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MCP23017 gpio i2c extension

Connect to RPi

Pinout

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Wiring

MCP23017 RPi note
VCC 1 3.3v
GND 6
SDA 3
SCL 5
RESET 1 3.3v

i2c

install
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sudo apt install i2c-tools

detect

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sudo i2cdetect -y 1
     0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f
00:                         -- -- -- -- -- -- -- -- 
10: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
20: 20 -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
30: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
40: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
50: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- 
60: -- -- -- -- -- -- -- -- -- -- -- -- -- -- -- --

Demo

Connect LED to port PA0

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# set all portA pin as output
sudo i2cset -y 1 0x20 0x00 0x00 

# set gister 0x0c
sudo i2cset -y 1 0x20 0x14 0x01
sudo i2cset -y 1 0x20 0x14 0x00

Registers

The MCP23017 supports two register addressing modes:

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Bank 0 (Sequential Addressing - Default Mode)
Bank 1 (Split Addressing Mode)

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demo

The demo above use register

  • register 0x00(IODIRA) as set portA all pin to output
  • register 0x14(OLATA) set as 0x00/0x01 to set port PA0 low/high

Datasheet

3.5.1 I/O DIRECTION REGISTER

Controls the direction of the data I/O. When a bit is set, the corresponding pin becomes an input. When a bit is clear, the corresponding pin becomes an output.

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3.5.11 OUTPUT LATCH REGISTER (OLAT)

The OLAT register provides access to the output latches. A read from this register results in a read of the OLAT and not the port itself. A write to this register modifies the output latches that modifies the pins configured as outputs.

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register address

The data sheet use bank1 address mode The address is 0x14 for OLATA register in bank0 mode

Permission

Check file owner and add you'r user to file group

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sudo usermod -aG i2c $USER

CPP Demo

Toggle PA0 on/off

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#include <iostream>
#include <fcntl.h>
#include <unistd.h>
#include <linux/i2c-dev.h>
#include <sys/ioctl.h>
#include <chrono>
#include <thread>
#include <cstdint>  // For fixed-width integers

int main() {
    int file;
    const char *i2cBus = "/dev/i2c-1"; // I2C bus
    const int deviceAddress = 0x20;    // MCP23017 I2C address

    // Open the I2C bus
    file = open(i2cBus, O_RDWR);
    if (file < 0) {
        std::cerr << "Failed to open I2C bus\n";
        return -1;
    }

    // Connect to the MCP23017 device
    if (ioctl(file, I2C_SLAVE, deviceAddress) < 0) {
        std::cerr << "Failed to connect to I2C device\n";
        close(file);
        return -1;
    }

    // Set IODIRA register (0x00) to 0x00 (all output)
    uint8_t config[2] = {0x00, 0x00};
    if (write(file, config, 2) != 2) {
        std::cerr << "Failed to write to I2C device\n";
    }

    // Toggle PA0 port using GPIOA (0x14)
    for (int i=0; i<10; i++)
    {
        uint8_t state = i%2;
        uint8_t output[2] = {0x14, state};
        if (write(file, output, 2) != 2) {
            std::cerr << "Failed to write GPIOA\n";
        }
        std::this_thread::sleep_for(std::chrono::seconds(1));
    }

    // // Read GPIOA (0x12)
    // uint8_t reg = 0x12;
    // if (write(file, &reg, 1) != 1) {
    //     std::cerr << "Failed to set register address\n";
    // }

    // uint8_t data;
    // if (read(file, &data, 1) != 1) {
    //     std::cerr << "Failed to read from I2C device\n";
    // } else {
    //     std::cout << "GPIOA Data: 0x" << std::hex << (int)data << std::dec << std::endl;
    // }

    close(file);
    return 0;
}

Python Demo

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pip3 install smbus2

Toggle PA0 on/off

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from smbus2 import SMBus
import time

I2C_BUS = 1         # Raspberry Pi uses I2C bus 1
I2C_ADDR = 0x20     # MCP23017 I2C address

# MCP23017 Register Addresses
IODIRA = 0x00       # I/O direction register for GPIOA
GPIOA = 0x12        # Register to read/write GPIOA
OLATA = 0x14        # Output latch register for GPIOA

with SMBus(I2C_BUS) as bus:
    # Set all GPIOA pins as output (0x00 means all pins are outputs)
    bus.write_byte_data(I2C_ADDR, IODIRA, 0x00)

    while True:
        # Turn PA0 ON (set bit 0 to 1)
        bus.write_byte_data(I2C_ADDR, OLATA, 0x01)
        print("PA0 ON")
        time.sleep(1)

        # Turn PA0 OFF (set bit 0 to 0)
        bus.write_byte_data(I2C_ADDR, OLATA, 0x00)
        print("PA0 OFF")
        time.sleep(1)

Reference