SHIFT REGISTERS
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, 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
Above we show a block diagram of a serial-in/serial-out shift register, which is 4-stages long. Data at the input will be delayed by four clock periods from the input to the output of the shift register.
Data at "data in", above, will be present at the Stage A output after the first clock pulse. After the second pulse stage A data is transfered to stage B output, and "data in" is transfered to stage A output. After the third clock, stage C is replaced by stage B; stage B is replaced by stage A; and stage A is replaced by "data in". After the fourth clock, the data originally present at "data in" is at stage D, "output". The "first in" data is "first out" as it is shifted from "data in" to "data out".
