Digital Register Quick Reference Guide

A register is a group of flip-flops used to store a binary value.

• Each flip-flop stores 1 bit.
• All flip-flops normally use a common clock.
• An n-bit register requires n flip-flops.

2-Bit Register Example

A 2-bit register contains two D flip-flops:

• D1 stores the MSB.
• D0 stores the LSB.
• Outputs are Q1 and Q0.

Both flip-flops receive the same clock signal.

Fig. 1: 2-bit Register

Working

• Before the active clock edge, the register keeps its previous value.
• At the active clock edge, D1D0 is stored in Q1Q0.
• The stored value remains unchanged until the next active clock edge.

Table 1: 2-Bit Register Example

X means the input value does not affect the already stored output.

A right shift register moves every stored bit one position toward the LSB.

For a 4-bit register:

Q3 → Q2 → Q1 → Q0

Here:

• Q3 is the MSB.
• Q0 is the LSB.
• New serial data enters Q3.
• Data leaves from Q0.

Fig. 2: 4-bit Right Shift Register

Connections

• D3 = Serial In
• D2 = Q3
• D1 = Q2
• D0 = Q1
• Serial Out = Q0

Working

At every active clock edge:

• Q3 receives the serial input.
• Q2 receives the previous Q3.
• Q1 receives the previous Q2.
• Q0 receives the previous Q1.

Table 2: Right Shift Example

The bits move from MSB to LSB.

A left shift register moves every stored bit one position toward the MSB.

For a 4-bit register:

Q0 → Q1 → Q2 → Q3

Here:

• New serial data enters Q0.
• Data leaves from Q3.

Fig. 3: 4-bit Left Shift Register

Connections

• D0 = Serial In
• D1 = Q0
• D2 = Q1
• D3 = Q2
• Serial Out = Q3

Working

At every active clock edge:

• Q0 receives the serial input.
• Q1 receives the previous Q0.
• Q2 receives the previous Q1.
• Q3 receives the previous Q2.

Table 3: Left Shift Example

The bits move from LSB to MSB.

An edge detector produces a short pulse when an input signal changes from one logic level to another.

It uses two clocked samples:

• Q0 – current sampled input
• Q1 – previous sampled input

The output pulse normally remains active for one clock cycle.

Table 4: Edge Detector Types

	Rising Edge 0 → 1	Falling Edge 1 → 0
Active-HIGH output	The output becomes 1 for one clock cycle when the input changes from 0 to 1.	The output becomes 1 for one clock cycle when the input changes from 1 to 0.
Active-LOW output	The output becomes 0 for one clock cycle when the input changes from 0 to 1.	The output becomes 0 for one clock cycle when the input changes from 1 to 0.

Applications

• Detecting push-button presses
• Generating counter pulses
• Capturing data in registers
• Starting serial data transmission
• Detecting changes in control signals
• Generating one-clock enable pulses

A bi-directional shift register can shift data in both directions.

• Right shift: MSB to LSB
• Left shift: LSB to MSB

A direction input DIR selects the shifting direction.

Fig. 4: 4-bit Bi-directional Shift Register with Two inputs

Circuit Structure

Each flip-flop input is connected to a 2-to-1 multiplexer.

The multiplexer selects:

• Data from the left-side flip-flop for right shifting
• Data from the right-side flip-flop for left shifting

Two serial inputs are used:

• SRin – enters the MSB during right shift
• SLin – enters the LSB during left shift

Table 5: Bi-Directional Register Connections

Working

When DIR = 0

• The multiplexers select the right-shift connections.
• Data moves from Q3 toward Q0.
• New data enters through SRin.

When DIR = 1

• The multiplexers select the left-shift connections.
• Data moves from Q0 toward Q3.
• New data enters through SLin.

The selected shift occurs at every active clock edge.

Registers are classified by how data enters and leaves them.

Table 6: Register Type Comparison

PIPO Register

PIPO means Parallel-In Parallel-Out.

All input bits are loaded together, and all output bits are available together.

Fig. 5: Parallel-in Parallel-out Shift Register

Connections

• Each parallel input is connected to one D flip-flop.
• All flip-flops share a common clock.
• Each flip-flop provides a separate output.

Working

At the active clock edge:

Q3Q2Q1Q0 = P3P2P1P0

The complete binary word is stored in one clock operation.

Functionality

• Temporary data storage
• Data buffering
• Processor registers
• Parallel data transfer

SISO Register

SISO means Serial-In Serial-Out.

Data enters one bit at a time and leaves one bit at a time.

Fig. 6: Serial-in Serial-out Shift Register

Connections

• The output of each flip-flop connects to the input of the next flip-flop.
• The first flip-flop receives the serial input.
• The last flip-flop provides the serial output.

Working

At each active clock edge:

• One new bit enters the register.
• Every stored bit moves one position.
• One bit leaves through the serial output.

For an n-bit SISO register, a bit reaches the output after n clock edges.

Functionality

• Serial data delay
• Digital delay lines
• Bit-by-bit data transfer

SIPO Register

SIPO means Serial-In Parallel-Out.

Data enters one bit at a time, but all stored bits can be read together.

Fig. 7: Serial-in Parallel-out Shift Register

Connections

• The flip-flops are connected as a serial shift register.
• Each flip-flop output is also available as a parallel output.

Working

• One serial bit enters during each clock edge.
• The bits move through the register.
• After the required number of clock edges, the complete word is available at the parallel outputs.

Functionality

• Serial-to-parallel conversion
• LED control
• Expanding processor output pins
• Receiving serial data

PISO Register

PISO means Parallel-In Serial-Out.

A complete binary word is loaded together and then shifted out one bit at a time.

Fig. 8: Parallel-in Serial-out Shift Register

Connections

Each flip-flop uses a multiplexer to select:

• Parallel input during load
• Previous flip-flop output during shift

A LOAD input controls the operation.

Working

When LOAD = 1

All parallel input bits are loaded together:

Q3Q2Q1Q0 = P3P2P1P0

When LOAD = 0

The stored data shifts one bit during each clock edge.

For right shifting:

• Data moves toward Q0.
• Serial data leaves from Q0.

Functionality

• Parallel-to-serial conversion
• Sending processor data over one wire
• Serial communication
• Reducing transmission wires

Parallel data can be transmitted through one serial line using:

PISO → Transmission Line → SIPO

Fig. 9: Parallel to Serial to Parallel Transmission

Working

1. A parallel data word is loaded into the PISO register.
2. The PISO register shifts out one bit during each clock cycle.
3. The transmission line carries the bits one at a time.
4. The SIPO register receives one bit during each clock cycle.
5. After all bits are received, the original word appears at the parallel outputs.

Table 7: Data Transmission Stages

Important Conditions

• The transmitter and receiver must use the same bit order.
• The receiver must sample each bit at the correct clock time.
• The number of transmitted bits must match the register size.

Benefits

• Requires fewer transmission wires
• Suitable for longer-distance communication
• Reduces circuit connections
• Allows parallel devices to communicate through a serial link

LED Belt

The LED belt uses a 4-bit right shift register to create a moving-light effect.

Each register output is connected to one active-HIGH LED:

• Q3 → LED3
• Q2 → LED2
• Q1 → LED1
• Q0 → LED0

An active-HIGH output falling-edge detector monitors input IN.

Fig. 10: LED Belt Circuit

Circuit Behaviour

• Normally, the edge-detector output remains 0.
• When IN changes from 1 → 0, the detector generates one HIGH pulse.
• This HIGH bit enters the serial input of the shift register.
• At every clock edge, the HIGH bit moves one position toward the LSB.
• The connected LEDs show the position of the stored HIGH bit.

Table 8: LED Belt Example

Only one HIGH pulse is inserted for one falling edge. Therefore, one LED moves through the belt after each clock.

Digital Lock

The digital lock stores a sequence of three button presses and compares it with the correct code:

5 → 1 → 7

Input Section

The circuit contains seven push buttons labelled 1 to 7.

• A pressed button generates HIGH.
• Buttons are connected to encoder inputs D1 to D7.
• The encoder produces outputs A, B, C, and D.
• Output D is unused.
• Outputs A, B, and C form the 3-bit button code.

Button-Press Detection

The signal: A + B + C

becomes HIGH when an encoded button value is present.

This signal is connected to an active-HIGH output rising-edge detector.

When a button is pressed:

• A + B + C changes from 0 → 1.
• The edge detector generates one HIGH pulse.
• This pulse is connected to the clock inputs of all PIPO registers.
• Only one register update occurs for each button press.

Sequence Storage

The circuit uses three 3-bit PIPO registers to store three encoded button values.

The registers store the entered button sequence. The stored values are checked after each entry.

Code Matching

A combinational circuit is connected after each PIPO register.

Each circuit produces HIGH only when its stored 3-bit value matches the required button value:

• First required value: 5
• Second required value: 1
• Third required value: 7

The three match signals are combined to generate UNLOCK.

Table 9: Digital Lock Code Check

When all three stored values match:

UNLOCK = 1

Reset Operation

The R push button shifts the code in next register. It works similar to "0".

Complete Working

1. The user presses one button from 1 to 7.
2. The encoder converts the button number into a 3-bit code.
3. The rising-edge detector generates one clock pulse.
4. The PIPO registers record the button entry.
5. The process repeats for three button presses.
6. The combinational circuits check the stored sequence.
7. UNLOCK becomes HIGH only when the entered code is 517.

Fig. 11: Digital Lock Circuit using Registers