File content as of revision 2:6d472c554758:

/* PIservo
 implement a PI controller with USB serial gain setting to stabilize dipole trap power.
 Two analog inputs (control and sensor), one analog output (to RF VCA).

20120806 Andreas Steffen
*/


#include "mbed.h"

DigitalOut adcCS(p8);
DigitalOut cyc(p10);
AnalogOut vca(p18);
SPI adcSPI(p5,p6,p7); //talk to ad7921
Serial pc(USBTX, USBRX);


Timer t;
Ticker commTick;
Ticker intTick;

float pGain = 0.5;
float iGain = 1000.;
float integrator = 0.; //the building up integrator value

void serialComm(){
    char c;
    float f;
    
    if (pc.readable()) { 
        c = pc.getc();
        pc.scanf("%f", &f);
        switch (c) {
            case 'p':
                pGain = f;   
                pc.printf("Changed p gain to %f.\n",pGain);
                break;
            case 'i':
                iGain = f;   
                pc.printf("Changed i gain to %f.\n",iGain);
                if (f==0)
                    integrator = 0.;
                break;
            default:
                pc.printf("Command not understood.\n",iGain);
                pc.printf("Read %c and %f.\n",c,f);
                break;
        }
    }
}

void limitIntegrator(){ 
    if (integrator > 3.3) {
            integrator = 3.3;
        }
    if (integrator < 0) {
        integrator = 0;
    }
}

int main() {
    float err=0;
    float out=0.;
    int ctl=0;
    int pd=0; 
    pc.printf("mbed restarted!\n");
    
    adcSPI.format(16,2);
    adcSPI.frequency(5000000); 
    
    commTick.attach(&serialComm,0.5); //check serial every half second  
    intTick.attach(&limitIntegrator,0.001); //check integrator overrun every ms  
    
    t.start();
    cyc = 0;    

    while(1) {
        cyc = cyc ^ 1; //toggle debug pin
        //read in from AD7921 over SPI
        adcCS = 0;
        pd = adcSPI.write(3<<13); //select ch 1  
        pd = (pd) & 0xFFF;
        adcCS = 1;
        
        adcCS = 0;
        ctl = adcSPI.write(1);       // select ch 0
        //take the last 12 bits that carry data
        ctl = (ctl) & 0xFFF;    
        adcCS = 1;  
              
        err = ctl*3.3/4095. - pd*3.3/4095.; //convert to volt   
        integrator += err * iGain*1.e-6 * t.read_us(); 
        t.reset(); //reset timer to get next integration time 
             
        out = 1.0* err * pGain + integrator;        //analog output
        vca.write(out); 
        
        //  pc.printf("Measuring ctl=%i and pd=%i, err=%f, vca is %f\n",ctl,pd,err,out);
        
    }
}