Design of 8-bit Bi-Directional Register

A shift register is a sequential circuit that stores digital data and moves stored bits in a specific direction. It is widely used in serial data transfer, data conversion (SIPO/PISO), arithmetic operations, and delay elements. It consists of cascaded flip-flops sharing a common clock.

The basic types of shift registers are:

• SIPO: Serial In, Parallel Out
• PISO: Parallel In, Serial Out
• PIPO: Parallel In, Parallel Out
• Bi-directional shift registers

Data Transfer Types

Serial Transfer: Data is transferred one bit at a time over a single line.

Parallel Transfer: Data is transferred simultaneously over multiple lines.

Shift Operations

Right Shift: Moves bits toward LSB (divides by 2).

Left Shift: Moves bits toward MSB (multiplies by 2).

Rotation: Circular shifting of data using feedback.

Left Shift Operation

Right Shift Operation

Bi-Directional Shift Register

A bidirectional shift register allows shifting data in both directions. It includes control logic to select left or right shifting using a mode control input.

Fig.1: Circuit Diagram of Shift Register

• Left shift multiplies a number by 2
• Right shift divides a number by 2
• Control input determines shift direction

Fig.2: Bi-Directional Shift Register

Operation Table

S.N.	Condition	Operation
1	M = 1 (Shift Right)	Data shifts right with each clock pulse
2	M = 0 (Shift Left)	Data shifts left with each clock pulse

Circuits / Block Diagram

Fig.3: Block Diagram of Bi-Directional Shift Register

Observations & Data

Write and simulate the VHDL code, generate RTL schematic, and observe waveform outputs using Test Bench Waveform.

Procedure

Follow Xilinx ISE steps for coding, simulation, and synthesis.

Conclusion

The bidirectional shift register was successfully implemented using VHDL. Simulation results matched expected behavior, confirming correct left and right shift operations.

Further Reading

1. Introduction to Tanner EDA Tools
2. CMOS Logic Gates: Inverter, NAND, and NOR
3. EDA Tools and VHDL for VLSI Design
4. Layout of CMOS Inverter
5. Standard Cell Design in VLSI
6. Design of CMOS XOR/XNOR Gates
7. Design of CMOS Full Adder
8. Design of CMOS Flip-Flops (SR, D, JK)
9. Design of 8-bit Synchronous Counter
10. Design of 8-bit Bi-Directional Register
11. Design of a 12-bit CPU with Basic Instructions
12. VHDL Logical Function & CMOS Inverter
13. CMOS Layout Color Codes - IC Design Guide