Daisy chaining and bus sharing with shift registers

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.

Shift registers circuit on breadboard

I/O Interfacing with current source and sink pins

Current source and current sink are two commonly used terms that you will find in datasheets of microcontrollers and digital circuits. Even though most input pins of an IC nowadays are internally connected to the gate of MOSFET transistors which are voltage driven, there are situations when you have to take into account current capabilities (i.e. high speed switching or something else than a CMOS input is connected to an output pin). I'm thinking about LEDs and bipolar transistor driven devices, such as relay modules.

These inputs are current driven and need to source or sink currents to change state. Fortunately, most output pins of contemporary microcontrollers are push-pull type and can source and sink currents. This means you don't have to worry too much about the way you are connecting an LED to a digital output. But there are still in use circuits which can only sink or source currents. And if you're not paying attention to datasheet, you might very well fail to design a working circuit.