A shift register is a digital circuit that is used to store and manipulate data in a sequential manner. It is composed of a series of flip-flops, which can be viewed as basic memory units able to store binary values. The outputs of each flip-flop are connected to the inputs of the next flip-flop in the sequence, such that the data is shifted from one flip-flop to the next with each clock pulse.
In this post I will use the 74HC595 serial-in-parallel-out shift register and its counterpart, 74HC165 parallel-in-serial-out shift register. Both are commonly employed when a microcontroller with limited available I/O pins has to control a large number of digital outputs or read a similar number of inputs (for example in home automation). What I want to show you in this post is how to daisy chain these ICs and how to make them share some control lines in order to keep the serial interface data lines to a minimum.
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Shift registers circuit on breadboard |