MCP23017 GPIO expander with ESP32

Specifications

Power and logic levels

3.3 V: safe to connect directly

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

Wiring

Arduino code

#include <Wire.h>

const int SDA_PIN = 8, SCL_PIN = 9;
const uint8_t ADDR = 0x20;

void writeReg(uint8_t reg, uint8_t value) {
  Wire.beginTransmission(ADDR);
  Wire.write(reg); Wire.write(value);
  Wire.endTransmission();
}

uint8_t readReg(uint8_t reg) {
  Wire.beginTransmission(ADDR);
  Wire.write(reg);
  Wire.endTransmission();
  Wire.requestFrom(ADDR, (uint8_t)1);
  return Wire.read();
}

void setup() {
  Serial.begin(115200);
  Wire.begin(SDA_PIN, SCL_PIN);
  writeReg(0x00, 0x00);               // port A: all outputs
  writeReg(0x01, 0x7F);               // port B: inputs, GPB7 output
  writeReg(0x0D, 0x7F);               // port B pull-ups
}

void loop() {
  bool pressed = !(readReg(0x13) & 0x01);   // button on GPB0 to GND
  writeReg(0x14, pressed ? 0x01 : 0x00);    // LED on GPA0
  delay(20);
}

No extra library needed. Compiled for ESP32-S3 with arduino-esp32 3.x. Not yet tested by us on a physical sensor. Change the pins to match your board.

On a display

Read the value in your sketch and pass it to a label, or send it over MQTT and show it with an MQTT value in the editor.

Boards with a ready connector

Only boards whose connectors we have surveyed. Others may still work on free GPIOs.

Good to know

• GPA7 and GPB7 are output only.
• Do not leave RESET or address pins floating.
• Too slow for PWM or LED strip data.
• Scan first: other chips may use the same address.

What it is good for

• Boards short of GPIO
• Banks of relays and buttons
• 4x4 keypads on two wires

FAQ

Sources

• https://cdn.shopify.com/s/files/1/0176/3274/files/MCP23017_Data_Sheet_DS20001952-2998473.pdf
• https://hackaday.com/2023/02/03/mcp23017-went-through-shortage-hell-lost-two-inputs
• https://github.com/mcauser/micropython-mcp23017/blob/master/README.md

Related parts

• 5 V relay module
• 4x4 matrix keypad