How to Use MCP23S17: Examples, Pinouts, and Specs

The MCP23S17 is a 16-bit I/O expander that communicates via the SPI (Serial Peripheral Interface) protocol. It is designed to provide additional GPIO (General Purpose Input/Output) pins for microcontroller-based applications. This component is highly versatile and can be configured for both input and output operations, making it ideal for projects requiring more I/O pins than what is available on the microcontroller.

• Expanding GPIO pins in microcontroller projects
• Driving LEDs, relays, or other output devices
• Reading multiple switches or sensors
• Home automation systems
• Industrial control systems
• Robotics and embedded systems

The MCP23S17 offers a range of features and specifications that make it a powerful and flexible I/O expander.

• Interface: SPI (up to 10 MHz)
• Operating Voltage: 1.8V to 5.5V
• I/O Pins: 16 (organized as two 8-bit ports: PORTA and PORTB)
• Maximum Sink Current per Pin: 25 mA
• Maximum Source Current per Pin: 25 mA
• Interrupt Capability: Configurable interrupt-on-change for each pin
• Package Options: PDIP, SOIC, SSOP, QFN
• Operating Temperature: -40°C to +125°C
• Addressing: Supports up to 8 devices on the same SPI bus (via hardware address pins)

The MCP23S17 has 28 pins, with the following key pin assignments:

The MCP23S17 is straightforward to use in a circuit. Below are the steps and considerations for integrating it into your project.

1. Power and Ground: Connect the VDD pin to your power supply (1.8V to 5.5V) and the VSS pin to ground.
2. SPI Connections:
   • Connect the SCK pin to the SPI clock pin of your microcontroller.
   • Connect the SI pin to the SPI MOSI (Master Out Slave In) pin.
   • Connect the SO pin to the SPI MISO (Master In Slave Out) pin.
   • Connect the CS pin to a GPIO pin on your microcontroller to act as the chip select.
3. Address Configuration: Use the A0, A1, and A2 pins to set the hardware address of the device. This allows up to 8 MCP23S17 devices to share the same SPI bus.
4. GPIO Configuration:
   • Configure the I/O pins as input or output by writing to the IODIRA and IODIRB registers.
   • Use the OLATA and OLATB registers to control output states.
   • Read input states from the GPIOA and GPIOB registers.
5. Interrupts (Optional): Connect the INTA and/or INTB pins to your microcontroller if you want to use interrupt-on-change functionality.

• Pull-up Resistors: Use pull-up resistors on input pins if required by your application.
• Decoupling Capacitor: Place a 0.1 µF decoupling capacitor close to the VDD pin to stabilize the power supply.
• SPI Speed: Ensure the SPI clock speed does not exceed 10 MHz.
• Reset Pin: Tie the RESET pin to VDD if not used, or connect it to a GPIO pin for manual reset control.

Below is an example of how to use the MCP23S17 with an Arduino UNO to toggle an LED connected to GPA0.

#include <SPI.h>

// MCP23S17 SPI settings
#define CS_PIN 10  // Chip Select pin for MCP23S17
#define OPCODE 0x40 // MCP23S17 opcode (A2, A1, A0 = 0)

// MCP23S17 register addresses
#define IODIRA 0x00 // I/O direction register for PORTA
#define OLATA  0x14 // Output latch register for PORTA

void setup() {
  pinMode(CS_PIN, OUTPUT); // Set CS pin as output
  digitalWrite(CS_PIN, HIGH); // Set CS pin high (inactive)

  SPI.begin(); // Initialize SPI
  SPI.setClockDivider(SPI_CLOCK_DIV4); // Set SPI clock speed (4 MHz for Arduino UNO)

  // Configure GPA0 as output
  writeRegister(IODIRA, 0xFE); // Set GPA0 as output, others as input
}

void loop() {
  // Toggle GPA0
  writeRegister(OLATA, 0x01); // Set GPA0 high
  delay(500); // Wait 500 ms
  writeRegister(OLATA, 0x00); // Set GPA0 low
  delay(500); // Wait 500 ms
}

// Function to write to MCP23S17 register
void writeRegister(byte reg, byte value) {
  digitalWrite(CS_PIN, LOW); // Select MCP23S17
  SPI.transfer(OPCODE); // Send opcode
  SPI.transfer(reg); // Send register address
  SPI.transfer(value); // Send data
  digitalWrite(CS_PIN, HIGH); // Deselect MCP23S17
}

1. No Response from MCP23S17:

   • Ensure the CS pin is correctly connected and toggled during SPI communication.
   • Verify the SPI clock speed does not exceed 10 MHz.
   • Check the hardware address pins (A0, A1, A2) for proper configuration.

2. Incorrect GPIO Behavior:

   • Double-check the IODIRA and IODIRB register configurations.
   • Ensure pull-up resistors are used for input pins if required.

3. Interrupts Not Working:

   • Verify that the INTA/INTB pins are connected to the microcontroller.
   • Ensure the interrupt-on-change feature is enabled in the configuration registers.

• Q: Can I use multiple MCP23S17 devices on the same SPI bus?
  A: Yes, up to 8 devices can be used by configuring the A0, A1, and A2 address pins.

• Q: What is the maximum current the MCP23S17 can handle?
  A: Each pin can source or sink up to 25 mA, with a total maximum current of 125 mA per port.

• Q: Do I need external pull-up resistors for the GPIO pins?
  A: Pull-up resistors are required for input pins if the connected device does not provide them.

This concludes the documentation for the MCP23S17.