How to expand the number of I/O pins using shift registers or I/O expanders?

#iopins #shiftregisters #74hc595 #mcp23017

This is a very common challenge when working with microcontrollers or FPGAs, because the number of available I/O pins often isn’t enough for all sensors, buttons, or displays you want to connect. The two classic solutions are shift registers and I/O expanders. Let’s go step by step:

1. Expanding I/O with Shift Registers
How It Works

A shift register takes serial data from the MCU and “shifts” it out bit by bit to multiple outputs.

You only need 3 MCU pins:

Data (SER)
Clock (SRCLK)
Latch (RCLK)

Each clock pulse moves one bit into the register, and the latch updates the outputs.

Common Chips

74HC595 (8-bit serial-in, parallel-out): For expanding output pins.
74HC165 (8-bit parallel-in, serial-out): For expanding input pins.

Example (74HC595 for Output Expansion)

Chain multiple 74HC595s together → 8, 16, 24, … outputs.
Send 16 bits of serial data, and 2 cascaded chips will output 16 parallel signals.
Useful for LED driving, digit displays, relays, etc.

Pros:

Very cheap, widely available.
Can cascade many for lots of outputs.

Cons:

One-direction only (output OR input, not both at the same time).
Requires fast MCU to keep updating outputs.

2. Expanding I/O with I/O Expanders
How It Works

An I/O expander is an IC that communicates over a serial bus (I²C or SPI) with the MCU and provides extra GPIOs that can be configured as input or output.

Common Chips

MCP23017 (I²C, 16 I/O pins, up to 8 devices = 128 pins).
PCF8574 (I²C, 8 I/O pins, very popular for LCD modules).
MCP23S17 (SPI version of MCP23017, faster).

Example (MCP23017 on I²C Bus)

Connect SCL and SDA to the MCU (just 2 pins).
Each chip provides 16 extra pins.
Address pins allow up to 8 MCP23017 devices on one bus = 128 pins total.
Each pin can be input, output, or even have pull-ups.

Pros:

Bi-directional I/Os.
Easy to configure (libraries available for Arduino, STM32, etc.).
Less CPU overhead compared to manually clocking shift registers.

Cons:

Slightly more expensive.
Slower than direct MCU I/O (because of I²C/SPI communication delays).

3. Comparison Table

4. When to Use Which

Shift registers (74HC595/165): Best when you need lots of simple, fast outputs/inputs (e.g., LED matrices, 7-seg displays, button panels).
I/O expanders: Best when you need true GPIO flexibility (input + output, pull-ups, interrupts), and want to save MCU pins with a simple I²C/SPI interface.

In short: