File
Creation is different depending on the programming techniques used. If the program is written
in Embedded C/ C++ using an IDE, the cross compiler included in the IDE converts it into
corresponding processor/ controller understandable HEX File. If you are following the
Assembly language based programming technique, you can use the utilities supplied by the
processor/ controller vendors to convert the source code into HEX File. Also third party tools
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 Microcontroller and Embedded Systems 2023

are available, which may be of free of cost, for this conversion.

For a beginner in the embedded software field, it is strongly recommended to use the high level
language based development technique. The reasons for this being: Writing codes in a high
level language is easy, the code written in high level language is highly portable which means
you can use the same code to run on different processor/ controller with little or less

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modification. The only thing you need to do is re-compile the program with the required
processor's IDE, after replacing the include files for that particular processor. Also the
programs written in high level languages are not developer dependent. Any skilled programmer
can trace out the functionalities of the program by just having a look at the program. It will be
much easier if the source code contains necessary comments and documentation lines. It is very

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easy to debug and the overall system development time will be reduced to a greater extent.
The embedded software development process in assembly language is tedious and time
consuming. The developer needs to know about all the instruction sets of the processor/
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controller or at least s/he should carry an instruction set reference manual with her/ him. A
programmer using assembly language technique writes the program according to his/ her view
and taste. Often he/ she may be writing a method or functionality which can be achieved
through a single instruction as an experienced person's point of view, by two or three
co

instructions in his/ her own style. So the program will be highly dependent on the developer. It
is very difficult for a second person to understand the code written in Assembly even if it is
well documented.
u

Other System Components

The other system components refer to the components/ circuits/ ICs which are necessary for the
vt

proper functioning of the embedded system. Some of these circuits may be essential for the
proper functioning of the processor/ controller and firmware execution. Watchdog timer, Reset
IC (or passive circuit), brown-out protection IC (or passive circuit) etc., are examples of circuits/
ICs which are essential for the proper functioning of the processors/ controllers. Some of the
controllers or SoCs, integrate these components within a single IC and doesn't require such
components externally connected to the chip for proper functioning.
Depending on the system requirement, the embedded system may include other integrated
circuits for performing specific functions, level translator ICs for interfacing circuits with
different logic levels, etc.

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 Microcontroller and Embedded Systems 2023

Reset Circuit: The reset circuit is essential to ensure that the device is not operating at a voltage
level where the device is not guaranteed to operate, during system power ON. The reset signal
brings the internal registers and the different hardware systems of the processor/ controller to a
known state and starts the firmware execution from the reset vector (Normally from vector
address 0x0000 for conventional processors/ controllers.

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The reset signal can be either active high (The processor undergoes reset when the reset pin of
the processor is at logic high) or active low (The processor undergoes reset when the reset pin of
the processor is at logic low). Since the processor operation is synchronized to a clock signal,
the reset pulse should be wide enough to give time for the clock oscillator to stabilize before the
internal reset state starts. The reset signal to the processor can be applied at power ON through

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an external passive reset
circuit comprising a
Capacitor and Resistor or
de
through a standard Reset IC
like MAX810 from Maxim
Dallas. Select the reset IC
based on the type of reset
co

signal and logic level

(CMOS/ TTL) supported
by the processor/ controller in use. Some microprocessors /controllers contain built-in internal
u

reset circuitry and they don't require external reset circuitry. Figure illustrates a resistor capacitor
based passive reset circuit for active high and low configurations. The reset pulse width can be
vt

adjusted by changing the resistance value R and capacitance value C.

Brown-out Protection Circuit: Brown-out protection circuit prevents the processor/ controller
from unexpected program execution behavior when the supply voltage to the processor/
controller falls below a specified voltage. It is essential for battery powered devices since there
are greater chances for the battery voltage to drop below the required threshold. The processor
behavior may not be predictable if the supply voltage falls below the recommended operating
voltage. It may lead to situations like data corruption.
A brown-out protection circuit holds the processor/ controller in reset state, when operating
voltage falls below the threshold, until it rises above the threshold voltage. Certain processors/

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 Microcontroller and Embedded Systems 2023

controllers support built in brown-out protection circuit which monitors the supply voltage
internally. If the processor/ controller don’t integrate a built-in brown-out protection circuit, the
same can be implemented using external passive circuits or supervisor ICs.
The Zener diode, Dz, and transistor, Q, forms the heart of this
circuit. The transistor conducts always when the supply

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voltage VCC is greater than that of the sum of VBE and VZ
(Zener voltage). The transistor stops conducting when the
supply voltage falls below the sum of VBE and VZ. Select the
Zener diode with required voltage for setting the low
threshold value for VCC. The values of Rl, R2, and R3 can be

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selected based on the electrical characteristics of the transistor
in use.
Oscillator Unit: A microprocessor/ microcontroller are a digital device made up of digital
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combinational and sequential circuits. The instruction execution of a microprocessor/ controller
occurs in synchronization with a clock signal. The oscillator unit of the embedded system is
responsible for generating the precise clock for the processor.
u co
vt

Certain processors/ controllers integrate a built-in oscillator unit and simply require an external
ceramic resonator/ quartz crystal for producing the necessary clock signals. Quartz crystals and
ceramic resonators are equivalent in operation, however they possess physical difference.
Certain devices may not contain built-in oscillator unit and require the clock pulses to be
generated and supplied externally. The speed of operation of a processor is primarily dependent
on the clock frequency. However we cannot increase the clock frequency blindly for increasing
the speed of execution. The logical circuits lying inside the processor always have an upper
threshold value for the maximum clock at which the system can run, beyond which the system
becomes unstable and nonfunctional.
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 Microcontroller and Embedded Systems 2023

The total system power consumption is directly proportional to the clock frequency. The power
consumption increases with increase in clock frequency. The accuracy of program execution
depends on the accuracy of the clock signal.
Real-Time Clock (RTC): Real-Time Clock is a system component responsible for keeping track
of time. RTC holds information like current time (In hours, minutes and seconds) in 12-hour/

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24-hour format, date, month, year, day of the week, etc. and supplies timing reference to the
system.
RTC is intended to function even in the absence of power. RTCs are available in the form of
Integrated Circuits from different semiconductor manufacturers like Maxim/Dallas, ST
Microelectronics etc. The RTC chip contains a microchip for holding the time and date related

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information and backup battery cell for functioning in the absence of power, in a single IC
package. The RTC chip is interfaced to the processor or controller of the embedded system.
For Operating System based embedded devices, a timing reference is essential for synchronizing
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the operations of the OS kernel. The RTC can interrupt the OS .kernel by asserting the interrupt
line of the processor/controller to which the RTC interrupt line is connected. The OS kernel
identifies the interrupt in terms of the Interrupt Request (IRQ) number generated by an interrupt
controller. One IRQ can be assigned to the RTC interrupt and the kernel can perform necessary
co

operations like system date time updating, managing software timers etc. when an RTC timer
tick interrupt occurs. The RTC can be configured to interrupt the processor at predefined
intervals or to interrupt the processor when the RTC register reaches a specified value (used as
u

alarm interrupt).
Watchdog Timer: In desktop Windows systems, if we feel our application is behaving in an
vt

abnormally or if the system

hangs up, we have the 'Ctrl +
Alt + Del' to come out of the
situation. For embedded
system a watchdog will
monitor the firmware
execution and reset the
system processor/ microcontroller when the program execution hangs up. A watchdog timer, or
simply a watchdog, is a hardware timer for monitoring the firmware execution. Depending on

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 Microcontroller and Embedded Systems 2023

the internal implementation, the watchdog timer increments or decrements a free running
counter with each clock pulse and generates a reset signal to reset the processor if the count
reaches zero for a down counting watchdog, or the highest count value for an up counting
watchdog.
If the watchdog counter is in the enabled state, the firmware can write a zero (for up counting

om
watchdog implementation) to it before starting the execution of a piece of code and the
watchdog will start counting. If the firmware execution doesn't complete due to malfunctioning,
within the time required by the watchdog to reach the maximum count, the counter will generate
a reset pulse and this will reset the processor. If the firmware execution completes before the
expiration of the watchdog, you can reset the count by writing a 0 (for an up counting watchdog

.c
timer) to the watchdog timer register.
If the processor/ controller doesn't contain a built in watchdog timer, the same can be
implemented using an external watchdog timer IC circuit. The external watchdog timer uses
de
hardware logic for enabling/ disabling, resetting the watchdog count, etc., instead of the
firmware based 'writing' to the status and watchdog timer register. The microprocessor
supervisor IC DS 1232 integrates a hardware watchdog timer in it.
co

Reference:
Shibu K V, “Introduction to Embedded Systems”, Tata McGraw Hill Education, Private
Limited, 2nd Edition
u

Notes By: Veena Bhat

vt

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