Documentation

// MCP23017 - Arduino Example
// SDA->A4, SCL->A5, addr 0x20 | Library: "Adafruit MCP23017"

#include <Adafruit_MCP23X17.h>

Adafruit_MCP23X17 mcp;

void setup() {
  Serial.begin(115200);
  if (!mcp.begin_I2C(0x20)) {
    Serial.println("MCP23017 not found - check wiring/address");
    while (1);
  }
  mcp.pinMode(0, OUTPUT);          // GPA0 = LED
  mcp.pinMode(8, INPUT_PULLUP);    // GPB0 = button (pins 8-15 = port B)
  Serial.println("16 extra GPIO online at 0x20");
}

void loop() {
  mcp.digitalWrite(0, HIGH);
  delay(250);
  mcp.digitalWrite(0, LOW);
  delay(250);

  if (mcp.digitalRead(8) == LOW) {
    Serial.println("Button on GPB0 pressed!");
  }
}

// MCP23017 - ESP32 Example: chase across all 16 pins
// SDA->21, SCL->22 | Library: "Adafruit MCP23017"

#include <Adafruit_MCP23X17.h>

Adafruit_MCP23X17 mcp;

void setup() {
  Serial.begin(115200);
  Wire.begin(21, 22);
  if (!mcp.begin_I2C(0x20)) {
    Serial.println("MCP23017 not found");
    while (1) delay(10);
  }
  for (int p = 0; p < 16; p++) mcp.pinMode(p, OUTPUT);
}

void loop() {
  for (int p = 0; p < 16; p++) {       // GPA0..7 then GPB0..7
    mcp.digitalWrite(p, HIGH);
    delay(60);
    mcp.digitalWrite(p, LOW);
  }
}

#!/usr/bin/env python3
# MCP23017 - Raspberry Pi Example (raw registers via smbus2)
# SDA->GPIO2, SCL->GPIO3 | Install: pip3 install smbus2

from smbus2 import SMBus
import time

ADDR   = 0x20
IODIRA = 0x00   # 1 = input, 0 = output
IODIRB = 0x01
GPPUB  = 0x0D   # port B pull-ups
GPIOA  = 0x12
GPIOB  = 0x13

bus = SMBus(1)
bus.write_byte_data(ADDR, IODIRA, 0x00)   # port A all outputs
bus.write_byte_data(ADDR, IODIRB, 0xFF)   # port B all inputs
bus.write_byte_data(ADDR, GPPUB,  0xFF)   # pull-ups on port B

print("GPA0 blinking, watching GPB0...")
try:
    led = False
    while True:
        led = not led
        bus.write_byte_data(ADDR, GPIOA, 0x01 if led else 0x00)

        b = bus.read_byte_data(ADDR, GPIOB)
        if not (b & 0x01):                # pulled low = pressed
            print("Button on GPB0 pressed!")
        time.sleep(0.25)
except KeyboardInterrupt:
    bus.write_byte_data(ADDR, GPIOA, 0x00)
    print("Stopped by user")

# MCP23017 - Pico MicroPython Example (raw registers)
# SDA->GP4, SCL->GP5

from machine import I2C, Pin
import time

i2c = I2C(0, sda=Pin(4), scl=Pin(5), freq=400000)
print("I2C scan:", [hex(a) for a in i2c.scan()])   # expect 0x20

ADDR = 0x20
def reg_write(reg, val): i2c.writeto_mem(ADDR, reg, bytes([val]))
def reg_read(reg):       return i2c.readfrom_mem(ADDR, reg, 1)[0]

reg_write(0x00, 0x00)   # IODIRA: port A outputs
reg_write(0x01, 0xFF)   # IODIRB: port B inputs
reg_write(0x0D, 0xFF)   # GPPUB: pull-ups on B

print("GPA0 blinking, watching GPB0...")
led = False
while True:
    led = not led
    reg_write(0x12, 0x01 if led else 0x00)   # GPIOA

    if not (reg_read(0x13) & 0x01):          # GPIOB bit 0 low = pressed
        print("Button on GPB0 pressed!")
    time.sleep(0.25)