MCP23017 16-bit I2C I/O Expander Module CJMCU-2317

This tutorial shows how to set up the MCP23017 CJMCU-2317 expander module using an Arduino Uno board, to control a LED and read the state of a button through the 16 additional pins added on the I2C bus. The module is suitable when your project needs more digital inputs or outputs than the pins available on the base board, for example for panels with multiple LEDs, buttons or relays.

How it works:

MCP23017 communicates with Arduino through internal registers, which the library writes and reads via I2C. Two direction registers, IODIRA and IODIRB, determine whether each of the 8 pins of a port is an input or output, while two GPIOA and GPIOB registers read or write the physical state of the pins. The I2C address of the module is formed from 4 fixed bits followed by the state of pins A2, A1 and A0, which gives an address range between 0x20, when all three are at GND, and 0x27, when all three are at VCC. In this tutorial A0, A1 and A2 remain at GND, so the module responds to address 0x20. The Adafruit_MCP23X17 library hides all these registers and exposes simple functions such as pinMode, digitalWrite and digitalRead for each of the 16 pins, numbered from 0 (GPA0) to 15 (GPB7).

What you need:

• 1 x MCP23017 I2C 16-Bit IO Expander Module
• 1 x Arduino Uno R3 development board with Wifi ESP8266, Arduino compatible
• 1 x Breadboard 830 contacts MB-102
• 1 x LED 5mm, White
• 1 x 220 ohm resistor
• 1 x 4-pin push button
• 1 x 40 Dupont wires 10cm, Male-Female

Connection:

The module uses only 4 active wires to Arduino, plus the RESET pin and address pins, which are fixed to power or ground.

Board and libraries:

Select the Arduino Uno board in Arduino IDE, from Tools > Board > Arduino AVR Boards, a package included by default with Arduino IDE, without needing an additional address in File > Preferences. Install from Library Manager the Adafruit MCP23017 Arduino Library, by author Adafruit, which contains the file Adafruit_MCP23X17.h, and let the manager also install the required Adafruit BusIO dependency. Basic I2C communication uses the Wire library, included by default in Arduino IDE, which does not need to be installed separately.

Code:

The following program initializes the module at default address 0x20, configures GPA0 as an output for the LED and GPB0 as an input with pull-up for the button, then lights up the LED while the button is pressed.

/*
  MCP23017 16-Bit I2C IO Expander - Button and LED Example
  Board: Arduino Uno (Arduino AVR Boards)
  Library: Adafruit MCP23017 Arduino Library (Adafruit_MCP23X17.h) by Adafruit
  Library: Wire.h (included with Arduino IDE)
  Wiring:
  MCP23017 VCC    -> Arduino 5V
  MCP23017 GND    -> Arduino GND
  MCP23017 SCL    -> Arduino A5 (SCL)
  MCP23017 SDA    -> Arduino A4 (SDA)
  MCP23017 RESET  -> Arduino 5V
  MCP23017 A0,A1,A2 -> GND (I2C address 0x20)
  MCP23017 GPA0   -> 220 ohm resistor -> LED anode, LED cathode -> GND
  MCP23017 GPB0   -> push button -> GND (other button terminal)
*/
#include <Adafruit_MCP23X17.h>
Adafruit_MCP23X17 mcp;
const uint8_t LED_PIN = 0;     // GPA0
const uint8_t BUTTON_PIN = 8;  // GPB0
void setup() {
  Serial.begin(9600);
  // Initialize the MCP23017 at I2C address 0x20
  if (!mcp.begin_I2C(0x20)) {
    Serial.println("MCP23017 not found, check wiring and RESET pin");
    while (1);
  }
  // Configure GPA0 as output for the LED
  mcp.pinMode(LED_PIN, OUTPUT);
  // Configure GPB0 as input with internal pull-up for the button
  mcp.pinMode(BUTTON_PIN, INPUT_PULLUP);
  Serial.println("MCP23017 initialized at address 0x20");
}
void loop() {
  // Read the button state, LOW means pressed (pulled to GND)
  int buttonState = mcp.digitalRead(BUTTON_PIN);
  if (buttonState == LOW) {
    mcp.digitalWrite(LED_PIN, HIGH);  // turn LED on
  } else {
    mcp.digitalWrite(LED_PIN, LOW);   // turn LED off
  }
  Serial.print("Button state: ");
  Serial.println(buttonState == LOW ? "pressed" : "released");
  delay(100);
}

How the code works:

At the beginning of the program, the Adafruit_MCP23X17.h library is included and the mcp object is created, through which all commands to the module will be made. In the setup function the serial port is initialized at 9600 baud, then mcp.begin_I2C is called with address 0x20 to establish I2C communication, and if the module does not respond the program displays an error message and stops in a while(1) loop. Also in setup, the LED_PIN pin (GPA0) is configured as OUTPUT and the BUTTON_PIN pin (GPB0) as INPUT_PULLUP, using the internal pull-up of MCP23017. In the loop function, the button state is read with mcp.digitalRead(BUTTON_PIN), and if the value is LOW, meaning the button is pressed, HIGH is written to LED_PIN to light up the LED, otherwise LOW is written. The button state is also sent to the serial monitor, followed by a short pause given by delay.

Result:

Open the serial monitor at 9600 baud, the LED connected on GPA0 should light up when you hold down the button on GPB0 and turn off when you release it, and the read button state appears on the monitor at the same time.

MCP23017 initialized at address 0x20
Button state: released
Button state: released
Button state: pressed
Button state: pressed
Button state: pressed
Button state: released
Button state: released

Common issues:

The message MCP23017 not found on the serial monitor usually appears when the RESET pin is left unconnected, because the module remains blocked in reset, the solution is to connect RESET directly to VCC or through a 10 kOhm resistor.

The module does not respond to the address used in the code means that the address set in mcp.begin_I2C does not match the state of pins A0, A1 and A2, check the connections of these pins and run an I2C scanner to confirm the actual address.

The button seems pressed all the time or button press is never detected means that the BUTTON_PIN pin was not configured as INPUT_PULLUP, but as simple INPUT, leaving the input floating.

The LED remains off even though the button is pressed may mean that the LED is mounted backwards or the 220 ohm resistor is missing or broken, check the polarity of the LED and the continuity of the resistor.

If you use multiple modules on the same bus and none respond correctly, make sure you do not have two modules with the same combination on A0, A1 and A2, each module needs a unique address.

Unstable communication or intermittent errors on wires longer than a few centimeters is resolved by adding external 4.7 kOhm pull-up resistors on SDA and SCL, in addition to the internal network on the module.

Limitations and tips:

MCP23017 does not offer PWM outputs or analog inputs, being suitable only for digital on-off signals, not for reading variable voltages or for direct brightness control. The maximum current of 25 mA per pin and 125 mA total on the package limits the load connected directly to the GPIO pins, for relays, motors or power LEDs a separate transistor or relay module is needed. Up to 8 modules can operate simultaneously on the same I2C bus, offering together 128 additional pins, useful for panels with many buttons, LEDs or digital sensors, and the INTA and INTB outputs allow handling changes through interrupts, without continuous bus polling.