Table of Contents

Key Matrix

An M×N key matrix driver.

(M is number of output pins and N is number of input pins.)


These key matrices look like this:


This is a 4×4 matrix. And here is the schematic

You can connect any M×N key matrix, theoretically, by using M+N GPIO pins.

You can also use any compatible GPIO controller like Mcp23xxx instead of native controller.


You need to create 2 lists of int, one for the input and one for the output.

int[] outputs = new int[] { 26, 19, 13, 6 };
int[] inputs = new int[] { 21, 20, 16, 12 };
KeyMatrix mk = new KeyMatrix(outputs, inputs, TimeSpan.FromMilliseconds(20));

You can use as well any GpioController like the MCP23017 in the following example

var settings = new System.Device.I2c.I2cConnectionSettings(1, 0x20);
var i2cDevice = System.Device.I2c.I2cDevice.Create(settings);
var mcp23017 = new Iot.Device.Mcp23xxx.Mcp23017(i2cDevice);
GpioController gpio = new GpioController(PinNumberingScheme.Logical, mcp23017);
int[] outputs = new int[] { 26, 19, 13, 6 };
int[] inputs = new int[] { 21, 20, 16, 12 };
KeyMatrix mk = new KeyMatrix(outputs, inputs, TimeSpan.FromMilliseconds(20), gpio, true);

Read on Key

To read a key, just the ReadKey function:

KeyMatrixEvent? key = mk.ReadKey();

KeyMatrixEvent contains the event that happened. Please note that ReadKey is blocked up to the moment an event is detected.

Event based approach

KeyMatrix supports events. Just subscribe to the event and have a function to handle the events:

Debug.WriteLine("This will now start listening to events and display them for 60 seconds.");
mk.KeyEvent += KeyMatrixEventReceived;


void KeyMatrixEventReceived(object sender, KeyMatrixEvent keyMatrixEvent)
    // Do something here, you have an event!

Tips and tricks

  • Using diodes(eg. 1N4148) for each button prevents "ghosting" or "masking" problem.
  • Input pins need pull-down resistors connect to ground if your MCU doesn't have it. So you need to have a pull-down on a the MCU for example.
  • If your key matrix doesn't work well, try to swap output and input pins. Some includes diodes and if they are used the reverse way won't work properly.

Key Matrix Samples

This shows how to connect the matrix.

Important: Please make you don't forget to place a pull down on the input matrix.

Connection using on-board GPIO

Connection using MCU

Connection using MCP23017

Connection using a MCP23017