Microchip MCP23S08-E/SS 8-Bit SPI I/O Expander: Features, Application Circuit, and Design Considerations

In modern embedded systems, microcontroller I/O pin limitations are a common challenge. The MCP23S08-E/SS from Microchip Technology provides an elegant solution as an 8-bit SPI I/O expander, enabling designers to add GPIOs using just a few MCU pins. This serial interface I/O expander is ideal for space-constrained and cost-sensitive applications requiring additional input/output ports.

Key Features

The MCP23S08-E/SS integrates an SPI serial interface (supporting up to 10 MHz clock speeds) and eight configurable I/O pins. Each pin can be independently set as input or output through the I/O direction register. The device includes software programmable pull-up resistors (approx. 100 kΩ) on each pin, eliminating the need for external resistors when connecting switches or buttons. It also features high sink/source current capability (25 mA per I/O) for driving LEDs directly. With an industrial temperature range (-40°C to +85°C) and 2.7V to 5.5V operating voltage, it suits diverse environments. The hardware address pins allow multiple devices to share the same SPI bus.

Application Circuit

A typical application circuit involves connecting the MCP23S08-E/SS to a host microcontroller via SPI. The four essential SPI signals—SCK (Serial Clock), SI (Serial Data In), SO (Serial Data Out), and CS (Chip Select)—are connected to the MCU’s SPI peripheral. The CS pin is driven by a GPIO to select the device. The I/O pins (GP0 to GP7) can be connected to sensors, switches, LEDs, or relays. For input applications, such as reading a DIP switch, internal pull-ups are enabled via software. For output, such as driving LEDs, current-limiting resistors are added externally. Power supply decoupling is critical: a 0.1 µF ceramic capacitor must be placed close to the VDD and VSS pins.

Design Considerations

1. SPI Bus Configuration: The MCP23S08 operates in SPI Mode 0,0 (CPOL=0, CPHA=0) or Mode 1,1 (CPOL=1, CPHA=1). Ensure the MCU’s SPI settings match.

2. Addressing Multiple Devices: Up to eight MCP23S08 devices can share the SPI bus using the A0, A1, and A2 address pins. Each device must have a unique address.

3. Noise Immunity: In electrically noisy environments, use series resistors (22-100Ω) on SPI lines and minimize trace lengths to reduce ringing and crosstalk.

4. Power Sequencing: Avoid I/O pin voltage exceeding VDD +0.6V to prevent latch-up. Ensure VDD stabilizes before applying signals to I/O pins.

5. Software Implementation: Accessing the device requires writing to configuration registers. Proper initialization (setting IODIR for direction, GPPU for pull-ups) is essential before reading or writing GPIOs.

ICGOOODFIND

The MCP23S08-E/SS is a highly versatile and efficient solution for expanding I/O capabilities via SPI. Its built-in features like programmable pull-ups and high drive current simplify design while reducing component count. Careful attention to SPI communication, hardware addressing, and noise mitigation ensures reliable performance in industrial and consumer applications.

Keywords:

SPI Interface, I/O Expansion, Programmable Pull-up, GPIO Expander, Hardware Addressing