Dealing with micros rollover in an ISR

Projects Programming
1

Have I implemented this algorithm correctly:

void doStartDisplay()
{
uint32_t lunTimeElapsed = micros(), lunPeriod = 0, lunMask = 0;

lunPeriod = lunTimeElapsed - lintLastTimeElasped;
lunMask = lunPeriod >> 31;
lunPeriod = (lunMask ^ lunPeriod) - lunMask;

lintLastTimeElasped = lunTimeElapsed;
lunTimeSlice = lunPeriod / (60 * 6);
lunCount = 0;
// Timer1.attachInterrupt(doDisplay, lunTimeSlice);
// doDisplay();
}

I am trying to get rid of the if statement I previously had which would probably consume more CPU cycles.

2

Why not reduce your ISR to

void  doStartDisplay() {
    lunTimeElapsed = micros();
}

and do the rest of the calculation outside the ISR

Your names are not very well chosen for that they relate to.

...R

3

You know, after 200+ posts you should know how to use code tags when posting source code here. If not, read the two posts by Nick Gammon at the top of this Forum.

4

Robin2:
Why not reduce your ISR to

void  doStartDisplay() {

lunTimeElapsed = micros();
}




and do the rest of the calculation outside the ISR

Your names are not very well chosen for that they relate to.

...R
5

Didn't occur to me to do it that - thanks.

6

lunMask = lunPeriod >> 31;

This is likely to be unnecessarily slow.

7

If you are measuring intervals less then 71 minutes you can use the same math as for millis():

unsigned long EventInterval = 3276700UL; // Microseconds between events

void  doStartDisplay() {
    static unsigned long lastEventTime = 0;

   if (micros() - lastEventTime >= EventInterval) {
       // do the thing
       lastEventTime += EventInterval;
  }
}