Today we will see how to interfacing the Raspberry Pi with a matrix keypad using Rpi-hw library.
The library provides the class keypad::matrix↱, defined in “rpi-hw/keypad/matrix.hpp”↱, with which it is possible to manage keypads of any size.
Its constructor method takes two ordered lists containing the GPIO pins used by the device:
- keypad::matrix(
- { COL0, COL1, COL2, … }, // Column pins
- { ROW0, ROW1, ROW2, … } // Rows pins
- )
Let’s take an example. The following image shows how to connect the Raspberry Pi with a standard 12-key keypad:
We used the following GPIO pins: 21, 10, and 4 for columns; 22, 14, 15, and 17 for rows. This is the internal circuit diagram of the keypad (where pin names refer to the keypad connector):
The instance of keypad::matrix will check periodically which buttons are pressed, looking for connection between rows and columns. The control algorithm can be summarized as follows:
- Set all GPIOs of rows as input.
- Set all GPIOs of columns as output (low).
- Set a column pin to high.
- If one of rows reads a high value, then the button between those column and row is pressed.
- Set the previous column to low.
- Repeat from line #3 with another column, until there are no more.
And finally, here is a sample code that shows you how to read the state of some buttons in the 12-key keypad:
- #include <iostream>
- // Include Rpi-hw headers
- #include <rpi-hw.hpp>
- #include <rpi-hw/time.hpp>
- #include <rpi-hw/keypad/matrix.hpp>
- // Use Rpi-hw namespace
- using namespace rpihw;
- int
- main( int argc, char *args[] ) {
- // Matrix keypad controller
- keypad::matrix dev( { 21, 10, 4 }, { 22, 14, 15, 17 } );
- // Main loop
- for ( ;; ) {
- // Check some keys state
- if ( dev.pressed(0) )
- std::cout << “You have pressed button 0!\n”;
- if ( dev.released(2) )
- std::cout << “You have released button 2!\n”;
- if ( dev.pressed(1) && dev.pressed(4) )
- std::cout << “You have pressed buttons 1 and 4!\n”;
- // Wait some time
- time::msleep( 100 );
- } return 0;}
To compile it (or any other programs using the library) use the command:
For more detail: Raspberry Pi with a keypad matrix
