Get zero for clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);

Ok, I think that was a very poor explanation, please see the below code for the PID I am using , I get zero for millis() function on pocketbeagle

#include <stdio.h>
#include <time.h>
#include <unistd.h>


/*working variables*/
unsigned long lastTime;
double Input, Output, Setpoint; // Setpoint = desired pressure, Ouput is PWM value, Input is measured current value
double errSum, lastErr;
double kp, ki, kd;
struct timespec start, stop;


double millis()
{
double result = (stop.tv_sec - start.tv_sec) * 1e6 + (stop.tv_nsec - start.tv_nsec) / 1e3;    // in microseconds
return result;
}
void Compute()
{
  clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
   /*How long since we last calculated*/
   unsigned long now = millis();
   double timeChange = (double)(now - lastTime);
   printf("now is : %lu\n",now);
   printf("time change is : %f\n",timeChange);

   /*Compute all the working error variables*/
   double error = Setpoint - Input;  // Proportional error
   errSum += (error * timeChange); // Integral error
   double dErr = (error - lastErr) / timeChange; // Differential error

   /*Compute PID Output*/
   Output = kp * error + ki * errSum + kd * dErr;

   /*Remember some variables for next time*/
   lastErr = error;
   lastTime = now;
   clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &start);
}

void SetTunings(double Kp, double Ki, double Kd)
{
   kp = Kp;
   ki = Ki;
   kd = Kd;
}


int main()
{
printf("time diff is : %f\n",millis());
while(1)
{
sleep(3);
Compute();
printf("time diff is : %f\n",millis());
sleep(3);
}
return 0;
}

Hi Chris,

I've not tried it on BBB or PB, but I've used this code in the past on other machines.. not saying it is great code, but it worked at the time:

#include <stdlib.h>
#include <time.h>

struct timespec ts_zero;

void
get_tzero(void)
{
  clock_gettime(CLOCK_MONOTONIC, &ts_zero);
}

long
get_time(void)
{
  long nsec, sec;
  struct timespec tts;
  clock_gettime(CLOCK_MONOTONIC, &tts);
  sec=tts.tv_sec-ts_zero.tv_sec;
  nsec=tts.tv_nsec-ts_zero.tv_nsec;
  if (sec>0)
  {
    nsec=nsec+1E9; // we don't expect times greater than 1 sec
  }
  ts_zero=tts;
  return(nsec);
}

in your code:
execute get_tzero() at the start of the event that you wish to time
At the end of the event, use:
long nsec;
nsec=get_time();
nsec will contain the duration of the event in nanosec.