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Serial-to-Parallel Shift Register

The term register can be used in a variety of specific applications, but in all cases it refers to a group of flip-flops operating as a coherent unit to hold data. This is different from a counter, which is a group of flip-flops operating to generate new data by tabulating it.

In this context, a counter can be viewed as a specialized kind of register, which counts events and thereby generates data, rather than just holding the data or changing the way it is handled. More commonly, however, counters are treated separately from registers. The two are then handled as separate concepts which work together in many applications, and which have some features in common.

The demonstration circuit below is known as a shift register because data is shifted through it, from flip-flop to flip-flop. If you apply one byte (8 bits) of data to the initial data input one bit at a time, and apply one clock pulse to the circuit after setting each bit of data, you will find the entire byte present at the flip-flop outputs in parallel format. Therefore, this circuit is known as a serial-in, parallel-out shift register. It is also known sometimes as a shift-in register, or as a serial-to-parallel shift register.

By standardized convention, the least significant bit (LSB) of the byte is shifted in first.


As you would no doubt expect, the counterpart to the shift register above is the parallel-in, serial-out shift register, somtimes called a shift-out register. That circuit is a bit more complex that the shift-in register shown above, but generally operates in a very similar fashion, as we'll see on the next page.

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