File content as of revision 1:516ea1ca7abb:

#include <mbed.h>
#define samplerate 16000
#define MAXFREQS 10

Serial pc(USBTX, USBRX);
Ticker tick1;
PwmOut pwm1(p21);
AnalogOut da1(p18);

const int scalefreqs[12*2]={440, 466,494,523,554,587,622,659,698,740,784,830,
        880,932,987,1046,1109,1175,1245,1319,1397,1480,1568,1661};
signed char bytesintab[samplerate];
int  freqlist[MAXFREQS];
int  seqlist[MAXFREQS]={0};
int  maxfreqs=0;

void make_sintab(int count) {
    int i;
    for (i=0; i<count; i++) {
        bytesintab[i]=sin( 2.0 * 3.141592 *((float) i)/((float)samplerate)) *120;
        }    }

void gentone() {
    float outval=0;
    int i;
    for (i=0; i<maxfreqs; i++) {
        outval += bytesintab[seqlist[i]];
        seqlist[i] += freqlist[i];
        seqlist[i] = seqlist[i] % samplerate; //clip at 160000
        }        
    da1.write((outval/(256.0*maxfreqs))+0.5); }

void setfreq(const int tone, const int scale) {
    freqlist[tone]=scalefreqs[scale];
    seqlist[tone]=0; }

main() {
    int ch, chord=0;
    make_sintab(samplerate);    setfreq(0,0);    maxfreqs=1;
    tick1.attach(&gentone, 1.0/(float)samplerate);
    while (1) {
      ch=pc.getc();
      if ('a'<=ch && ch<='l') {
        setfreq(0, ch-'a');
        if (chord<=1) maxfreqs=1;
        else if (chord>1) { setfreq(1,ch-'a'+7); maxfreqs=2;}
        if (chord>2) { setfreq(2,ch-'a'+4); maxfreqs=3;}
        if (chord>3) { setfreq(2,ch-'a'+3); /*minor*/ }
        if (chord==7) { setfreq(3, ch-'a'+10); maxfreqs=4; }/*seventh*/
        }
      if ('1'<=ch && ch<='7') chord=ch-'0'; 
      pc.printf("%d %d\n\r", freqlist[0], chord); }
    };