A GPIO Expander is a device that provides a designer the ability to implement additional inputs and outputs (I/O) on a microprocessor (MPU) or microcontroller (MCU) system. They have an efficient data bus interface to reduce the I/O requirements of the MPU or MCU. This is typically a synchronous serial interface like SPI or I²C. They enable a designer to incorporate more I/O into a system than is available on the processor device they are using. They also enable I/O to be physically located on a system in a different location from the processor, facilitating PCB layout and improving signal integrity.

 

Serial port interfaces are usually selected because these have few data lines. SPI has four data lines, and I²C has two. As the I/O Expander typically has more than eight I/O ports, there is a net I/O gain on the microprocessor alone. The addition of further I/O Expanders can usually utilize the same bus, enabling a system to increase the I/O gained by using them. Expanders are controlled by a software driver writing and reading from registers internal to the device with the serial data stream. In many cases, the Expander can generate configurable interrupt signals to indicate to the processor when a signal change event has occurred.

 

Another benefit of a GPIO Expander is that they usually have higher drive capability than the I/O on an MPU or MCU. This can enable them to drive higher load devices like LEDs. The inputs of such devices can also have higher levels of electrostatic discharge (ESD) protection than processors, and may support much higher voltage tolerance. Some I/O Expanders integrate key debounce circuitry to remove glitches from keyboards, and may also incorporate a keyboard scanning algorithm for evaluating key presses on keypad arrays. The more special purpose I/O Expanders are available with serial interfaces that fit the application. For example, automotive I/O Expanders tend to have serial CAN bus interfaces. Parallel GPIO devices exist and are designed to sit on a parallel address and data buses as memory mapped peripherals. These usually have high I/O expansion counts of up to 24 or more ports. These are used in MPU systems when higher speed I/O access may be is required, or a serial port is not available.