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Registers

CPE 49

RMUTI

KOTAT

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Shift Registers

Capability to shift bits In one or both directions

Why?

Part of standard CPUinstruction set

Cheap multiplication

Serial communications

Just a chain of flip-flops

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Simple 4-Bit Shift Register

Clocked in common

Just serial in and serial out

Not quite a FIFO

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Serial In - Serial Out Shift Registers

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Serial In - Parallel Out Shift Registers

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Parallel In - Serial Out Shift Registers

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Parallel In - Parallel Out Shift Registers

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Bidirectional Shift Registers

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Shift Register Counters

Ring Counters

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Shift Register Counters

Johnson Counters

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Applications

To p roduce time delayThe ser ial in -ser ial ou t sh if t register can be us ed as a t ime delay

device. The amoun t of delay can be contro l led by:

- the num ber of stages in the register

- the clock frequ ency

To simpl i fy com binat ional logicThe ring counter techniq ue can be effect ively ut i l ized to implement

sync hrono us sequent ial c i rcu i ts . A major problem in the real ization of

sequent ial c i rcu i ts is the assignment of binary cod es to the internal

states of the circu i t in o rder to reduce the comp lexi ty of c i rcu i ts

required. By assig nin g one f l ip-f lop to one internal state, i t is po ssib le

to sim pl i fy the com binat ional logic required to realize the complete

sequent ial circuit . When the circu it is in a part icular state, the f l ip-f lop

co rrespon din g to th at state is set to HIGH and al l oth er f lip- f lops

remain LOW.

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Applications

To convert serial data to paral lel dataA com puter or m icroprocessor-based system

commonly requi res incom ing data to be in

paral lel form at. Bu t frequently, these systems

must commun icate w ith external devices that

send or receive serial data. So, serial-to -

paral lel conversion is requ ired. As shown inthe previous sections , a serial in - paral lel ou t

register can ach ieve this.

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Symbol

we could gate the clock

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Parallel Load

Can provide parallel outputs fromflip-flops

And also parallel inputs

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Schematic

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Why is this useful?

Basis for serial communications

Keyboard

Serial port Initially to connect to terminals

Now mainly for modem

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Bidirectional Shift Register

Shift either way

Now we have following possible

inputs Parallel load

Shift from left

Shift from right

Also no change

Schematic next

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Schematic

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Verilog for Shift Registermodule srg_4_r_v (CLK, RESET, SI, Q,SO);

input CLK, RESET, SI;output [3:0] Q;

output SO;

reg [3:0] Q;assign SO = Q[3];

always@(posedge CLK or posedge RESET)

begin

if (RESET)

Q

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Serial Transfer

Parallel transfer over as manywires as word (for example)

Serial transfer over a single wire Trade time for wires

Takes n times longer

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Example

Clocked 4 times

Why do this? Maybe

these are far apart

Could shift

data in

Whats on wire at each clock?

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Table Showing Shift

Hopefully weve figured it out

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Serial Addition

Shift value in serially

Then shift through

adder into A. Added

to 0 if A is empty.

At same time, new

value going into B

Adds one bit at a time

Stores carry one clock

Initially reset all

registers

Register A

accumulates

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Hardware Comparison

Serial vs. parallel adder

One full adder vs. n adders

Serial takes nunits of time, parallelonly one

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Arbitrary Count

One more type of counter is useful

Count an arbitrary sequence

Maybe you need a sequence of states

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Circuit and State Diagram

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Shift Registers

N-bit register with the provision for

shifting its stored data by a bit position

each tick of the clock.

Serial input specifies a new bit to

shifted into the register.

Serial output specifies the bits being

shifted out of the register

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References

http://www.eelab.usyd.edu.au/digital_tutorial/part2/hpage.html

http://

www.allaboutcircuits

.com

/vol_4

/index.html

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