File content as of revision 1:2c75ba53ee40:

#include "mbed.h"

#if !defined(TARGET_LPC1549)
#error This program uses 4ch PWMs of the LPC1549 platfrom
#endif

DigitalIn  btn_0(D0);
DigitalIn  btn_1(D1);
DigitalIn  btn_2(D2);
DigitalIn  btn_3(D3);
DigitalIn  btn_4(D4);
DigitalIn  btn_5(D5);
DigitalIn  btn_6(D6);
DigitalIn  btn_7(D7);
DigitalIn  btn_8(D8);
DigitalIn  btn_9(D9);
DigitalIn  btn_10(D10);
DigitalIn  btn_11(D11);
DigitalIn  btn_12(D12);

PwmOut tone_0(A0);
PwmOut tone_1(A1);
PwmOut tone_2(A2);
PwmOut tone_3(A3);

const int tone_table_us[] = {
0,
3820, // C    1
3610, // C#   2
3410, // D    3
3210, // D#   4
3030, // E    5
2860, // F    6
2700, // F#   7
2550, // G    8
2410, // G#   9
2270, // A   10
2150, // A#  11
2020, // B   12
1910  // C   13
};

#define MAX_PWM_CHANNEL 4
#define NUM_KEYS        13

int ch[MAX_PWM_CHANNEL] = {0};
PwmOut *tone[MAX_PWM_CHANNEL];

int key_index[NUM_KEYS];
DigitalIn *key[NUM_KEYS];

int getFreeChannel()
{
    for (int i = 0; i <MAX_PWM_CHANNEL; i++) {
        if (ch[i] == 0) {
            ch[i] = 1;
            return i;
        }
    }
    return -1;
}

int main()
{
    memset(key_index, -1, (NUM_KEYS)*4);

    tone_0.period(0);
    tone_1.period(0);
    tone_2.period(0);
    tone_3.period(0);

    tone[0] = &tone_0;
    tone[1] = &tone_1;
    tone[2] = &tone_2;
    tone[3] = &tone_3;

    key[0] = &btn_0;
    key[1] = &btn_1;
    key[2] = &btn_2;
    key[3] = &btn_3;
    key[4] = &btn_4;
    key[5] = &btn_5;
    key[6] = &btn_6;
    key[7] = &btn_7;
    key[8] = &btn_8;
    key[9] = &btn_9;
    key[10] = &btn_10;
    key[11] = &btn_11;
    key[12] = &btn_12;

    while(1) {
        for (int i=0; i<NUM_KEYS; i++) {
            volatile uint32_t dummy = key[i]->read();
            if (key[i]->read() != 1) {
                if (key_index[i] == -1) {
                    int num = getFreeChannel();
                    if (num != -1) {
                        key_index[i] = num;
                        tone[num]->period_us(tone_table_us[i+1]);
                        tone[num]->write(0.5f);
                    }
                }
            }
            else {
                if (key_index[i] != -1) {
                    tone[key_index[i]]->period(0);
                    ch[key_index[i]] = 0;
                    key_index[i] = -1;
                }
            }
        }
    }
}