18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

MODULE–5
RTOSANDIDEFOREMBEDDEDSYSTEMDESIGN

IRTOS-BASEDEMBEDDEDSYSTEMDESIGN
The super loop based task execution model for firmware executes the tasks sequentially in order in which
the tasks are listed within the loop. Here every task is repeated at regular intervals and the task execution
is non-real time. Also, any response delay is acceptable and it will not create any operational issues or
potential hazards.
But, certain applications demand time critical response to tasks/ events and delay in the response may be
catastrophic. Examples: Flight control systems, Air bag control, Anti-lock Brake Systems (ABS) for
vehicles, Nuclear monitoring devices, etc.
In embedded systems, the time critical response for tasks/events may be addressed by–
 Assigning priority to tasks and execute the high priority task.
 Dynamically change the priorities of tasks, if required on a need basis.
The introduction of operating system based firmware execution in embedded devices can address these
needs to a greater extent.

OPERATINGSYSTEM(OS)BASICS:
The Operating System (OS) acts as a bridge between the user applications/ tasks and the underlying
system resources through a set of system functionalities and services. The primary functions of operating
systems are
 Make the system convenient to use
 Organize and manage the system resources efficiently and correctly.
The following Figure gives an insight into the basic components of an operating system and their
KernelServices

interfaces with rest of the world.

User Applications
ApplicationProgramming
Interface(API)
MemoryManagement

ProcessManagement

TimeManagement

FileSystemManagement
I/OSystemManagement
DeviceDriver
Interface
Underlying Hardware

1
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The Kernel:
The kernel is the core of the operating system. It is responsible for managing the s system resources and
the communication among the hardware and other system services. Kernel acts as the abstraction layer
between system resources and user applications.
 Kernel contains a set of system libraries and services. For a general purpose OS, the kernel
contains different services like memory management, process management, time management,
file system management, I/O system management.

Process Management: The process management deals with managing the process/ tasks. Process
management includes –
 Setting up a memory for the process
 Loading process code into memory
 Allocating system resources
 Scheduling and managing the execution of the process
 Setting up and managing Process Control Block(PCB)
 Interprocess communication and synchronization
 Process termination/deletion, etc.

Primary Memory Management: Primary memory refers to a volatile memory (RAM), where processes
are loaded and variables and shared data are stored.
The Memory Management Unit(MMU)of the kernel is responsible for–
 Keeping a track of which part of the memory area is currently used by which process
 Allocating and De-allocating memory space on a need basis.

File System Management: File is a collection of related information. A file could be a program (source
code or executable), text files, image files, word documents, audio/ video files, etc. A file system
management service of kernel is responsible for –
 The creation, deletion and alteration of files
 Creation, deletion, and alteration of directories
 Saving of files in the secondary storage memory
 Providing automatic allocation of file space based on the amount off rerunning space available
 Providing flexible naming conversion for the files.

I/O System(Device)Management:KernelisresponsibleforroutingtheI/Orequestscomingfrom
differentuserapplicationstotheappropriateI/Odevicesofthesystem.InawellstructuredOS,direct
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
access to I/O devices is not allowed; access to them is establish through Application Programming
Interface (API). The kernel maintains list of all the I/O devices of the system. The service „ Device
Manager‟ of the kernel is responsible for handling all I/O related operations. The Device Manager is
responsible for –
 Loading and unloading of deviced rivers
 Exchanging information and the system specific control signals to and from the device.

Secondary Storage Management: The secondary storage management deals with managing the
secondary storage memory devices (if any) connected to the system. Secondary memory is used a backup
medium for programs and data, as main memory is volatile. In most of the systems secondary storage is
kept in disks (hard disks). The secondary storage management service of kernel deals with –
 Diskstorage allocation
 Disk scheduling
 Free disk space management

Protection Systems: Modern operating systems are designed in such way to support multiple users with
different levels of access permissions. The protection deals with implementing the security policies to
restrict the access of system resources and particular user by different application or processes and
different user.

Interrupt Handler: Kernel provides interrupt handler mechanism for all external/ internal interrupt
generated by the system.

The important services offered by the kernel of an OS:

 Kernel Space and User Space: The program code corresponding to the kernel applications/
services are kept ina contiguous area of primary(working) memory and is protected from the un-
authorized access by user programs/ applications.
o The memory space at which the kernel code is located is known as „ Kernel Space‟. All
user applications are loaded to a specific area of primary memory and thismemory areais
referred as „User Space‟.
o The partitioning of memory into kernel and user space is purely Operating System
dependent.
o Most of the operating systems keep the kernel application code in main memory and it is
not swapped out into the secondary memory.
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Monolithic Kernel and Microkernel: Kernel forms the heart of OS. Different approaches are adopted for
buildinganoperatingsystemkernel.Basedonthekerneldesign,kernelscanbeclassifiedinto
„Monolithic‟and„Micro‟.
 Monolithic Kernel: In monolithic kernel architecture, all kernel services run in the kernel space.
All kernel modules run within the same memory space under a single kernel thread.
 Themajordrawbackofmonolithickernelisthatanyerrororfailureinanyoneofthekernel modules leads
to the crashing of the entire kernel application.
o LINUX,SOLARIS,MS-DOSkernelsareexamplesofmonolithickernel.

 Microkernel: The micro kernel design incorporates only essential set of operating system
services into the kernel. The rest of the operating systems services are implemented in program
known as
„Servers‟ which runs in user space. The memory management, timer systems and interrupt
handlers are the essential services, which forms the part of the microkernel. The benefits of micro
kernel based designs are –
o Robustness: If a problem is encountered in any of the services, which runs as a server can
be reconfigured and restarted without the restarting the entire OS. Here chances of
corruption of kernel services are ideally zero.
o Configurability: Any services, which runs as a server application can be changed
without the need to restart the whole system. This makes the system dynamically
configurable.

TYPESOFOPERATINGSYSTEMS:
Depending on the type of kernel and kernel services,purpose and type of computing system, Operating
Systems are classified into different types.
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General Purpose Operating System(GPOS):
Theoperatingsystems,whicharedeployedingeneralcomputingsystems,arereferredasGPOS.The GPOSs are
often quite non-deterministic in behavior.
 Windows10/8.x/XP/MS-DOS, etc., are examples of GPIOs.

Real Time Operating System(RTOS):

Real T time implies deterministic in timing behavior.
 RTOS services consumes only known and expected amounts of time regardless the number of
services.
 RTOSimplementspoliciesandrulesconcerningtime-criticalallocationofasystem‟sresources.
 RTOSdecideswhichapplicationsshouldruninwhichorderandhowmuchtimeneedstobe allocated for
each application.
o WindowsEmbeddedCompact,QNX,VxWorksMicroC/OS-II,etc.,areexamplesof RTOSs.

The Real-Time kernel: The kernel of a Real-Time OS is referred as Real-Time kernel. The Real-Time
kernel is highly specialized and it contains only the minimal set of services required for running user
applications/ tasks. The basic functions of a Real-Time kernel are listed below:
 Task/Process management
 Task/Process scheduling
 Task/Process synchronization
 Error/Exception handling
 Memory management
 Interrupt handling
 Time management.

 Task/ Process Management: Deals with setting up the memory space for the tasks, loading the
task‟s code into the memory space, allocating system resources and setting up a Task Control
Block (TCB) for the task and task/process termination/deletion.
o A Task Control Block(TCB) is used for holding the information corresponding to a task.
TCB usually contains the following set of information:
 Task ID: Task Identification Number
 Task State: The current state of the task. (E.g. State= „Ready‟ for a task which is
ready to execute)
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 Task Type: Task type. Indicates what is the type for this task. The task can be a
hard real time or soft real time or background task.
 Task Priority: Task priority(E.g. Task priority=1fortaskwithpriority= 1)
 Task Context Pointer: Context pointer. Pointer for context saving
 Task Memory Pointers: Pointers to the code memory, data memory and stack
memory for the task
 TaskSystemResourcePointers:Pointerstosystemresources(semaphores,mutex,
etc.) used by the task
 Task Pointers: Pointers to other TCBs(TCBs for preceding ,next and waiting
tasks)
 Other Parameters: Other relevant task parameters.
o The parameters and implementation of the TCB is kernel dependent. The TCB
parameters vary across different kernels based on the task management implementation.

 Task/ Process Scheduling: Deals with sharing the CPU among various tasks/ processes. A kernel
application called „Scheduler‟ handles the task scheduling. Scheduler is an algorithm
implementation, which performs the efficient and optimal scheduling of tasks to provide a
deterministic behavior.

 Task/ Process Synchronization: Deals with synchronizing the concurrent access of a resource,
which is shared across multiple tasks and the communication between various tasks.

 Error/ Exception Handling: Deals with registering and handling the errors occurred/ exceptions
rose during the execution of tasks.
o Insufficient memory, timeouts, deadlocks, deadline missing, bus error, divide by zero,
unknown instruction execution etc, are examples of errors/exceptions.
o Errors/Exceptions can happen at the kernel level services or at task level.
 Deadlock is an example for kernel level exception, whereas timeout is an
example for a task level exception.
 Dead lock is a situation where a set of processes are blocked because
each process is holding a resource and waiting for another resource
acquired by some other process.
 Timeouts and retry are two techniques used together. The tasks retries an
event/ message certain number of times; if no response is received after
exhausting the limit, the feature might be aborted.
o T form of a system call(API).
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 Memory Management: The memory management function of an RTOS kernel is slightly

different compared to the General Purpose Operating Systems.
o In general, the memory allocation time increases depending on the size of the block of
memory need to be allocated and the state of the allocated memory block. RTOS achieves
predictable timing and deterministic behavior, by compromising the effectiveness of
memory allocation.
o RTOS generally uses „block‟ based memory allocation technique, instead of the usual
dynamic memory allocation techniques used by the GPOS. RTOS kernel uses blocks of
fixed size of dynamic memory and the block is allocated for a task on a need basis. The
blocks are stored in a „Free buffer Queue‟.
o Most of the RTOS kernels allow tasks to access any of the memory blocks without any
memory protection to achieve predictable timing and avoid the timing overheads. Some
commercial RTOS kernels allow memory protection as optional and the kernel enters a
fail-safe mode when an illegal memory access occurs.
o The memory management function a block of fixed memory is always allocated for tasks
on need basis and it is taken as a unit. Hence, there will not be any memory
fragmentation issues.

 Interrupt Handling: Deals with the handling of various interrupts. Interrupts inform the
processor that an external device or an associated task requires immediate attention of the CPU.
o Interrupts can be either Synchronous or Asynchronous.
 Interrupts which occurs in sync with the currently executing task is known as
Synchronous interrupts. Usually the software interrupts fall under the
Synchronous Interrupt category.
 Divide by zero, memory segmentation error etc are examples of
Synchronous interrupts.
 For synchronous interrupts, the interrupt handler runs in the same context of the
interrupting task.
 Interrupts which occurs at any point of execution of any task, and arenot in sync
with the currently executing task are Asynchronous interrupts.
 Timer overflow interrupts, serial data reception/ transmission interrupts
etc., are examples for asynchronous interrupts.
 For asynchronous interrupts, the interrupt handler is usually written as separate
task (depends on OS Kernel implementation) and it runs in a different context.
Hence, a context switch happens while handling the asynchronous interrupts.
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o Priority levels can be assigned to the interrupts and each interrupts can be enabled or
disabled individually. Most of the RTOS kernel implements „Nested Interrupts‟
architecture.

 Time Management: Accurate time management is essential for providing precise time reference
for all applications. The time reference to kernel is provided by a high-resolution Real Time
Clock (RTC) hardware chip (hardware timer).
o The hardware timer is programmed to interrupt the processor/ controller at a fixed rate.
This timer interrupt is referred as „Timer tick‟. The „Timer tick‟ is taken as the timing
reference by the kernel. The „Timer tick‟ interval may vary depending on the hardware
timer. Usually, the „Timer tick‟ varies in the microseconds range. The time parameters
for tasks are expressed as the multiples of the „Timer tick‟.
o The System time is updated based on the „Timer tick‟. If the System time register is 32
bits wide and the „Timer tick‟ interval is 1 microsecond, the System time register will
reset in;
232*10–6/(24*60*60)=~0.0497Days = 1.19 Hours
o Ifthe„Timertick‟intervalis1millisecond,theSystemtimeregisterwillresetin 2 32 *
10–3 / (24 * 60 * 60) = 49.7 Days = ~ 50 Days
o The„Timertick‟interruptishandledbythe„TimerInterrupt‟handlerofkernel.The
„Timertick‟interruptcanbeutilizedforimplementingthefollowingactions:
 Save the current context(Context to the currently executing task)
 Increment the System time register by one. Generate timing error and reset the
System time register if the timer tick count is greater than the maximum range
available for System time register.
 Update the timers implemented in kernel (Increment or decrement the timer
registers for each timer depending on the count direction setting for each register.
Increment registers with count direction setting = „count up‟ and decrement
registers with count direction setting = „count down‟)
 Activate the periodic tasks, which are in the idle state
 Invoke the scheduler and schedule the tasks again based on the scheduling
algorithm
 Delete all the terminated tasks and the is associated data structures (TCBs)
 Load the context for the first task in the ready queue. Due to the re-scheduling,
the ready task might be changed to a new one from the task, which was pre-
empted by the „Timer Interrupt‟ task.
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 Hard Real-Time: A Real Time Operating Systems which strictly adheres to the timing
constraints for a task is referred as hard real-time systems. A Hard Real Time system must meet
the deadlines for a task without any slippage. Missing any deadline may produce catastrophic
results for Hard Real Time Systems, including permanent data lose and irrecoverable damages to
the system/users.
o Hard real-time systems emphasize on the principle „A late answer is a wrong answer‟.
 For example, Air bag control systems and Anti-lock Brake Systems (ABS) of
vehicles are typical examples of Hard Real Time Systems.
o Most of the Hard Real Time Systems are automatic.

 Soft Real-Time: Real Time Operating Systems that does not guarantee meeting deadlines, but,
offer the best effort to meet the deadline are referred as soft real-time systems. Missing deadlines
for tasks are acceptable if the frequency of deadline missing is within the compliance limit of the
Quality of Service (QoS).
o Soft real-time system emphasizes on the principle„ A late answer is an acceptable
answer, but it could have done bit faster‟.
o Automatic Teller Machine (ATM) is a typical example of Soft Real Time System. If the
ATM takes a few seconds more than the ideal operation time, nothing fatal happens.

TASKS,PROCESSESANDTHREADS:
The term „task‟ refers to something that needs to be done. In the Operating System context, a task is
defined as the program in execution and the related information maintained by the Operating system for
the program. Task is also known as „Job‟ in the operating system context. A program or part of it in
execution is also called a „Process‟.
 The terms „Task‟, „Job‟ and „Process‟ refer to the same entity in the Operating System context
and most often they are used interchangeably.

Process:
A Process‟ is a program, or part of it, in execution. Process is also known as an instance of a program in
execution. A process requires various system resources like CPU for executing the process, memory for
storing the code corresponding to the process and associated variables, I/O devices for information
exchange etc.

 Structure of a Processes: The concept of „Process‟ leads to concurrent execution of tasks and
thereby, efficient utilization of the CPU and other system resources. Concurrent execution is
achieved through the sharing of CPU among the processes.
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o A process mimics a processor in properties and holds a set of registers, process status, a
Program Counter (PC) to point to the next executable instruction of the process, a stack
for holding the local variables associated with the process and the code corresponding to
the process. This can be visualized as shown in the following Figure.

o A process, which inherits all the properties of the CPU, can be considered as a virtual
processor, awaiting its turn to have its properties switched into the physical processor.
When the process gets its turn, its registers and Program Counter register becomes
mapped to the physical registers of the CPU.
o The memory occupied by the process is segregated into three regions namely; Stack
memory, Data memory and Code memory (Figure, shown above).
 The „Stack‟ memory holds all temporary data such as variables local to the
process.
 The„ Data‟ memory holds all global data for the process.
 The „Code‟ memory contains the program code (instructions) corresponding to
the process.
o On loading a process into the main memory, a specific area of memory is allocated forthe
process. The stack memory usually starts at the highest memory address from the
memory area allocated for the process.

 Process States& State Transition: The creation of a process to its termination is not a single step
operation. The process traverses through a series of states during its transition from the newly
created state to the terminated state.
o The cycle through which a process changes its state from „newly created‟ to „execution
completed‟ is known as „Process Life Cycle‟.
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o The various states through which a process traverses through during a Process Life Cycle
indicates the current status of the process with respect to time and also provides
information on what it is allowed to do next.
o The transition of a process from one state to another is known as „State transition‟. The
Process states and state transition representation are shown in the following Figure.

 Created State: The state at which a process is being created is referred as

„Created State‟. The Operating System recognizes a process in the „Created
State‟ but no resources are allocated to the process.
 Ready State: The state, where a process is incepted in to the memory and
awaiting the processor time for execution, is known as „Ready State‟. At this
stage, the process is placed in the „Ready list‟ queue maintained by the OS.
 Running State: The state where in the source code instructions corresponding to
the process is being executed is called„ Running State‟. Running state is the state
at which the process execution happens.
 Blocked State/ Wait State: Refers to a state where a running process is
temporarily suspended from execution and does not have immediate access to
resources. The blocked state might have invoked by various conditions like- the
process enters a wait state for an event to occur (E.g. Waiting for user inputs such
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

As keyboard input) or waiting for getting access to a shared resource like semaphore,
mutex etc.
 Completed State: A state where the process completes its execution.

Threads:
A thread is the primitive that can execute code. A thread is a single sequential flow of control within a
process. A thread is also known as lightweight process.
 A process can have many threads of execution. Different threads, which are part of a process,
share the same address space; meaning they share the data memory, code memory and heap
memory area.
 Threads maintain their own thread status (CPU register values), Program Counter (PC)and stack.
The memory model for a process and its associated threads are given in the following figure.

 The Concept of Multithreading: The process is split into multiple threads, which executes a
portion of the process; there will be a main thread and rest of the threads will be created within
the main thread.
o The multithreaded architecture of a process can be visualized with the thread-process
diagram, shown below.
o Use of multiple threads to execute a process brings the following advantage:
 Better memory utilization: Multiple threads of the same process share the address
space for data memory. This also reduces the complexity of inter thread
communication since variables can be shared across the threads.
 Since the process is split into different threads, when one thread enters a wait
state, the CPU can be utilized by other threads of the process that do not require
the event, which the other thread is waiting, for processing. This speeds up the
execution of the process.
 Efficient CPU utilization. The CPU is engaged all time.
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 Thread Standards: Thread standards deal with the different standards available for thread
creation and management. These standards are utilized by the Operating Systems for thread
creation and thread management. It is a set of thread class libraries. The commonly available
thread class libraries are –
o POSIX Threads: POSIX stands for Portable Operating System Interface. The POSIX.4
standard deals with the Real Time extensions and POSIX.4a standard deals with thread
extensions.ThePOSIXstandardlibraryforthreadcreationandmanagementis
„P threads‟. „P threads‟ library defines the set of POSIX thread creation and
management functions in „C‟ language. (Example 1 – Self study).
o Win32 Threads: Win32 threads are the threads supported by various flavors of Windows
Operating Systems. The Win32 Application Programming Interface (Win32 API)
libraries provide the standard set of Win32 thread creation and management functions.
Win32 threads are created with the API.
o Java Threads: Java threads are the threads supported by Java programming Language.
The java thread class „Thread‟ is defined in the package„ java. lang‟. This pack age
needs to be imported for using the thread creation functions supported by the Java thread
CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

There are two ways of creating threads in Java: Either by extending the base „Thread‟
class or by implementing an interface. Extending the thread class allows inheriting the
methods and variables of the parent class (Thread class) only whereas interface allows a
way to achieve the requirements for a set of classes.

 Thread Pre-emption: Thread pre-emption is the act of pre-empting the currently cunning thread
(stopping temporarily). It is dependent on the Operating System. It is performed for sharing the
CPU time among all the threads. The execution switching among threads are known as „Thread
context switching‟. Threads falls into one of the following types:
o User Level Thread: User level threads do not have kernel/ Operating System support and
they exist only in the running process. A process may have multiple user level threads;
but the OS threats it as single thread and will not switch the execution among the
different threads of it. It is the responsibility of the process to schedule each thread as and
when required. Hence, user level threads are non-preemptive at thread level from OS
perspective.
o Kernel Level/ System Level Thread: Kernel level threads are individual units of
execution, which the OS treats as separate threads. The OS interrupts the execution of the
currently running kernel thread and switches the execution to another kernel thread based
on the scheduling policies implemented by the OS.
 The execution switching (thread context switching) of user level threads happen
only when the currently executing user level thread is voluntarily blocked.
Hence, no OS intervention and system calls are involved in the context switching
of user level threads. This makes context switching of user level threads very
fast.
 Kernel level threads involve lots of kernel overhead and involve system calls for
context switching. However, kernel threads maintain a clear layer of abstraction
and allow threads to use system calls independently.
 There are many ways for binding user level threads with kernel/ system level
threads; which are explained below:
 Many-to-One Model: Many user level threads are mapped to a single
kernel thread. The kernel treats all user level threads as single thread and
the execution switching among the user level threads happens when a
currently executing user level thread voluntarily blocks itself or
relinquishes the CPU. Solaris Green threads and GNU Portable Threads
are examples for this.
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 One-to-One Model: Each user level thread is bonded to a kernel/ system

level thread. Windows XP/NT/2000 and Linux threads are examples of
One-to-One thread models.
 Many-to-Many Model: In this model many user level threads are allowed
to be mapped to many kernel threads. Windows NT/2000 with Thread
Fiber package is an example for this.
 Thread versus Process:
Thread Process
Thread is a single unit of execution and is part of Process is a program in execution and contains one
process. Or more threads.
A thread does not have its own data memory and Process has its own code memory, data memory,
heap memory. And stack memory.
A thread cannot live independently ;it lives within
A process contains at least one thread.
The process.
There can be multiple threads in a process; the first Threads within a process share the code, data and
(main)thread calls the main function and occupies heap memory; each thread holds separate memory
The start of the stack memory of the process. Area for stack.
Processes are very expensive to create; involves
Threads are very inexpensive to create.
Many OS overhead.
Context switching is complex and involves lots of
Context switching is inexpensive and fast.
OS over head and comparatively slow.
If a process dies, the resource allocated to it are
If a thread expires, its stack is reclaimed by the
reclaimed by the OS and all associated threads of
process.
The process also dies.

MULTIPROCESSINGANDMULTITASKING:
The ability to execute multiple processes simultaneously is referred as multiprocessing. Systems which
are capable of performing multiprocessing are known as multiprocessor systems.
 Multiprocessor systems possess multiple CPUs and can execute multiple processes
simultaneously.
 The ability of the Operating System to have multiple programs in memory, which are ready for
execution, is referred as multiprogramming. In a uni-processor system, it is not possible to
execute multiple processes simultaneously.
Multitasking refers to the ability of an operating system to hold multiple processes in memory and switch
the processor (CPU) from executing one process to another process.
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 Multitaskinginvolves„Contextswitching‟(seethefollowingFigure),„Contextsaving‟and
„Contex tretrieval‟.
o The act of switching CPU among the processes or changing the current execution context
is known as „Context switching‟.
o The act of saving the current context (details like Register details, Memory details,
System Resource Usage details, Execution details, etc.) for the currently running
processes at the time of CPU switching is known as „Context saving‟.
o The process of retrieving the saved context details for a process, which is going to be
executed due to CPU switching, is known as „Context retrieval‟.

Types of Multitasking:
Depending on how the task/ process execution switching act is implemented, multitasking can is
classified into –
 Co-operative Multitasking: Co-operative multitasking is the most primitive form of multitasking
in which a task/ process gets a chance to execute only when the currently executing task/ process
voluntarily relinquishes the CPU. In this method, any task/ process can avail the CPU as much
time as it wants. Since this type of implementation involves the mercy of the tasks each other for
getting the CPU time for execution, it is known as co-operative multitasking. If the currently
executing task is non-cooperative, the other tasks may have to wait for a long time to get the
CPU.
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 Preemptive Multitasking: Preemptive multitasking ensures that every task/ process gets a chance
to execute. When and how much time a process gets is dependent on the implementation of the
preemptive scheduling. As the name indicates, in preemptive multitasking, the currently running
task/process is preempted to give a chance to other tasks/process to execute. The preemption of
task may be based on time slots or task/ process priority.
 Non-preemptive Multitasking: The process/ task, which is currently given the CPU time, is
allowed to execute until it terminates (enters the„ Completed‟ state) or enters the„ Blocked/ Wait‟
state, waiting for an I/O. The co-operative and non-preemptive multitasking differs in their
behavior when they are in the „Blocked/Wait‟ state. In co-operative multitasking, the currently
executing process/task need not relinquish the CPU when it enters the „Blocked/ Wait‟ sate,
waiting for an I/O, or a shared resource access or an event to occur whereas in non-preemptive
multitasking the currently executing task relinquishes the CPU when it waits for an I/O.

TASK COMMUNICATION:
In a multitasking system, multiple tasks/ processes run concurrently (in pseudo parallelism) and each
process mayor may not interact between. Based onthe degree of interaction, the processes/ tasks running
on an OS are classified as –
 Co-operating Processes: In the co-operating interaction model, one process requires the inputs
from other processes to complete its execution.
 Competing Processes: The competing processes do not share anything among themselves but
they share the system resources. The competing processes compete for the systemresources such
as file, display device, etc.
o The co-operating processes exchanges information and communicate through the
following methods:
 Co-operationthroughsharing:Exchangedatathroughsomesharedresources.
 Co-operation through Communication: No data is shared between the processes.
But they communicate for execution synchronization.

The mechanism through which tasks/ processes communicate each other is known as Inter Process/ Task
Communication (IPC). IPC is essential for process co-ordination. The various types of IPC mechanisms
adopted by process are kernel (Operating System) dependent. They are explained below.

IPC Mechanism-Shared Memory:

Processes share some area of the memory to communicate among them (see the following Figure).
Information to be communicated by the process is written to the shared memory area. Processes which
require this information can read the same from the shared memory area.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

 The implementation of shared memory is kernel dependent. Different mechanisms are adopted by
different kernels for implementing this, a few among are s follows:
1. Pipes: „Pipe‟ is a section of the shared memory used by processes for communicating. Pipes
follow the client-server architecture. A process which creates a pipe is known as pipe server
and a process which connects to a pipe is known as pipe client. A pipe can be considered as a
medium for information flow and has two conceptual ends. It can be unidirectional, allowing
information flow in one direction or bidirectional allowing bi-directional information flow. A
unidirectional pipe allows the process connecting at one end of the pipe to write to the pipe
and the process connected at the other end of the pipe to read the data, whereas a bi-
directional pipe allows both reading and writing at one end. The unidirectional pipe can be
visualized as

o The implementation of „Pipes‟ is OS dependent. Microsoft® Windows Desktop

Operating Systems support two types of „Pipes‟ for Inter Process Communication.
Namely;
o Anonymous Pipes: The anonymous pipes are unnamed, unidirectional pipes used for
data transfer between two processes.
o Named Pipes: Named pipe is a named, unidirectional or bi-directional pipe for data
exchange between processes. Like anonymous pipes, the process which creates the
named pipe is known as pipe server. A process which connects to the named pipe is
known as pipe client. With named pipes, any process can act as both client and server
allowing point-to-point communication. Named pipes can be used for communicating
between processes running on the same machine or between processes running on
different machines connected to a network.

2. Memory Mapped Objects: Memory mapped object is a shared memory technique adopted by
certain Real Time Operating Systems for allocating a shared block of memory which can be
accessed by multiple process simultaneously. In this approach, a mapping object is created
and physical storage for it is reserved and committed. A process can map the entire
committed physical area or a block of it to its virtual address space. All read and write
operation to this virtual address space by a process is directed to its committed physical area.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

Any process which wants to share data with other processes can map the physical memory
area of the mapped object to its virtual memory space and use it for sharing the data. The
concept of memory mapped object is shown bellow.

IPC Mechanism-Message Passing:

Message passing is a/an synchronous/ asynchronous information exchange mechanism for Inter Process/
Thread Communication. The major difference between shared memory and message passing technique is
 Through shared memory lots of data can be shared whereas only limited amount of info/ data is
passed through message passing.
 Messagepassingisrelativelyfastandfreefromthesynchronizationoverheadscomparedto shared
memory.

Based on the message passing operation between the processes, message passing is classified into–
1. Message Queues: Process which wants to talk to another process posts the message to a
First-In-First-Out (FIFO) queue called „Message queue‟, which stores the messages
temporarily in a system defined memory object, to pass it to the desired process. Messages
are sent and received through send (Name of the process to which the message is to be sent,
message) and receive (Name of the process from which the message is to be received,
message) methods. The messages are exchanged through a message queue. The
implementation of the message queue, send and receive methods are OS kernel dependent.
MICRO CONTROLLERANDEMBEDDEDSYSTEMS

2. Mailbox: Mailbox is a special implementation of message queue. Usually used for one way
communication, only a single message is exchanged through mailbox whereas „message
queue‟ can be used for exchanging multiple messages. One task/process creates the mailbox
and other tasks/process can subscribe to this mailbox for getting message notification. The
implementation of the mailbox is OS kernel dependent. The MicroC/ OS-II RTOS
implements mailbox as a mechanism for inter task communication

3. Signalling: Signals are used for an asynchronous notification mechanism. The signal mainly
used for the execution synchronization of tasks process/ tasks. Signals do not carry any data
and are not queued. The implementation of signals is OS kernel dependent and VxWorks
RTOS kernel implements „signals‟ for inter process communication.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
IPC Mechanism - Remote Procedure Call (IPC) and Sockets: Remote Procedure Call is the Inter
Process Communication (IPC) mechanism used by a process, to call a procedure of another process
running on the same CPU or on a different CPU which is interconnected in a network. In the object
oriented language terminology, RPC is also known as Remote Invocation or Remote Method Invocation
(RMI). The CPU/ process containing the procedure which needs to be invoked remotely is known as
server. The CPU/ process which initiates an RPC request is known as client.
 In order to make the RPC communication compatible across all platforms, it should stick on to
certain standard formats.
 Interface Definition Language (IDL) defines the interfaces for RPC. Microsoft Interface
Definition Language (MIDL) is the IDL implementation from Microsoft for all Microsoft
platforms.
 The RPC communication can be either Synchronous (Blocking) or Asynchronous (Non-
blocking).

Sockets are used for RPC communication. Socket is a logical endpoint in a two-way communication link
between two applications running on a network. A port number is associated with a socket so that the
network layer of the communication channel can deliver the data to the designated application. Sockets
are of different types namely; Internet sockets (INET), UNIX sockets, etc.
 The INET Socket works on Internet Communication protocol. TCP/ IP, UDP, etc., are the
communication protocols used by INET sockets.
 INET sockets are classified into:
o Stream Sockets: are connection oriented and they use TCP to establish a reliable
connection.
o Data gram Sockets: rely on UDP for establishing a connection.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
TASK SYNCHRONIZATION:
In a multitasking environment, multiple processes run concurrently and share the system resources. Also,
each process may communicate with each other with different IPC mechanisms. Hence, there may be
situations that; two processes try to access a shared memory area, where one process tries to write to the
memory location when the other process is trying to read from the same memory location. This will lead
to unexpected results.
The solution is, make each process aware of access of a shared resource. The act of making the processes
aware of the access of shared resources by each process to avoid conflicts is known as “ Task/ Process
Synchronization”.
Task/Process Synchronization is essential for–
1. Avoiding conflicts in resource access(racing, deadlock,etc.)in multitasking environment.
2. Ensuring proper sequence of operation across processes.
3. Establish proper communication between processes.

The code memory area which holds the program instructions (piece of code) for accessing a shared
resource is known as „Critical Section‟. In order to synchronize the access to shared resources, the access
to the critical section should be exclusive.

Task Communication/ Synchronization Issues:

Various synchronization issues may arise in a multitasking environment, if processes are not
synchronized properly in shared resource access, such as:
1. Racing: Look into the following piece of code:
#include<stdio.h>
//****************************************************************
//counterisanintegervariableandBufferisabytearrayshared
//betweentwoprocessesProcessAandProcessB. char
Buffer [10] = {1,2,3,4,5,6,7,8,9,10};
shortintcounter = 0;
//****************************************************************
//ProcessA
VoidProcess_A(void)
{
inti;
for(i=0; i<5;i++)
{
if(Buffer[i]>0)
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

counter++;
}
}
//****************************************************************
//ProcessB
VoidProcess_B(void)
{
intj;
for(j=5;j<10;j++)
{
if(Buffer[j]>0) counter++;
}
}
//****************************************************************
//MainThread.
int main()
{
DWORDid;
CreateThread (NULL, 0, (LPTHREAD_START_ROUTINE) Process_A,
(LPVOID)0,0, &id);
CreateThread (NULL, 0, (LPTHREAD_START_ROUTINE) Process_B,
(LPVOID)0,0, &id);
Sleep (100000);
return 0;
}
 From a programmer perspective, the value of counter will be 10 at the end of execution of
processes A & B. But it need not be always.
o The program statement counter++; looks like a single statement from a high level
programming language (C Language) perspective. The low level implementation of this
statement is dependent on the under lying processor instruction set and the(cross) compiler in
use. The low level implementation of the high level program statement counter++; under
Windows XP operating system running on an Intel Centrino Duo processor is given below.
mov eax, dword ptr [ebp-4] ;Load counter in Accumulator
add eax, 1 ; Increment Accumulator by 1
mov dword ptr [ebp-4], eax ;StorecounterwithAccumulator
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

o At the processor instruction level, the value of the variable counter is loaded to the
Accumulator register (EAX Register).The memory variable counter is represented using a
pointer. The base pointer register (EBP Register) is used for pointing to the memory variable
counter. After loading the contents of the variable counter to the Accumulator, the
Accumulator content is incremented by one using the add instruction. Finally the content of
Accumulator is loaded to the memory location which represents the variable counter. Both
the processes; Process A and Process B contain the program statement counter++;
Translating this into the machine instruction.
ProcessA ProcessB

moveax,dwordptr[ebp-4] add moveax,dwordptr[ebp-4] add

eax, 1 eax, 1

movdwordptr[ebp-4],eax movdwordptr [ebp-4], eax

o Imagine a situation where a process switching(context switching) happens from Process A to

Process B when Process A is executing the counter++; statement. Process A accomplishes
the counter++;statement through three different low level instructions. Now imagine that the
process switching happened at the point, where Process A executed the low level instruction
mov eax, dword ptr [ebp-4] and is about to execute the next instruction add eax, 1. The
scenario is illustrated in the following Figure.

o Process B increments the shared variable „counter‟ in the middle of the operation where
Process A tries to increment it. When Process A gets the CPU time for execution, it starts
from the point where it got interrupted (If Process B is also using the same registers eax and
ebp for executing counter++; instruction, the original content of these registers will be saved
as part of context saving and it will be retrieved back as part of the context retrieval, when
Process A gets the CPU for execution. Hence the content of eax and ebp remains intact
irrespective of context switching). Though the variable counter is incremented by Process B,
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

Process A is unawareof it and it increments the variable with the old value. This leads to the
loss of one increment for the variable counter.

2. Deadlock: Deadlock is the condition in which a process is waiting for a resource held by another
process which is waiting for a resource held by the first process; hence, none of the processes are
able to make any progress in their execution.
o Process A holds a resource „x‟ and it wants a resource „y‟ held by Process B. Process B is
currentlyholdingresource„y‟anditwantstheresource„x‟whichiscurrentlyheldbyProcess
A. Both hold the respective resources and they compete each other to get there source held by
the respective processes.

o Conditions Favoring Deadlock:

 Mutual Exclusion: The criteria that only one process can hold a resource at a time.
Meaning processes should access shared resources with mutual exclusion. Typical
example is the accessing of display device in an embedded device.
 Hold &Wait: The condition in which a process holds a shared resource by acquiring
the lock controlling the shared access and waiting for additional resources held by
other processes.
 No Resource Preemption: The criteria that Operating System cannot take back a
resource from a process which is currently holding it and the resource can only be
released voluntarily by the process holding it.
 Circular Wait: A process is waiting for a resource which is currently held by another
process which in turn is waiting for a resource held by the first process. In general
there exists a set of waiting process P0, P1 …. Pn with P0 is waiting for a resource
held by P1 and P1 is waiting for a resource held by P0, ……,Pn is waiting for a
resource held by P0 and P0 is waiting for a resource held by Pn and so on… This
forms a circular wait queue.
o Handling Deadlock: The OS may adopt any of the following techniques to detect and
prevent deadlock conditions.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

 Ignore Deadlocks: Always assume that the system design is deadlock free. This is
acceptable for the reason the cost of removing a deadlock is large compared to the
chance of happening a deadlock. UNIX is an example for an OS following this
principle. A life critical system cannot pretend that it is deadlock free for any reason.
 Detect and Recover: This approach suggests the detection of a deadlock situation and
recovery from it.
 This is similar to the deadlock condition that may arise at a traffic junction.
When the vehicles from different directions compete to cross the junction,
deadlock (traffic jam) condition is resulted. Once a deadlock (traffic jam) is
happened at the junction, the only solution is to back up the vehicles from
one direction and allow the vehicles from opposite direction to cross the
junction. If the traffic is too high, lots of vehicles may have to be backed upto
resolve the traffic jam. This technique is also known as „back up cars‟
technique.
 Operating Systems keep a resource graph in their memory. The resource
graph is updated on each resource request and release. A deadlock condition
can be detected by analyzing the resource graph by graph analyzer
algorithms. Once a deadlock condition is detected, the system can terminate a
process or preempt the resource to break the deadlocking cycle.
o Avoid Deadlocks: Deadlock is avoided by the careful resource allocation techniques by the
Operating System. It is similar to the traffic light mechanism at junctions to avoid the traffic
jams.
o Prevent Deadlocks: Prevent the deadlock condition by negating one of the four conditions
favoring the deadlock situation.
o Ensure that a process does not hold any other resources when it requests a resource. This can
be achieved by implementing the following set of rules/ guidelines in allocating resources to
processes.
1. A process must request all its required resource and the resources should be allocated
before the process begins its execution.
2. Grant resource allocation requests from processes only if the process does not hold a
resource currently.
o Ensure that resource preemption (resource releasing) is possible at operating system level.
This can be achieved by implementing the following set of rules/ guidelines in resources
allocation and releasing:
1. Releasealltheresourcescurrentlyheldbyaprocessifarequestmadebytheprocessfor
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18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

2. Addtheresourceswhicharepreempted(released)toaresourcelistdescribingthe resources
which the process requires to complete its execution.
3. Reschedule the process for execution only when the process gets its old resources and the
new resource which is requested by the process.

Task Synchronization Techniques:

The technique used for task synchronization in a multitasking environment is mutual exclusion. Mutual
exclusion blocks a process. Based on the behavior of blocked process, mutual exclusion methods can be
classified into two categories: Mutual exclusion through busy waiting/ spin lock & Mutual exclusion
through sleep & wakeup.
 Semaphore: Semaphore is a sleep and wakeup based mutual exclusion implementation for shared
resource access. Semaphore is a system resource; and a process which wants to access the shared
resource can first acquire this system object to indicate the other processes which wants the
shared resource that the shared resource is currently in use by it.
 The resources which are shared among a process can be either for exclusive use by a process or
for using by a number of processes at a time.
 The display device of an embedded system is a typical example of a shared resource which needs
exclusive access by a process. The Hard disk(secondary storage) of a system is a typical example
for sharing the resource among a limited number of multiple processes.
 Based on the implementation, Semaphores can be classified into Binary Semaphore and Counting
Semaphore.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

o Binary Semaphore: Implements exclusive access to shared resource by allocating the

resource to a single process at a time and not allowing the other processes to access it
when it is being used by a process.
 „Only one process/thread‟ can own the binary semaphore at a time.
 The state of a „binary semaphore‟ object is set to signaled when it is not owned
by any process/ thread, and set to non-signaled when it is owned by any process/
thread.
 The implementation of binary semaphore is OS kernel dependent. Under certain
OS kernel it is referred as mutex.
O Counting Semaphore: Maintains a count between zero and a maximum value. It limits the
usage of resource by a fixed number of processes/ threads.
o The count associated with a „Semaphore object‟ is decremented by one when a process/
thread acquires it and the count is incremented by one when a process/ thread releases the
„Semaphore object‟.
o The state of the counting semaphore object is set to „signaled‟ when the count of the
object is greater than zero.
o The state of the „Semaphore object‟ is set to non-signaled when the semaphore is
acquired by the maximum number of processes/ threads that the semaphore can support
(i.e. when the count associated with the „Semaphore object‟ becomes zero).
o Thecreationandusageof„countingsemaphoreobject‟isOSkerneldependent.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
HOW TO CHOOSE AN RTOS:
The decision of choosing an RTOS for an embedded design is very crucial. A lot of factors need to be
analyzed carefully before making a decision on the selection of an RTOS. These factors can be either
functional or non-functional.

Functional Requirements:
 Processor Support: It is not necessary that all RTOS‟ support all kinds of process or architecture.
It is essential to ensure the processor support by the RTOS.
 Memory Requirements: The OS requires ROM memory for holding the OS files and it is
normally stored in a non-volatile memory like FLASH. OS also requires working memory RAM
for loading the OS services. Since embedded systems are memory constrained, it is essential to
evaluate the minimal ROM and RAM requirements for the OS under consideration.
 Real-time Capabilities: It is not mandatory that the operating system for all embedded systems
need to be Real-time and all embedded Operating systems-are 'Real-time' in behavior. The task/
process scheduling policies play an important role in the 'Real-time' behavior of an OS. Analyze
the real-time capabilities of the OS under consideration and the standards met by the operating
system for real-time capabilities.
 Kernel and Interrupt Latency: The kernel of the OS may disable interrupts while executingcertain
services and it may lead to interrupt latency. For an embedded system whose response
requirements are high, this latency should be minimal.
 Inter Process Communication and Task Synchronization: The implementation of Inter Process
Communication and Synchronization is OS kernel dependent. Certain kernels may provide a
bunch of options whereas others provide very limited options. Certain kernels implement policies
for avoiding priority inversion issues in resource sharing.
 Modularization Support: Most of the operating systems provide a bunch of features. At times it
may not be necessary for an embedded product for its functioning. It is very useful if the OS
supports moclularisation where in which the developer can choose the essential modules and re-
compile the OS image for functioning. Windows CE is an example for a highly modular
operating system.
 Support for Networking and Communication: The OS kernel may provide stack implementation
and driver support for a bunch of communication interfaces and networking. Ensure that the OS
under consideration provides support for all the interfaces required by the embedded product.
 Development Language Support: Certain operating systems include the run time libraries required
for running applications written in languages like Java and C#. A Java Virtual Machine (JVM)
customized for the Operating System is essential for running java applications. Similarly
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

the.NET Compact Framework(.NETCF)is require for running Microsoft .NET applications on top
of the Operating System. The OS may include these components as built-in component, if not;
check the availability of the same from a third party vendor or the OS under consideration.

Non-functional Requirements:
 Custom Developed or Off the Shelf: Depending on the OS requirement, it is possible to go for the
complete development of an operating system suiting the embedded system needs or use an off
the shelf, readily available operating system, which is either a commercial product or an Open
Source product, which is in close match with the system requirements. Sometimes it may be
possible to build the required features by customizing an Open source OS. The decision on which
to select is purely de- pendent on the development cost, licensing fees for the OS, development
time and availability of skilled resources.
 Cost: The total cost for developing or buying the OS and maintaining it in terms of commercial
product and custom build needs to be evaluated before taking a decision on the selection of OS.
 Development and Debugging Tools Availability: The availability of development and debugging
tools is a critical decision making factor in the selection of an OS for embedded design. Certain
Operating Systems may be superior in performance, but the availability of tools for supporting
the development may be limited. Explore the different tools available for the OS under
consideration.
 Ease of Use: How easy it is to use a commercial RTOS is another important feature that needs to
be considered in the RTOS selection.
 After Sales: For a commercial embedded RTOS ,after sales inthefom1of e-mail, on-call services
etc., for bug fixes, critical patch updates and support for production issues, etc., should be
analyzed thoroughly.

INTREGRATION AND TESTING OF EMBEDDED HARDWARE AND FIRMWARE

Integration testing of the embedded hardware and firmware is the immediate step following theembedded
hardware and firmware development.
 The final embedded hardware constitute of a PCB with all necessary components affixed to it as
per the original schematic diagram.
 Embedded firmware represents the control algorithm and configuration data necessary to
implement the product requirements on the product. Embedded firmware will be in a target
processor/ controller understandable format called machine language (sequence of ls and 0s-
Binary).
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

 The target embedded hardware without embedding the firmware is a dumb device and cannot
function properly. If you power up the hardware without embedding the firmware, the device
may behave in an unpredicted manner.
 Both embedded hardware and firmware should be independently tested (Unit Tested) to ensure
their proper functioning.
 Functioning of individual hardware sections can be done by writing small utilities which checks
the operation of the specified part.
 The functionalities of embedded firmware can easily be checked by the simulator environment
provided by the embedded firmware development tool's IDE. By simulating the firmware, the
memory contents, register details, status of various flags and registers can easily be monitoredand
it gives an approximate picture of "What happens inside the processor/ controller and whatare the
states of various peripherals" when the firmware is running on the target hardware. The IDE gives
necessary support for simulating the various inputs required from the external world, like
inputting data on ports, generating an interrupt condition, etc.

INTEGRATION OF HARDWARE AN DFIRMWARE:

Integration of hardware and firmware deals with the embedding of firmware into the target hardware
board. It is the process of 'Embedding Intelligence' to the product.
 The embedded processors/ controllers used in the target board may or may not have built in code
memory. For non-operating system based embedded products, if the processor/ controller contain
internal memory and the total size of the firmware is fitting into the code memory area, the code
memory is downloaded into the target controller/ processor.
 If the processor/ controller does not support built in code memory or the size of the firmware is
exceeding the memory size supported by the target processor/ controller, an external dedicated
EPROM/ FLASH memory chip is used for holding the firmware. This chip is interfaced to the
processor/ controller.
A variety of techniques are used for embedding the firmware into the target board. The commonly used
firmware embedding techniques for a non-OS based embedded system are explained below. The non-OS
based embedded systems store the firmware either in the on-chip processor/ controller memory or off-
chip memory (FLASHI/ NVRAM, etc.).

Out-of-Circuit Programming:
Out-of-circuit programming is performed outside the target board. The processor or memory chip into
which the firmware needs to be embedded is taken out of the target board and it is programmed with the
help of a programming device.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
The programming device is a dedicated unit which contains the necessary hardware circuit to generate the
programming signals. Most of the programming devices available in the market are capable of
programming different family of devices.
The programming device will be under the control of a utility program running on a PC. Usually the
programming device is interfaced to the PC through RS-232C/USB/Parallel Port Interface. Thecommands
to control the programmer are sent from the utility program to the programmer through the interface (see
the following Figure).

The sequence of operations for embedding the firmware with a programmer is listed below:
1. Connect the programming device to the specified port of PC(USB/COM port/Parallel port)
2. Power up the device (Most of the programmers incorporate LED to indicate Device power up.
Ensure that the power indication LED is ON)
3. Execute the programming utility on the PC and ensure proper connectivity is established between
PC and programmer. In case of error turn off device power and try connecting it again
4. Unlock the ZIF socket by turning the lock pin
5. Insert the device to be programmed into the open socket as per the insert diagram shown on the
programmer
6. Lock the ZIF socket
7. Select the device name from the list of supported devices
8. Load the hex file which is to be embedded into the device
9. Program the device by' Program 'option of utility program
10. Waittillthecompletionofprogrammingoperation(TillbusyLEDofprogrammerisoff)
11. Ensure that programming is success by checking the status LED on the programmer (Usually
'Green' for success and 'Red' for error condition) or by noticing the feedback from the utility
program
12. Unlock the ZIF socket and take the device out of programmer.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Now the firmware is successfully embedded into the device. Insert the device into the board, power up the
board and test it for the required functionalities. It is to be noted that the most of programmers support
only Dual Inline Package (DIP) chips, since its ZIF socket is designed to accommodate only DIP chips.
Option for setting firmware protection will be available on the programming utility. If you really want the
firmware to be protected against unwanted external access, and if the device is supporting memory
protection, enable the memory protection on the utility before programming the device.
The programmer usually erases the existing content of the chip before programming the chip. Only
EEPROM and FLASH memory chips are erasable by the programmer.
The major drawback of out-of-circuit programming is the high development time. Whenever the firmware
is changed, the chip should be taken out of the development board for re-programming. This is tedious
and prone to chip damages due to frequent insertion and removal.
The out-of-system programming technique is used for firmware integration for low end embedded
products which runs without an operating system. Out-of-circuit programming is commonly used for
development of low volume products and Proof of Concept (PoC) product Development.

In System Programming (ISP):

With ISP, programming is done 'within the system', meaning the firmware is embedded into the target
device without removing it from the target board. It is the most flexible and easy way of firmware
embedding. The only pre-requisite is that the target device must have an ISP support. Apart from the
target board, PC, ISP cable and ISP utility, no other additional hardware is required for ISP.
The target board can be interfaced to the utility program running on PC through Serial Port/ Parallel Port/
USB. The communication between the target device and ISP will be in a serial format. The serial
protocols used for ISP may be 'Joint Test Act Group (JTAG)' or 'Serial Peripheral Interface (SPI)' or any
other proprietary protocol.

In System Programming with SPI Protocol: Devices with SPI (Serial Peripheral Interface) ISP (In
System Programming) support contains a built-in SPI interface and the on-chip EEPROM or FLASH
memory. The primary I/O lines involved in SPI-In System Programming are listed below:
MOSI–Master Out Slave In
MISO–Master In Slave Out
SCK – System Clock
RST – Reset of Target Device
GND–Ground of Target Device
PC acts as the master and target device acts as the slave in ISP. The program data is sent to the MOSI pin
oftargetdeviceandthedeviceacknowledgementisoriginatedfromtheMISOpinofthedevice.SCKpin
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
acts as the clock for data transfer. A utility program can be developed on the PC side to generate theabove
signal lines.
Standard SPI-ISP utilities are feely available on the internet and, there is no need for going for writing
own program. For ISP operations, the target device needs to be powered up in a pre-defined sequence.The
power up sequence for In System Programming for Atmel's AT89S series microcontroller family is listed
below:
1. Apply supply voltage between VCC and GND pin soft target chip
2. Set RST pin to "HIGH" state
3. If a crystal is not connected across pins XTAL 1andXTAL2, apply a 3MHzto24MHz clock to
XTALl pin and wait for at least 10 milliseconds
4. Enable serial programming by sending the Programming Enable serial instruction to pin MOSI/
Pl.5. The frequency of the shift clock supplied at pin SCK/ P1.7 needs to be less than the CPU
clock at XTALl divided by 40
5. The Code or Data array is programmed one byte at a time by supplying the address and data
together with the appropriate Write instruction. The selected memory location is first erased
before the new data is written. The write cycle is self-timed and typically takes less than2.5 ms at
5V
6. Any memory location can be verified by using the Read instruction, which returns the content at
the selected address at serial output MISO/ Pl.6
7. After successfully programming the device, set RST pin low or turn off the chip power supplyand
turn it ON to commence the normal operation.

The key player behind ISP is a factory programmed memory (ROM) called 'Boot ROM‟. The Boot ROM
normally resides at the top end of code memory space and it varies in the order of a few Kilo Bytes (For a
controller with 64K code memory space and lK Boot ROM, the Boot ROM resides at memory location
FC00H to FFFFH). It contains a set of Low-level Instruction APIs and these APIs allow the processor/
controller to per form the FLASH memory programming, erasing and Reading operations. The contents
of the Boot ROM are provided by the chip manufacturer and the same is masked into every device.

In Application Programming(IAP):
In Application Programming is a technique used by the firmware running on the target device for
modifying a selected portion of the code memory. It is not a technique for first time embedding of user
written firmware. It modifies the program code memory under the control of the embedded application.
Updating calibration data, look-up tables, etc., which are stored in code memory, are typical examples of
IAP.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Use of Factory Programmed Chip:
It is possible to embed the firmware into the target processor/ controller memory at the time of chip
fabrication itself. Such chips are known as 'Factory Programmed Chips'. Once the firmware design isover
and the firmware achieved operational stability, the firmware files can be sent to the chip fabricator to
embed it into the code memory.
Factory programmed chips are convenient for mass production applications and it greatly reduces the
product development time. It is not recommended to use factory programmed chips for development
purpose where the firmware undergoes frequent changes. Factory programmed ICs are bit expensive.

Firmware Loading for Operating System Based Devices:

The OS based embedded systems are programmed using the In System Programming (ISP)technique. OS
based embedded systems contain a special piece of code called 'Boot loader' program which takes control
of the OS and application firmware embedding and copying of the OS image to the RAM of the system
for execution.
The 'Boot 1oader' for such embedded systems comes as pre-loaded or it can be loaded to the memory
using the various interface supported like JTAG. The boot loader contains necessary driver initialization
implementation for initializing the supported interfaces like UART/ I2C, TCP/ IP, etc. Boot loader
implements menu options for selectingthesourcefor OS imageto load(Typical menu item examples are
Load from FLASH ROM, Load from Network, Load through UART, etc).
Once a communication link is established between the host and target machine, the OS image can be
directly downloaded to the FLASH memory of the target device.

BOARD BRING UP:

Oncethefirmwareisembeddedintothetargetboardusingoneoftheprogrammingtechniques,then power up the
board. You may be expecting the device functioning exactly in a way as you designed. But in real
scenario it need not be and if the board functions well in the first attempt itself you are very lucky.
Sometimes the first power up may end up in a messy explosion leaving the smell of burned components
behind. It may happen due to various reasons, like Proper care was not taken in applying the power and
powerappliedinreversepolarity(+veofsupplyconnectedto–veofthetargetboardandviceversa),
componentswerenot placedinthe correct polarityorder (E.g. a capacitor onthetarget boardisconnected to
the board with +ve terminal to –ve of the board and vice versa), etc ... etc ...

Theprototype/evaluation/productionversionmustpassthroughavariedsetofteststoverifythat embedded
hardware and firmware functions as expected. Bring up process includes –
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

 basic hardware spot checks/ validations to make sure that the individual components and busses/
interconnects are operational – which involves checking power, clocks, and basic functional
connectivity;
 basic firmware verification to make sure that the processor is fetching the code and the firmware
execution is happening in the expected manner;
 runningadvancedvalidationssuchasmemoryvalidations,signalintegrityvalidation,etc.

THE EMBEDDED SYSTEM DEVELOPMENT ENVIRONMENT

The embedded system development environment consists of –
 DevelopmentComputer(PC)orHost–actsastheheartofthedevelopmentenvironment
 IntegratedDevelopmentEnvironment(IDE)Tool–forembeddedfirmwaredevelopmentand
debugging
 Electronic Design Automation(IDA)Tool–for embedded hardware design
 A nemulator hardware–for debugging the target board
 Signal sources(like CRO, Multimeter, Logic Analyzer ,etc.)
 Target hardware.

THE INTEGRAT EDDEVELOPMENT ENVIRONMENT (IDE):

In embedded system development context, Integrated Development Environment (IDE) stands for an
integrated environment for developing and debugging the target processor specific embedded firmware.
IDE is a software package which bundles –
 a“ Text Editor (Source Code Editor)”,
 “Cross-complier(forcrossplatformdevelopmentandcomplierforsameplatformdevelopment)”,
 “Linker”, and
 a “Debugger”.
Some IDEs may provide–
 interface to target board emulators,
 target processor’s /controller’s Flash memory programmer,
etc. IDE may be command line based or GUI based.
NOTE:TheKeilµVisionIDE& AnOverviewofIDEs–lestasanexercise/ selfstudytopic.

DISASSEMBLER / DECOMPLIER:
Disassembler is a utility program which converts machine codes into target processor specific Assembly
codes/ instructions. The process of converting machine codes into Assembly code is known as
'Disassembling'. In operation, disasseri1bling is complementary to assembling/ cross-assembling.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Decompiler is the utility program for translating machine codes into corresponding high level language
instructions. Decompiler performs the reverse operation of compiler/ cross-compiler.
The disassemblers/ decompilers for different family of processors/ controllers are different.
Disassemblers/ Decompilers are deployed in reverse engineering. Reverse engineering is the process of
revealing the technology behind the working of a product. Reverse engineering in Embedded Product
development is employed to find out the secret behind the working of popular proprietary products.
Disassemblers /decompilers help the reverse engineering process by translating the embedded firmware
into Assembly/ high level language instructions.
Disassemblers/ Decompilers are powerful tools for analyzing the presence of malicious codes (virus
information) in an executable image. Disassemblers/ Decompilers are available as either freeware tools
readily available for free download from internet or as commercial tools.
It is not possible for a disassembler/ decompiler to generate an exact replica of the original assemblycode/
high level source code in terms of the symbolic constants and comments used. However disassemblers/
decompilers generate a source code which is somewhat matching to the original source code from which
the binary code is generated.

SIMULATORS, EMULATORS AND DEBUGGING:

Simulators and emulators are two important tools used in embedded system development.
 Simulator is a software tool use for simulating the various conditions for checking the
functionality of the application firmware. The Integrated Development Environment (IDE) itself
will be providing simulator support and they help in debugging the firmware for checking its
required functionality. In certain scenarios, simulator refers to a soft model (GUI model) of the
embedded product.
o For example, if the product under development is a handheld device, to test the
functionalities of the various menu and user interfaces, a soft form model of the product
with all UI as given in the end product can be developed in software. Soft phone is an
example for such a simulator.
 Emulator is hardware device which emulates the functionalities of the target device and allows
real time debugging of the embedded firmware in a hardware environment.

Simulators:
Simulators simulate the target hardware and the firmware execution can be inspected using simulators.
The features of simulator based debugging are listed below.
1. Purely software based
2. Doesn't require area target system
3. Very primitive(Lack of featured I/O support. Everything is a simulated one)
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

4. Lack of Real-time behavior.

Advantages of Simulator Based Debugging: Simulator based debugging techniques are simple and
straightforward .The major advantages of simulator based firmware debugging techniques are explained
below.
 No Need for Original Target Board: Simulator based debugging technique is purely software
oriented. IDE's software support simulates the CPU of the target board. User only needs to know
about the memory map of various devices within the target board and the firmware should be
written on the basis of it. Sincethereal hardware is not required, firmware development can start
wellinadvanceimmediatelyafter thedeviceinterfaceand memory maps are finalized. This saves
development time.
 Simulate I/O Peripherals: Simulator provides the option to simulate various I/O peripherals.
Using simulator's I/O support you can edit the values for I/O registers and can be used as the
input/ output value in the firmware execution. Hence it eliminates the need for connecting I/O
devices for debugging the firmware.
 Simulates Abnormal Conditions: With simulator's simulation support you can input any desired
value for any parameter during debugging the firmware and can observe the control flow of
firmware. It really helps the developer in simulating abnormal operational environment for
firmwareand helps thefirmware developer tostudythe behavior of thefirmwareunder abnormal
input conditions.

Limitations of Simulator Based Debugging: Though simulation based firmware debugging technique is
very helpful in embedded applications, they possess certain limitations and we cannot fully rely on the
simulator-based firmware debugging. Some of the limitations of simulator-based debugging are explained
below:
 Deviation from Real Behavior: Simulation-based firmware debugging is always carried out in a
development environment where the developer may not be able to debug the firmware under all
possible combinations of input. Under certain operating conditions, we may get some particular
result and it need not be the same when the firmware runs in a production environment.
 Lack of Real Timeliness: The major limitation of simulator based debugging is that it is not real-
time in behavior. The debugging is developer driven and it is no way capable of creating a real
time behavior. Moreover in a real application the I/O condition may be varying or unpredictable.
Simulation goes for simulating those conditions for known values.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Emulators and Debuggers:
Debugging in embedded application is the process of diagnosing the firmware execution, monitoring the
target processor's registers and memory, while the firmware is running and checking the signals from
various buses of the embedded hardware. Debugging process in embedded application is broadly
classified into two, namely; hardware debugging and firmware debugging.
 Hardware debugging deals with the monitoring of various bus signals and checking the status
lines of the target hardware.
 Firmware debugging deals with examining the firmware execution, execution flow, changes to
various CPU registers and status registers on execution of the firmware to ensure that the
firmware is running as per the design.

Firmware debugging is performed to figure out the bug or the error in the firmware which creates the
unexpected behavior. The following section describes the improvements over firmware debugging
starting from the most primitive type of debugging to the most sophisticated On Chip Debugging (OCD):

 Incremental EEPROM Burning Technique: This is the most primitive type of firmware
debugging technique where the code is separated into different functional code units. Instead of
burning the entire code into the EEPROM chip at once, the code is burned in incremental order,
where the code corresponding to all functionalities are separately coded, cross-compiled and
burned into the chip one by one.

 Inline Breakpoint Based Firmware Debugging: Inline breakpoint based debugging is another
primitive method of firmware debugging. Within the firmware where you want to ensure that
firmware execution is reaching up to a specified point, insert an inline debug code immediately
after the point. The debug code is a printf() function which prints a string given as per the
firmware. You can insert debug codes (printf()) commands at each point where you want to
ensure the firmware execution is covering that point. Cross-compile the source code with the
debug codes embedded within it. Burn the corresponding hex file into the EEPROM.

 Monitor Program Based Firmware Debugging: Monitor program based firmware debugging is
the first adopted invasive method for firmware debugging (see the following Figure). In this
approach a monitor program which acts as a supervisor is developed. The monitor program
controls the downloading of user code into the code memory, inspects and modifies register/
memory locations; allows single stepping of source code, etc. The monitor program implements
the debug functions as per a pre-defined command set from the debug application interface. The
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

monitor program always listens to the serial port of the target device and according to the
command received from the serial interface it performs command specific actions like firmware
downloading, memory inspection/ modification, firmware single stepping and sends the debug
information (various register and memory contents) back to the main debug program running on
the development PC, etc.

o The first step in any monitor program development is determining a set of commands for
performing various operations like firmware downloading, memory/ register inspection/
modification, single stepping, etc. The entire code stuff handling the command reception
and corresponding action implementation is known as the "monitor program". The most
common type of interface used between target board and debug application is RS-232C
Serial interface.
o The monitor program contains thef ollowing set of minimal features:
1. Command set interface to establish communication with the debugging
application
2. Firmware download option to codememory
3. Examineandmodifyprocessor registersandworkingmemory(RAM)
4. Singlestep programexecution
5. Setbreakpointsinfirmwareexecution
6. Senddebuginformationtodebugapplicationrunningonhostmachine.

 In Circuit Emulator (ICE) Based Firmware Debugging: The terms 'Simulator' and 'Emulator'
are little bit confusing and sounds similar. Though their basic functionality is the same-"Debugthe
target firmware", the way in which they achieve this functionality is totally different. The
simulator 'simulates' the target board CPU and the emulator 'emulates' the target board CPU.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

o 'Simulator' is a software application that precisely duplicates (mimics) the target CPU and
simulates the various features and instructions supported by the target CPU.
o 'Emulator' is a self-contained hardware device which emulates the target CPU. The
emulator hardware contains necessary emulation logic and it is hooked to the debugging
application running on the development PC on one end and connects to the target board
through some interface on the other end.
o The Emulator POD (see the following Figure) forms the heart of any emulator system
and it contains the following functional units.

o Emulation Device: is a replica of the target CPU which receives various signals from the
target board through a device adaptor connected to the target board and performs the
execution of firmware under the control of debug commands from the debug application.
o Emulation Memory: is the Random Access Memory(RAM) incorporated in the Emulator
device. It acts as a replacement to the target board's EEPROM where the code is
supposed to be downloaded after each firmware modification. Hence the original
EEPROM memory is emulated by the RAM of emulator. This is known as 'ROM
Emulation'. ROM emulation eliminates the hassles of ROM burning and it offers the
benefit of infinite number of reprogramming.
o Emulator Control Logic: is the logic circuits used for implementing complex hardware
breakpoints, trace buffer trigger detection, trace buffer control, etc. Emulator control
logic circuits are also used for implementing logic analyzer functions in advanced
emulator devices. The 'Emulator POD' is connected to the target board through a 'Device
adaptor' and signal cable.
o Device Adaptors: act as an interface between the target board and emulator POD. Device
adaptors are normally pin-to-pin compatible sockets which can be inserted/ plugged into
thetargetboardforroutingthevarioussignalsfrompinsassignedforthetarget
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS

processor. The device adaptor is usually connected to the emulator POD using ribbon cables.

 On Chip Firmware Debugging (OCD): Advances in semiconductor technology has brought out
new dimensions to target firmware debugging. Today almost all processors/controllers in-
corporate built in debug modules called On Chip Debug (OCD) support. Though OCD adds
silicon complexity and cost factor, from a developer perspective it is a very good feature
supporting fast and efficient firmware debugging. The On Chip Debug facilities integrated to the
processor/ controller are chip vendor dependent and most of them are proprietary technologies
like Background Debug Mode (BDM), OnCE, etc.

TARGET HARDWARE DEBUGGING:

Even though the firmware is bug free and everything is intact in the board, your embedded product need
not function as per the expected behavior in the first attempt for various hardware related reasons like dry
soldering of components, missing connections in the PCB due to any un-noticed errors in the PCB layout
design, misplaced components, signal corruption due to noise, etc. The only way to sort out these issues
and figure out the real problem creator is debugging the target board.
Hardware debugging is not similar to firmware debugging. Hardware debugging involves the monitoring
of various signals of the target board (address/ data lines, port pins, etc.), checking the inter connection
among various components, circuit continuity checking, etc.
The various hardware debugging tools used in Embedded Product Development are explainedbelow.

Magnifying Glass(Lens):
You might have noticed watch repairer wearing a small magnifying glass while engaged -in repairing a
watch. They use the magnifying glass to view the minute components inside the watch in an enlarged
manner so that they can easily work with them.

Similar to a watch repairer, magnifying glass is the primary hardware debugging tool for an embedded
hardware debugging professional.
A magnifying glass is a powerful visual inspection tool. With a magnifying glass (lens), the surface of the
target board can be examined thoroughly for dry soldering of components, missing components, improper
placement of components, improper soldering, track (PCB connection) damage, short of tracks, etc.
Nowadays high quality magnifying stations are available for visual inspection.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Multimeter:
A multimeter is used for measuring various electrical quantities like voltage (Both AC and DC), current
(DC as well as AC), resistance, capacitance, continuity checking, transistor checking, cathode and anode
identification of diode, etc.
Any multimeter will workover a specific rangefor each measurement. A multimeter is the most valuable
tool in the tool kit of an embedded hardware developer. It is the primary debugging tool for physical
contact based hardware debugging and almost all developers start debugging the hardware with it.

Digital CRO:
Cathode Ray Oscilloscope (CRO) is a little more sophisticated tool compared to a multimeter. CRO is
used for waveform capturing and analysis, measurement of signal strength, etc. By connecting the point
under observationonthetarget boardtotheChannels of theOscilloscope, the waveforms can becaptured and
analyzed for expected behavior.
CRO is a very good tool in analyzing interference noise in the power supply line and other signal lines.
Monitoringthecrystaloscillator signal fromthetarget boardis a typical example oftheusageofCRO for
waveform capturing and analysis in target board debugging.
CROs are available in both analog and digital versions. Though Digital CROs are costly, feature-wisethey
are best suited for target board debugging applications. Digital CROs are available for highfrequency
support and they also incorporate modem techniques for recording waveform over a period of time,
capturing waves on the basis of a configurable event (trigger) from the target board.
Various measurements like phase, amplitude, etc. are also possible with CROs. Tektronix, Agilent,
Philips, etc. are the manufacturers of high precision good quality digital CROs.

Logic Analyzer:
A logic analyzer is the big brother of digital CRO. Logic analyzer is used for capturing digital data (logic
1 and0)from a digital circuitry whereas CRO is employed in capturing all kinds of waves including logic
signals. Another major limitation of CRO is that the total number of logic signals/ waveforms that canbe
captured with a CRO is limited to the number of channels.
A logic analyzer contains special connectors and clips which can be attached to the target board for
capturing digital data. In target board debugging applications, a logic analyzer captures the states of
various port pins, address bus and data bus of the target processor/ controller, etc.
Logic analyzers give an exact reflect on of what happens when a particular line of firmware is running.
This is achieved by capturing the address line logic and data line logic of target hardware. Most modem
logic analyzers contain provisions for storing captured data, selecting a desired region of the captured
waveform, zooming selected region of the captured waveform, etc. Tektronix, Agilent, etc. are the giants
in the logic analyzer market.
18CS44 MICROCONTROLLERANDEMBEDDEDSYSTEMS
Function Generator:
Function generator is not a debugging tool. It is a input signal simulator tool. A function generator is
capable of producing various periodic waveforms like sine wave, square wave, saw-tooth wave, etc. with
different frequencies and amplitude.
Sometimes the target board may require some kind of periodic waveform with a particular frequency as
input to some part of the board. Thus, in a debugging environment, the function generator serves the
purpose of generating and supplying required signals.

BOUNDARY SCAN:
As the complexity of the hardware increase, the number of chips present in the board and the
interconnection among them may also increase. The device packages used in the PCB become miniature
to reduce the total board space occupied by them and multiple layers may be required to route the
interconnections among the hips. With miniature device packages and multiple layers for the PCB it will
be very difficult to debug the hardware using magnifying glass, multimeter, etc. to check the
interconnection among the various chips.

Boundary scan isa technique used for testing the interconnection among the various chips, which support
JTAG interface, present in the board. Chips which support boundary scan associate a boundary scan cell
with each pin of the device.

A JTAG port contains the five signal lines, namely, TDI, TDO, TCK, TRST and TMS form the Test
Access Port (TAP) for a JTAG supported chip. Each device will have its own TAP. The PCB also
contains a TAP for connecting the JTAG signal lines to the external world.

A boundary scan path is formed inside the board by interconnecting the devices through JTAG signal
lines. The TDI pin of the TAP of the PCB is connected to the TDI pin of the first device.

The TDO pin of the first device is connected to the TDI pin of the second device. In this way all devices
are interconnected and the TDO pin of the last JTAG device is connected to the TDOpin of the TAP of
the PCB. The clock line TCK and the Test Mode Select (TMS) line of the devices are connected to the
clock line and Test mode select line of the Test Access Port of the PCB respectively. This forms a
boundary scan path.

The following Figure illustrates the same.

MICROCONTROLLERANDEMBEDDEDSYSTEMS

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