Page 189 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Digital Circuits and Logic Design

Notes • Define the memory decoding
• Explain the semiconductor memories

Introduction

Shift registers, like counters, are a form of sequential logic. Sequential logic, unlike combinational
logic is not only affected by the present inputs, but also, by the prior history. In other words,
sequential logic remembers past events.

Shift registers produce a discrete delay of a digital signal or waveform. A waveform synchronized
to a clock, a repeating square wave, is delayed by “n” discrete clock times, and where “n” is the
number of shift register stages. Thus, a four stage shift register delays “data in” by four clocks to
“data out”. The stages in a shift register are delay stages, typically type “D” Flip-Flops or type
“JK” Flip-flops.
Formerly, very long (several hundred stages) shift registers served as digital memory. This obsolete
application is reminiscent of the acoustic mercury delay lines used as early computer memory.
Serial data transmission, over a distance of meters to kilometers, uses shift registers to convert
parallel data to serial form. Serial data communications replaces many slow parallel data wires
with a single serial high speed circuit.

Serial data over shorter distances of tens of centimeters, uses shift registers to get data into and
out of microprocessors. Numerous peripherals, including analog to digital converters, digital
to analog converters, display drivers, and memory, use shift registers to reduce the amount of
wiring in circuit boards.
Some specialized counter circuits actually use shift registers to generate repeating waveforms.
Longer shift registers, with the help of feedback generate patterns so long that they look like
random noise, pseudo-noise.
Basic shift registers are classified by structure according to the following types:
• Serial-in/serial-out
• Parallel-in/serial-out
• Serial-in/parallel-out
• Universal parallel-in/parallel-out

• Ring counter

11.1 Shift Registers

Shift registers are a type of sequential logic circuit, mainly for storage of digital data. They are a
group of flip-flops connected in a chain so that the output from one flip-flop becomes the input
of the next flip-flop. Most of the registers possess no characteristic internal sequence of states.
All flip-flops are driven by a common clock, and all are set or reset simultaneously.

The basic types of shift registers are studied, such as Serial-In/Serial-Out, Serial-In/Parallel-Out,
Parallel-In/Serial-Out, Parallel-In/Parallel-Out, bidirectional shift registers. A special form of
counter—the shift register counter, is also introduced.
Register:
• A set of n flip-flops
• Each flip-flop stores one bit

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