PAPER NAME: Digital Circuit Design

PAPER CODE: ECE2002

MOD 4

1. Implement the function 𝑌 = (𝐴 + 𝐵)′ using CMOS logic circuit.

2. Describe a CMOS inverter circuit and discuss its advantage over nMOS and pMOS circuits.
3. Using CMOS gates construct a two input NAND gate and explain its working principle with a truth table.
4. Draw the circuit of two input NOR gates using a CMOS transistor.
5. Realize the following logic expression using CMOS logic F= (AB+CD)’
6. Realize the following logic expression using CMOS logic F= A+AC+BC
7. Construct an Exclusive-NOR circuit using a MOS transistor and expand its operation.
8. Mention two advantages of CMOS logic.
9. CMOS switching speed is greater than PMOS/NMOS: Explain.
10. Implement a NOT gate using a CMOS transistor?
11. Implement a XOR gate using CMOS logic for both with and without the incorporation of inverter at the final stage
and explain the logic operation in each case.
12. Identify the reasons for minimum power dissipation in CMOS logic.
13. Explain the working principle of CMOS 2 input NAND gate.
14. Explain the working principle of CMOS 2 input NOR gate along with the circuit diagram.
15. Explain the working principle of SERIAL-IN, PARALLEL-OUT shift register with suitable logic diagram.
(Ans. Below)
16. Define ROM and RAM. Explain the basic differences between EPROM and EEROM?
17. Define functions does a PLD programmer perform. Explain the applications of PLA?
18. Explain with a schematic the working principle of a 1 bit SRAM and a DRAM and compare their relative merits and
demerits.
19. Discuss the differences between Programmable Logic Arrays (PLA) and Programmable Array Logic (PAL).
20. Explain the difference among ROM, EPROM and EEPROM.
21. Implement the logic function 𝑓 = 𝐴𝐵′+A’B using CMOS logic
22. Realise a two input NAND gate using CMOS logic
23. How do the registers differ from each other in terms of data handling and operations?
(Ans. Below)
24. Explain the functionalities of programmable logic devices (PLDs) highlighting their applications in digital systems.
25. Design a 4-bit bidirectional shift register
(Ans. Below)
Ans. Q15
Explain the working principle of SERIAL-IN, PARALLEL-OUT shift register with suitable logic
diagram.

SIPO Shift Register: Circuit, Working, Truth Table & Its

Applications
Generally, a register can be defined as a device used to store the binary data but if you want to store multiple data
bits then a set of Flip flops are used which are connected in series. The data which is stored in the registers can
be shifted by using shift registers on either the right side or left side by providing CLK pulses. Shift Register is a
group of flip flops used to store multiple bits of data. Similarly, a shift register with n-bits can be formed by simply
connecting n flip-flops wherever every flip-flop simply stores a single data bit. Once the register shifts the bits to
the right side it is the right shift register whereas if it shifts to the left side then it is known as a left shift register.
This article discusses an overview of one of the types of shift register namely serial in parallel out shift register
or SIPO shift register.

What is SIPO Shift Register?

The shift register which allows serial input parallel output is known as the SIPO shift register. In the SIPO register,
the term SIPO stands for serial input parallel output. In this type of shift register, the input data is given bit by bit
serially. For each clock pulse, the input data at all the FFs can be shifted by a single position. The o/p at every
flip-flop can be received parallel.

Circuit Diagram

The SISO shift register circuit diagram is shown below. This circuit can be built with 4 D flip-flops which are
connected as shown in the diagram where the CLR signal is given additionally to the CLK signal to all FFs o
RESET them. In the above circuit, the first FF output is given to the second FFs input. All these four D flip-flops
are connected with each other serially because the same CLK signal is given to every flip-flop.

SIPO Shift Register Diagram

Working of SIPO Shift Register

The working of the SIPO shift register is; that it takes the serial data input from the first flip flop of the left side
and generates a parallel data output. The 4-bit SIPO shift register circuit is shown below. The operation of this
shift register is, first all the flip flops from the circuit from FF1 to FF4 have to RESET so that all the outputs of
FFs like QA to QD will be at logic zero level so there is no parallel data output.
The construction of the SIPO shift register is shown above. In the diagram, the first flip flop output ‘QA’ is
connected to the second flip flop input ‘DB’. The second flip flops output ‘QB’ is connected to the third flip flops
input DC, and the third flip flops output ‘QC’ is connected to the fourth flip flops input ‘DD. Here, QA, QB,
QC, and QD are data outputs.

Initially, all the output will become zero so without CLK pulse; all the data will become zero. Let’s take a 4-bit
data input example like 1101. If we apply the first clock pulse ‘1’ to the first flip flop, the data to be entered into
the FF and QA becomes ‘1’, and remaining all the outputs like QB, QC and QD will become zero. So the first
data output is ‘1000’

If we apply the second clock pulse as ‘0’ to the first flip flop then QA becomes ‘0’, QB becomes ‘0’, QC becomes
‘0’ and QD becomes ‘0’. So the second data output will become ‘0100’ due to the shift right process.

If we apply the third clock pulse as ‘1’ to the first flip flop then QA becomes ‘1’, QB becomes ‘0’, QC becomes
‘1’ and QD becomes ‘0’. So the third data output will become ‘1011’ due to the shift right process.
If we apply the fourth clock pulse as ‘1’ to the first flip flop then QA becomes ‘1’, QB becomes ‘1’, QC becomes
‘0’ and QD becomes ‘1’. So the third data output will become ‘1101’ due to the shift right process.

Timing Diagram

The timing diagram of the SIPO shift register is shown below.

Timing Diagram
Here we are using a positive edge CLK i/p signal. In a first clock pulse the input data becomes QA = ‘1’ and all
other values like QB, QC, and QD become ‘0’. So the output will become ‘1000’. In the second clock pulse, the
output will become ‘0101’. In the third clock pulse, the output will become ‘1010’ and in the fourth clock pulse,
the output will become ‘1101’
Ans. Q23
How do the registers differ from each other in terms of data handling and operations?
Ans. Q 25 :
Design a 4-bit bidirectional shift register

What is a Bidirectional Shift Register?

A type of shift register that allows for shifting of data to both the left and right directions is referred to as
a bidirectional shift register. Therefore, the bidirectional shift registers offer greater flexibility to shift and
manipulate data within the register.

In the bidirectional shift register, the direction of shifting of the data is controlled by a control signal and it depends
on the desired operation. For this purpose, an additional control circuit is provided within the register.

A bidirectional shift register is typically constructed by cascading a series of flip-flops. In this series, the flip-flops
are connected such that the output of each flip-flop is connected to the input of the next flip-flop. It is important
to note that the input of the first flip-flop is connected to the data input line.

The control/clock signal generated by the control circuit determines the direction of data shift. These control
signals are generally named as shift-left (to shift data towards the left side) and shift-right (to shift data towards
the right side).

Additionally, the bidirectional register also has a control signal, referred to as parallel-load (PL), that allows the
register to accept data in parallel and load them into the flip-flops at the same time.

Circuit Diagram and Working of Bidirectional Shift Register

The circuit diagram of a 4-bit bidirectional shift register is depicted in the following figure −

It consists of four D-flip-flops cascaded together along with a control circuit consisting of two AND gates and
one OR gate for each flip-flop. It has a control input signal R/L' that controls the direction of the shift.
From the control signal R/L', it is clear that R is an active high signal, while L' is an active low signal. When R/L'
signal is HIGH, the shift register acts as a shift-right shift register. When the R/L' is LOW, then the shift register
acts as a shift-left shift register.

How Do Bidirectional Shift Registers Operate?

Case 1 – Shift-Right Operation

When the control signal R/L' is HIGH, then the AND gates 1, 3, 5, and 7 are enabled, and the AND gates 2, 4,
6, and 8 are disabled. The output of the flip-flop A is connected to the input of the flip-flop B, the output of the
flip-flop B is connected to the input of the flip-flop C, and the output of the flip-flop C is connected to the input
of the flip-flop D. Hence, when a clock signal occurs, the data bits are shifted one place to the right.

Case 2 – Shift-Left Operation

When the control signal R/L' is LOW, then the AND gates 2, 4, 6, and 8 are enabled, and the AND gates 1, 3, 5,
and 7 are disabled. The output Q of each flip-flop connected to the D input of the following flip-flop. Thus, when
a clock signal occurs, the data bits are shifted one place to the left.

This is how a bidirectional shift-register is constructed and its operation takes place.

Advantages of Bidirectional Shift Register

The use of bidirectional shift registers provides several benefits in digital electronics circuits. Some key advantages
of bidirectional shift registers are listed below −

 Bidirectional shift registers allow to shift data in both left and right directions. This improves the flexibility
of the digital circuit.
 Bidirectional shift registers reduce the circuit complexity by integrating both right-shift and left-shift
operations in a single unit.
 Bidirectional shift registers also optimize the memory utilization.

Applications of Bidirectional Shift Register

The following some major applications of bidirectional shift registers −

 Bidirectional shift registers are widely used in microprocessors for faster data processing.
 Bidirectional shift registers are used in arithmetic circuits.
 They are also used in memory units to store and manipulate data.
 Bidirectional shift registers find application in the field of image processing to perform tasks like image
scanning, rotation, etc.
 Bidirectional shift registers are also employed in data compression and encryption applications.
 They are also used in digital signal processing to reduce noise, analyze signals, filtering operation, etc.