source: avrstuff/kbd/matrixtops2/main.c@ c0a8b2a

#include <usart.h>

#include <avr/io.h>
#include <avr/interrupt.h>
#include <avr/wdt.h>
#include <avr/pgmspace.h>
#include <util/delay.h>

#include <string.h>
#include <stdbool.h>

// Cnfigure the I/Os. We will use port C for columns (we put one of those to 0
// at a time) and port ?? for lines (we read this one and look for pressed keys
// in the column).
#define DDRCOL DDRC
#define PORTCOL PORTC
#define PINCOL PINC

#define PORTLINE PORTA
#define PINLINE PINA
#define DDRLINE DDRA

static uint8_t matrix[8];

int main() {
        // Enable the watchdog to reboot in case of crashes.
        wdt_enable(WDTO_2S);

        // By default, columns are configured as input (high impedance). We force
        // just one column at a time to the output low state. This way, even in
        // case of keyboard interferences, there is no short-circuit situation.
        PORTCOL = 0;
        DDRCOL = 0;

        // configure lines as inputs, with pull-up resistors.
        DDRLINE = 0;
        PORTLINE = 0xFF;

        // Let's configure the USART.
        USARTInit();

        // Let's rock!
        uint8_t column = 0;
        for(;;) {
                // We are alive
                wdt_reset();

                // Scan a column
                DDRCOL = 1 << column;

                // Let things settle after setting up the pin. We rely on internal
                // pull-ups to raise pins which are left unconnected, and this takes
                // some time.
                _delay_us(6);

                // - Read input (8 rows) - we invert is so that pressed key are "1"
                uint8_t read = ~PINLINE;

                // Check if there were any changes since previous scan
                if (read ^ matrix[column]) {
                        // There was a change in this colum!
                        // TODO scan each set bit and report press/release according to
                        // the keyboard matrix
                        USARTWriteChar('c');
                        USARTWriteHex(column);
                        USARTWriteChar(':');
                        USARTWriteHex(read);

                        // Update the matrix for next time so we don't re-report the same keys
                        matrix[column] = read;

                        if (read == 0) {
                                USARTWriteChar('\r');
                                USARTWriteChar('\n');
                        } else
                                USARTWriteChar(' ');
                }

                // Prepare next column
                column++;
                if (column > 7) {
                        column = 0;
                }
        }

        return 0;
}