File content as of revision 0:f99e355ba60d:

#include "mbed.h"

/* IO pin defines*/
DigitalOut led1(LED1);
PwmOut pwm(p21);
AnalogIn mic(p20);
AnalogOut spk(p18);
Ticker readtick, writetick;
// Serial pc(USBTX, USBRX);

#define WAVMAX 7000  /*buffer size; SAMPFREQ*DEALY<WAVMAX*/
#define SAMPFREQ 16000.0
#define DELAY 0.25 /*s*/

unsigned short wave[WAVMAX];
int readindex=0, writeindex=0;

class KBline : public Serial
{
public:
    char line[100];
    int  index;
    int  receiving; //status 0=not started, 1=waiting, 2=receiving, 3=end
private:
    void serialirq()
    {
        char ch;
        if (this->readable()) {
            ch=this->getc();
            switch(ch) { //simple line editing
                case '\n':
                case '\r':  //LF and CR terminate reading
                    line[index++]=ch;
                    line[index++]=0;
                    this->printf("\r\n");
                    receiving=3;
                    break;
                case 21:  /* control-U cancels entire line*/
                    this->printf("\n\r");
                    index=0;
                    break;
                case 0x08:
                case 0x7f: /*BS and DEL delete the last character*/
                    if (index>0) {
                        index--;
                        this->putc(0x08);
                        this->putc(0x20);
                        this->putc(0x08);
                    }
                    break;
                default:
                    receiving=2;
                    line[index++]=ch;
                    this->putc(ch);
                    break;
            }
        }
    }
public:
    KBline(PinName tx, PinName rx) : Serial(tx, rx)
    {
        receiving=0;
    }
    //  KBline(Serial *pc) { term=pc; }; //constructor
    void start_getline()
    {
        index=0;
        receiving=1;
        this->attach(this, &KBline::serialirq);
    }
    // int receiving() { return(receiving);}
    char *getline()
    {
        while (receiving!=3) wait(0.1);
        receiving=0;
        return(line);
    }
} ;

void read_wave()    /*timer driven*/
{
    wave[readindex++]= mic.read_u16();  //mic.read()*30000;
    if (readindex>=WAVMAX) readindex=0;
}

void new_writeindex(int wi)
{
    if (wi<0) wi += WAVMAX;
    writeindex=wi % WAVMAX;
}

void write_wave()
{
    unsigned short val=wave[writeindex];
    pwm.write(val*0.001);
    spk.write_u16(val); // (val/30000.0  );
    new_writeindex(writeindex+1);
    // wave[index] +=val<<1;
}

// main() runs in its own thread in the OS
int main()
{
    KBline pc(USBTX, USBRX);
    float delay=DELAY;

    pc.baud(9600);
    pwm.period_us(50);
    delay=DELAY;
    readindex=0;
    new_writeindex(readindex-delay*SAMPFREQ);

    readtick.attach(read_wave, 1.0/SAMPFREQ);
    writetick.attach(write_wave, 1.0/SAMPFREQ);
    pc.printf("echo machine  delay=%fs\n\r", delay);
    while (true) {
        pc.start_getline();
        while (pc.receiving!=3) {
            led1 = !led1;
            wait(0.2);
        }
        delay=(pc.line[0]-'0') *0.1;  /* multiple of 0.1s */
        new_writeindex(readindex-delay*SAMPFREQ);
        pc.printf("new delay %5.2fs  %d \n\r", delay, (readindex-writeindex)% WAVMAX);
    }
}