Registers

By : Ali Mustafa

Registers & Counter

• Circuits that include flip-flops are usually classified by the function they perform– Registers– Counters

• Register is a group of flip-flops.– Each flip-flop is capable of storing one bit of

information.– An n-bit register consists of a group of n flip-flops.– Register is a group of binary cells suitable for holding

binary information.

• A counter is essentially a register that goes through a predetermined sequence of states.

4-Bit Register

• A simple 4-bit register can be made with 4 D-FF• Data is loaded in parallel

• Common Clock• At each positive-edge, 4 bits are loaded

in parallel• Previous data is overwritten• Entering data is called loading

• Common Clear• Asynchronous active-low clear• When Clear = 0, all FFs are cleared; i.e.

0 is stored.

4-Bit Register (cont.)

• Question: How to modify this register to enable/disable loading new data (overwriting previous) ?

4-Bit Register (cont.)

• Question: How to modify this register to enable/disable loading new data (overwriting previous) ?

• Answer: When Load=0, the clock input to the FFs will never take a transition (0 to 1, 1 to 0), no new data will be loaded. When Load=1, normal data loading takes place

• This is called clock gating

4-Bit Register (cont.)

• Clock Skew Problem:

• It results from clock gating.

4-Bit Register (cont.)

• Better Solution: Register with Parallel Load

• Use a 2x1 MUX as shown:

4-Bit Register (cont.)

clockload

• A 4-bit Parallel Load Register

• When Load = 0, the data is not changed (no loading)

• When Load = 1, the data is loaded in parallel at the rising edge (+ve)

Register with Parallel Load

• Synchronous digital systems have a master clock generator that supplies a continuous train of clock pulses.

• The transfer of new information into a register is referred to as loading the register.

• If all the bits of the register are loaded simultaneously with a common clock pulse, we say that the loading is done in parallel.

• The load input determines whether the next pulse will accept new information or leave the information in the register intact

Register with Parallel Load• Load = 1 ; the I inputs

are transferred into the register

• Load = 0 ; maintain the content of the register

• Because the D flip-flop does not have a “no change”

Register with Parallel Load

Shift Registers

• Capable of shifting its binary information in one or both directions

Shift Register Applications• Shift Registers are an important Flip-Flop configuration

with a wide range of applications, including:

– Computer and Data Communications

– Serial and Parallel Communications

– Multi-bit number storage

– Sequencing

– Basic arithmetic such as scaling (a serial shift to the left or right will change the value of a binary number a power of 2)

– Logical operations

Parallel versus Serial• Serial communications: provides a binary number as a

sequence of binary digits, one after another, through one data line.

• Parallel communications: provides a binary number as binary digits through multiple data lines at the same time.

Shift Registers

• Shift Registers are devices that store and move data bits in serial (to the left or the right),

• ..or in parallel,

• ..or a combination of serial and parallel.

Configuration

• In Shift Registers, the binary digit transfers (shifts) from the output of one flip-flop to the input of the next individual Flip-Flop at every clock edge.

• Once the binary digits are shifted in, the individual Flip-Flops will each retain a bit, and the whole configuration will retain a binary number.

Shift Register Construction• Shift registers are comprised of D Flip-Flops

that share a common clock input.

D Q

Q

D Q

Q

D Q

Q

Combinations of Data Transfer Methods

• SISO: Serial In, Serial Out

• SIPO: Serial In, Parallel Out

• PISO: Parallel In, Serial Out

• PIPO: Parallel In, Parallel Out

10110 10110

10110

10110

10110

1011010110

10110

SISO Flip-Flop Shift Register

• Serial In Serial Out shift register has a single input and a single output

D Q

Q

D Q

Q

D Q

Q

Input Output

SIPO Flip-Flop Shift Register• Serial In Parallel Out shift register has a single

input and access to all outputs

D Q

Q

D Q

Q

D Q

Q

Input

Output Output Output

PISO Flip-Flop Shift Register• a Parallel In Serial Out shift register requires

additional gates, and the parallel input must revert to logic low.

Input

D Q

Q

Input

Output

Input

D Q

Q

D Q

Q

PIPO Flip-Flop Shift Register• Parallel In Parallel Out register has the

simplest configuration. It represents a memory device.

D Q

Q

Input

Output

D Q

Q

Output

D Q

Q

Output

Input Input

Shift Registers: Serial Transfer

Shift Register A Shift Register BS0 S1S1 S0

CLK

Example: Serial Transfer

Universal Shift Register

If the register has both shifts and parallel load capabilities, it is referred to as a universal shift register.

• A clear control to clear the register to 0.• A clock input to synchronize the operations.• A shift-right control to enable the shift right operation and

the serial input and output lines associated with the shift right.

• A shift-left control to enable the shift left operation and the serial input and output lines associated with the shift left.

• A parallel-load control to enable a parallel transfer and the n input lines associated with the parallel transfer.

• n parallel output lines• A control state that leaves the information in the register

unchanged in the presence of the clock.

Universal Shift Register

Universal Shift Register (cont.)

Universal Shift Register (cont.)

• How does it work?

• 4 D-FF and 4 MUXs with selection S0,S1

• S0S1=00, FF output is feedback to its input

• S0S1=01, FF input comes from left FF or serial-in (shift-right)

• S0S1=10, FF input comes from right FF or serial-in (shift-left)

• S0S1=11, parallel data transferred in

• Applications:

• Parallel ↔ Serial conversions

• Arithmetic multiplication/division

• Delaying input sequence

Counters• Counter: A register (sequential circuit) that goes through a pre-

determined sequence of states upon the application of input (clock or other source) pulses

• Binary Counter: The sequence of the states follows the binary number sequence (e.g. 000 001 010 011 etc.)• n-bit binary counter requires n flip-flops – counts from 0 to 2n-1

• Sequences can be binary, BCD, random, etc. • Counting can be up, down• A modulo-n counter goes through values 0,1,2, …, (n-1)

• e.g. modulo-10 up counter counts: 0,1,…9

• Two Types of Counters:

• Ripple counter (asynchronous):• Flip-flop output transition serves as source for triggering the other flip-

flops

• Synchronous counter:• common clock for all flip-flops (same design procedure)