File content as of revision 2:eb447fcd8284:

#include "mbed.h"

AnalogIn rx(p20);

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

static const int N_PPM = 4;
static const int PULSE_TIME = 1000;
static const int SAMPLE_RATE = 10; // must be an even divisor of PULSE_TIME
static const int SAMPLE_GAP = PULSE_TIME / SAMPLE_RATE;
static const int HALF_SAMPLE = SAMPLE_RATE / 2;
static const int BUF_SIZE = 1024;
static const float THRESH = 0.005;

int buf[BUF_SIZE];
int idx = 0;
int nzeros = 0;
int start = 0;
bool sample_lock = false;
int offCount, onCount;

char* decodechar(int stopIdx){
    int len = (stopIdx - start + BUF_SIZE) % BUF_SIZE;
    bool* data = new bool[len * N_PPM];
    for(int i = 0; i < len; i++)
    {
        int c = buf[start++];
        if(start == BUF_SIZE) start = 0;
        for(int j = 0; j < N_PPM; j++)
        {
            data[i * N_PPM + j] = (c >> (N_PPM - j - 1));
        }
    }
    return (char*) data;
}

void sample()
{
    if(sample_lock) pc.printf("\r\nWarning: Sampling too fast\r\n");
    sample_lock = true;
    float v = rx.read();
    if(v < 0.001){
        offCount++;
        if(offCount > HALF_SAMPLE) onCount = 0;
    }else{ // Pulse
        if(onCount > HALF_SAMPLE && offCount > HALF_SAMPLE)
        {
            int offPulses = (offCount + HALF_SAMPLE) / SAMPLE_RATE;
            pc.printf("%d:%d ", offCount, offPulses);
            if(offPulses <= N_PPM)
            {
                buf[idx] = offPulses - 1;
                idx++;
                if(idx == BUF_SIZE) idx = 0;
                if(idx == start) pc.printf("\r\nWarning: Buffer too small\r\n");
            }
            offCount = 0;
        }
        onCount++;
    }
    sample_lock = false;
}

int main()
{
    pc.printf("3 CM Link Board - Recieve\r\n");
    bool calib = true;
    if(calib){
        while(true){
            pc.printf("%f\r\r\n", rx.read());
            wait_ms(100);
            if(pc.readable()) break;
        }
    }
    idx = 0;
    offCount = 0;
    offCount = 0;
    pc.printf("Ready.\r\n");
    float v = rx.read();
    while(v < THRESH)
    {
        v = rx.read();
        wait_us(SAMPLE_GAP);
    }
    pc.printf("Starting...\r\n");
    Ticker sampler;
    sampler.attach_us(&sample, SAMPLE_GAP);
    
    while(true){
        if((offCount + HALF_SAMPLE) / SAMPLE_RATE > N_PPM){ // synchronize
            char* data = decodechar(idx);
            pc.printf(data);
            delete[] data;
            offCount = 0;
        }
    }
}