Mbed extension board alternative keypad reading.

keys.cpp

#include "mbed.h"

Serial pc(USBTX, USBRX); // tx, rx

BusOut cols_out(p26, p25, p24); 

// Checking extension board with multimeter shows that p14,p13,p12,p11 are connected 
// to row indicating pins of the keypad. The extension board schematic is incorrect.
BusIn rows_in(p14, p13, p12, p11);

char Keytable[][4] = {
      {'1', '2', '3', 'F'},
      {'4', '5', '6', 'E'},
      {'7', '8', '9', 'D'},
      {'A', '0', 'B', 'C'}
    };
                  
char getKey() {
    int i,j;
    for (i = 0; i <= 3; i++) {
        cols_out = i;
        
        // for each bit in rows
        for (j = 0; j <= 3; j++) {
            
            // if j'th bit of "rows_in" is LOW
            if (~rows_in & (1 << j)) {
                return Keytable[j][3-i];
            }
        }
    }
    return ' ';
}

int main() {
    char key = ' ';
    while(1) {
        key = getKey();
        if(key != ' ') {
            pc.printf("Key = %c\r\n", key);
            wait(0.3);
        }
    }
}