File content as of revision 0:901c98b3591a:

#include "mbed.h"
#include "math.h"
Ticker flipper;
Ticker flipper2;
DigitalOut led2(LED2);
AnalogIn sensor(p20);
AnalogIn sensor_visible(p19);
AnalogIn sensor_invisible(p18); 

//680 nm
#define visible 68 
//940 nm
#define invisible 94

 

float data =0; 
float previous_data =0;
int count1=-1;
int count2=-1;
float max1 =0.0;
float max2 =0.0;
int running_count=0;  
bool foundMax = false;
#define difference 0.05 
#define count_period 0.1
void flip() {
    led2 = !led2;
    previous_data = data; 
   // printf("Data = %f\t",data);
    data = sensor.read() * 3.3;
    running_count++;  
   // printf("data = %f\n",data);
    if (data - previous_data > 0){
        //increasing 
        printf("Increasing \n");
        //do nothing
    }     
    else if(data - previous_data < 0 && !foundMax){
        //check if foundMax 
        printf("found max \n");        
        foundMax = true;
        count1 = running_count; 
        max1 = data;    
    }
    else{
        printf("Decrease\n");    
    }
    if(max1-data < difference && data-previous_data >0 && foundMax){
        //found second max; 
        printf("found second max \n");
        count2 = running_count;     
        float period = (count2 - count1)* count_period; 
        
        float BPM = (1/period) * 60;
        printf("BPM = %f",BPM);
        exit(1);
        //reset 
        count1 = 0;
        count2 =0; 
        foundMax = false; 
        max1 =0;
        running_count =0; 
    }
    
}

void findOxygenSaturation(){
    
    float v = log10f(sensor_visible.read()*3.3) * visible;
    float iv = log10f(sensor_invisible.read()*3.3) * invisible; 
    printf("visible = %f\n",v);
    printf("invisible = %f\n",iv);
    float ratio = v/iv;
    printf("ratio = %f\n",ratio);
    float spo = -0.3393 * ratio + 1.1595 ;
    printf("spo = %f\n",spo);
}


int main() {
    led2 = 1;
    //comment out the below line to test heart rate 
    //the heart rate test subroutine, once it find the heart rate, it will terminate the program.
    //so make sure you reset the MBED
    //flipper.attach(&flip, count_period); // the address of the function to be attached (flip) and the interval (1 seconds)    
    
    
    //read OxygenSaturation Every two second. 
    //use pin 19 for visible light input
    //use pin 18 for invisible light input
    flipper2.attach(&findOxygenSaturation,2.0);
    while(1) {
    
    }
}