Microchip MCP23S08T-E/ML 8-Bit SPI I/O Expander: Features and Application Design Guide

In modern embedded systems, microcontroller unit (MCU) pin count is often a limiting factor in design. The MCP23S08T-E/ML from Microchip Technology provides an elegant solution, offering 8 additional general-purpose I/O pins using a industry-standard high-speed SPI interface, making it an ideal choice for space-constrained and cost-sensitive applications.

This device is a member of Microchip's extensive I/O expander family. Housed in a compact 16-pin QFN (5x5 mm) ML package, it is designed for robust performance in industrial and consumer environments. Its key features are centered on simplicity, flexibility, and integration.

Key Features and Capabilities

SPI Interface: The MCP23S08 communicates via a simple and fast SPI serial interface (up to 10 MHz), requiring only four MCU pins (SCK, MOSI, MISO, CS) to control all eight I/O ports. This significantly reduces the wiring complexity and pin usage on the host microcontroller.

Hardware Address Pins: The device features three address pins (A0, A1, A2), allowing up to eight MCP23S08 devices to be connected on a single SPI bus, expanding a single SPI port to 64 I/O pins. This daisy-chaining capability is crucial for large-scale systems.

Configurable I/O Ports: Each of the 8 I/O pins is individually configurable as an input or output through the IODIR register. Input pins can be configured with internal 100 kΩ pull-up resistors (enabled via the GPPU register), eliminating the need for external components in switch interfacing.

Interrupt Output: The INTA pin provides an interrupt signal to the host MCU upon a change of state on any input pin, eliminating the need for the MCU to continuously poll the device. This greatly improves system efficiency by allowing the MCU to remain in a low-power sleep mode until an event occurs.

Industrial Temperature Range: The MCP23S08T-E/ML is specified for an operating temperature range of -40°C to +125°C, making it suitable for harsh industrial and automotive applications.

Application Design Guide

Integrating the MCP23S08 into a system is straightforward. Here are the key design considerations:

1. SPI Bus Configuration: Connect the SCK, MOSI, and MISO lines from the MCU's SPI peripheral to the corresponding pins on the MCP23S08. A dedicated Chip Select (CS) pin from the MCU must be used for each MCP23S08 on the bus. Ensure the SPI mode is set to mode 0 (CPOL=0, CPHA=0) or mode 1 (CPOL=0, CPHA=1) as per the datasheet.

2. Address Configuration: Set the hardware address of each device using the state of the A0, A1, and A2 pins (tied to VDD or VSS). Each device on the bus must have a unique address. The first byte of every SPI command is the device opcode, which includes this 3-bit address.

3. Input Design (e.g., for Button Press):

Configure the relevant pin as an input (set bit in IODIR register).

Enable the internal pull-up resistor for that pin (set bit in GPPU register).

Connect a push button from the pin to ground. When the button is open, the pin is read as high. When pressed, it is pulled low.

Enable the interrupt-on-change feature for that pin. The INTA pin will go active low when the button is pressed, alerting the MCU to read the input register and determine which button changed state.

4. Output Design (e.g., for Driving an LED):

Configure the pin as an output (clear bit in IODIR register).

Simply write a '1' or '0' to the output latch (OLAT or GPIO register) to turn an connected LED on or off. A current-limiting resistor is required in series with the LED.

5. Power Supply Decoupling: A 0.1 μF ceramic decoupling capacitor should be placed as close as possible to the VDD pin of the MCP23S08 to ensure stable operation and mitigate noise on the power supply line.

ICGOOODFIND

The Microchip MCP23S08T-E/ML is an exceptionally versatile and robust SPI I/O expander. Its combination of a simple interface, hardware addressing, built-in pull-ups, and interrupt functionality makes it a superior choice for efficiently extending the I/O capabilities of virtually any microcontroller. It solves the common problem of pin-limited MCUs, enabling more complex and feature-rich designs without migrating to a larger, more expensive host processor.

Keywords: SPI Interface, I/O Expander, Interrupt Output, Hardware Addressing, Industrial Temperature