File content as of revision 0:cfa060762ad7:

#include "mbed.h"

PwmOut buzzer(p21);
PwmOut speaker(p26);

Serial pc(USBTX, USBRX); 
DigitalOut cols[4]= {p20,p19,p18,p17};
InterruptIn rows[4]={p25,p22,p23,p24};

double HZc[] = { 1209.0,   1336.0, 1477.0, 1633.0}; // Colum
double HZr[] = {697.0, 770.0, 852.0, 941.0}; //Row
char Keytable[] = { '1', '2', '3', 'A',
                     '4', '5', '6', 'B',
                     '7', '8', '9', 'C',
                     '*', '0', '#', 'D'
                   };
 
void start(int row){
    double high = 0, low = 0;
    if(rows[row].read() == 0) return;
    for(int i = 0; i < 4; ++i){
        cols[i].write(0);
        if(rows[row].read() == 0){
            high = HZc[i];
            low = HZr[row];
            pc.printf("input row %d col %d %c \n", row, i, Keytable[row * 4 + i]);
            break;
        }
    }
    buzzer.period(low);
    speaker.period(high);
    buzzer = 0.5;
    speaker = 0.5;
}    

void stop(){
    buzzer.period(0);
    speaker.period(0);
    buzzer = 0;
    speaker = 0;
    for(int i = 0; i < 4; ++i) cols[i] = 1;
}



void row0Rise(){
    start(0);
}
void row1Rise(){
    start(1);
}
void row2Rise(){
    start(2);
}
void row3Rise(){
    start(3);
}

void rowFall(){
    stop();
}

int main() {
    rows[0].rise(&row0Rise);
    rows[1].rise(&row1Rise);
    rows[2].rise(&row2Rise);
    rows[3].rise(&row3Rise);
    for(int i = 0; i < 4; ++i) rows[i].fall(&rowFall);
    while(1) {
        ;
    }
}