Microchip MCP23S08-E/SO 8-Bit SPI I/O Expander: Features and Application Design Guide

In modern embedded system design, the scarcity of GPIO (General-Purpose Input/Output) pins on microcontrollers often presents a significant constraint. The MCP23S08-E/SO, an 8-bit I/O expander from Microchip Technology, provides an elegant solution by adding eight configurable I/O ports via a high-speed SPI (Serial Peripheral Interface) bus. This device is instrumental in extending system capabilities while minimizing pin count on the host controller.

Key Features of the MCP23S08-E/SO

The MCP23S08-E/SO stands out due to its robust integration of features tailored for flexible I/O expansion. It operates over a wide voltage range (1.8V to 5.5V), making it compatible with various logic levels and suitable for both 3.3V and 5V systems. The device supports high-speed SPI communication at 10 MHz, enabling rapid data transfer between the host microcontroller and the I/O expander.

Each of the eight I/O pins is individually configurable as either an input or an output. When set as an input, the pin can be enabled with a software-configurable internal pull-up resistor, eliminating the need for external components in many applications. For output configurations, each pin can source or sink up to 25 mA of current, allowing it to drive LEDs or small relays directly.

A critical feature for enhanced system control is the hardware-configurable address pins, permitting up to eight MCP23S08 devices to share the same SPI bus. This allows for a theoretical expansion of up to 64 additional I/O lines from a single SPI port, providing immense scalability. Furthermore, the device includes an interrupt output pin that can be configured to signal the host controller upon changes on any input pin, reducing the need for constant polling and improving system efficiency.

Application Design Guide

Integrating the MCP23S08 into a new or existing design is straightforward. The primary connection involves the four SPI signals: Serial Data Out (SDO), Serial Data In (SDI), Serial Clock (SCK), and Chip Select (CS). The CS pin, in particular, is crucial for selecting the device on the bus, especially when multiple expanders are used.

Power supply decoupling is essential for stable operation. A 0.1µF ceramic capacitor should be placed as close as possible to the VDD and VSS pins of the IC to filter high-frequency noise. For applications with long SPI traces or in electrically noisy environments, series resistors on the clock and data lines can help dampen signal ringing and improve integrity.

The device's interrupt functionality is a powerful tool for designing responsive systems. The INT pin can be connected to an external interrupt pin on the host microcontroller. When an input pin on the MCP23S08 changes state, the INT pin is asserted. The host can then quickly query the expander's interrupt capture register to determine which specific input caused the event, facilitating immediate and efficient processing.

Software implementation involves initializing the SPI peripheral on the host microcontroller and then configuring the MCP23S08's control registers. Key registers include the I/O Direction register (IODIR) to set pins as inputs or outputs, the GPIO register to read or write pin states, and the GPPU register to enable internal pull-up resistors. The interrupt control registers (GPINTEN, INTCON, DEFVAL) provide fine-grained control over the interrupt generation logic.

ICGOODFIND Summary

The MCP23S08-E/SO is a highly versatile and efficient solution for GPIO expansion. Its SPI interface, configurable interrupts, and high drive capability make it an indispensable component for designers looking to maximize functionality without upgrading to a larger microcontroller. Its ability to operate in diverse voltage environments and support daisy-chaining simplifies system architecture and reduces overall design complexity.

Keywords

SPI Interface, GPIO Expander, Interrupt Function, I/O Port, Hardware Address