Shift registers are sequential circuits made up of flip-flops that move data from input to output using a clock. They can convert parallel data to serial, delay data signals, and convert analog to digital. There are five types, including ring counters, which have recirculating data patterns. They were once used for computer memory but are now used in display drivers, digital-to-analog converters, and serial data memory.
As applied to digital circuits, a shift register is a series of flip-flops based on sequential clock timing. Flip-flops facilitate moving data from input to output using sequential logic. A clock, in the form of a repeating wave in a square pattern, is used to synchronize how data travels through shift registers, creating a short delay in the transmission of a digital signal. Most often, variable length shift registers are used to convert parallel data to serial, but they can also be used for data flow in microprocessors or to convert analog data to digital and vice versa.
Shift registers are high speed circuits. First, a shift register shifts bits of data left or right along a circuit, depending on the specific structure of the circuit. In its simplest form, a shift register receives data in the first stage and shifts bits one stage left or right when the clock signals the need for data advance. Registers are identified by the number of temporary memory slots available after each phase between input and output. Temporary storage slots allow a shift register to delay data signals until clock signals advance appropriate data. An 8-bit register, for example, has eight stages and thus eight temporary storage slots for the bits in a data string.
Structurally, there are five basic types of shift registers. Serial-in/serial-out and universal parallel-in/parallel-out shift registers make it easy to input and output data in serial or parallel form, respectively, without the need for conversions. Parallel-in/serial-out refers to shift registers that process parallel data transmission inputs and convert those transmissions to output in serial form. Serial-in/parallel-out shift registers are nearly identical to parallel-in/serial-out, except that serial-form input data is converted to parallel-form output.
A ring counter is a type of shift register structure that has recirculating or repeating data patterns. When a shift register completes processing of a data string and returns the last stage to the initial data entry stage, a circular model results. Ring counters are used when a specific function is required on a set repeating pattern. For example, an LED display set to repeat chosen displays might use a ring counter structure for the shift register so that the output repeats at a predetermined clock pulse.
During the early days of computers, a shift register with several hundred stages was used for computer memory. The use of shift registers replaced mercury delay lines, speeding up data processing and allowing for smaller computer components and peripherals. Today, shift registers as primary computer storage are considered antiquated. Printed circuit boards, however, still have shift registers to reduce the amount of wiring required, especially in display drivers, digital-to-analog converters, and serial data memory.
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