DIGITAL Registers

7/28/2019 DIGITAL Registers

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Registers


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Registers

Registers hold larger quantities of data than individual flip-flops.

Registers are central to the design of modern processors.

There are many different kinds of registers.

Well show some applications of

these special registers.

Flip-flops are limited because they can store only one bit.

We had to use two flip-flops for our two-bit counter examples.

Most computers work with integers and single-precisionfloating-point numbers that are 32-bits long.

A register is an extension of a flip-flop that can store multiple bits.

Registers are commonly used as temporary storage in aprocessor.

They are faster and more convenient than main memory.

More registers can help speed up complex calculations.

Registers 1.2


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Basic registers are easy to build. We can store multiple bits just by putting a

bunch of flip-flops together!A 4-bit register internal implementation is below.This register uses D flip-flops, so its easy to store data without worryingabout flip-flop input equations. All the flip-flops share a common CLK and CLRsignal.

Registers 1.3


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Shift Register Applications

Shift Registers are an important Flip-Flopconfiguration 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 leftor right will change the value of a binary number a powerof 2)

Logical operations


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A shift registershifts its output once every clock cycle.

SI is an input that supplies a new bit to shift intothe register.

For example, if on some positive clock edge we have:

SI = 1Q

0-Q

3= 0110

then the next state will be:

Q0-Q3 = 1011

The current Q3 (0 in this example) will be lost on the next cycle.

Shift registers

Q0(t+1) = SIQ1(t+1) = Q0(t)Q2(t+1) = Q1(t)Q3(t+1) = Q2(t)


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Registers 1.6Registers 6

Shift direction

The circuit and example make it look like the register shifts

right.

But it really depends on your interpretation of the bits. If you

consider Q3 to be the most significant bit instead, then theregister is shifting in the opposite direction!

Q0(t+1) = SIQ

1(t+1) = Q

0(t)

Q2(t+1) = Q1(t)Q3(t+1) = Q2(t)

Present Q0-Q3 SI Next Q0-Q3

ABCD X XABC

Present Q3-Q0 SI Next Q3-Q0

DCBA X CBAX


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Registers

Shift registers with parallel load

We can add a parallel load, just like we did for regular registers.

When LD = 0, the flip-flop inputs will be SIQ0Q1Q2, so the register shifts on the nextpositive clock edge.

When LD = 1, the flip-flop inputs are D0-D3, and a new value is loaded into the shiftregister, on the next positive clock edge.


8/27Registers 1.8

Basic Shift RegisterShift register consist of arrangements of flip-flop

and are important in applications involving thestorage and transfer of data in a digital system.

Serial in/serial out shift registersSerial in/parallel out shift registersParallel in/serial out shift registersParallel in/parallel out shift registers

Bidirectional shift registers


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Parallel versus Serial

Serial communications: provides a binarynumber as a sequence of binary digits, oneafter another, through one data line.

Parallel communications: provides a binarynumber as binary digits through multiple datalines at the same time.


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Shift Registers

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

..or in parallel,

..or a combination of serial and parallel.


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Registers 1.11


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Configuration

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

Once the binary digits are shifted in, the individualFlip-Flops will each retain a bit, and the wholeconfiguration will retain a binary number.


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Construction

Shift registers are constructed from flip-flops due to their

characteristics: Edge-triggered devices

Output state retention

Each Flip-Flop in a shift register can retain one binary digit.

For instance, if a 5-bit binary number needs to be stored andshifted, 5 flip-flops are required.

Each binary digit transfer operation requires a clock edge.

Asynchronous inputs are useful in resetting the wholeconfiguration.


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Shift Register Construction

Shift registers are comprised of D Flip-Flops thatshare a common clock input.

D Q

Q

D Q

Q

D Q

Q


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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

How many clock edges are required for each operation?

10110 10110

10110

10110

10110

1011010110

10110


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SISO Flip-Flop Shift Register

a Serial In Serial Out shift register has a singleinput and a single output

D Q

Q

D Q

Q

D Q

Q

Input Output


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Registers 1.17

4-bit version One-bit at a time on asingle line


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Registers 1.18

Four Bits(1010) beingenteredserially intothe

register


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Registers 1.19

Four Bits (1010)beingserially shifted outof theregister and

replaced byall zeros


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SIPO Flip-Flop Shift Register

a Serial In Parallel Out shift register has a single inputand access to all outputs

D Q

Q

D Q

Q

D Q

Q

Input

Output Output Output

Registers 1.20


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PISO Flip-Flop Shift Register

aParallel In Serial Out

shift register requiresadditional gates, and the parallel input must revert tologic low.

Input

D Q

Q

Input

Output

Input

D Q

Q

D Q

Q


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PIPO Flip-Flop Shift Register

a Parallel In Parallel Out register has the simplestconfiguration. It represents a memory device.

D Q

Q

Input

Output

D Q

Q

Output

D Q

Q

Output

Input Input


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Registers 1.24

Universal Shift Registers

Look up the 74LS194 and describe its function

by looking at the schematic. Fill in the table.

S0 S1 Mode

0 0

0 1

1 0

1 1

24

l ll l f h


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Application: Parallel transferring thecontents of a Register to another register.

Describe where this

circuit combinationmay be used.


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JK Shift Registers

J-K Shift registers are seldom used, as two inputs (J,K)are required to load the first flip-flop (note all othersreceive only set or reset inputs).

Input OutputJ Q

K Q

J Q

K Q

J Q

K QInput


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Registers summary

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A register is a special state machine that stores multiple

bits of data.

Several variations are possible:

Parallel loading to store data into the register.

Shifting the register contents either left or right.

Counters are considered a type of register too! One application of shift registers is converting between

serial and parallel data.

Most programs need more storage space than registersprovide.

Well introduce RAM to address this problem. Registers are a central part of modern processors, as we

will see in coming weeks.

DIGITAL Registers

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