How to make sure, that a compare event will be generated

[Torsten Robitzki]

Hello, I want to generate timer events as exact as possible, based on a time base that is defined by a runing timer. For example, I've setup timer0 to count up every 1µs and the time point, where I've started the timer is my anchor/T0 in time. When I want to have an interrupt that is 600µs after T0, I could simply load 600 into a capture/compare register and wait for the coresponding compare event.

When the value of the counter is already very close to my target time, it could be that I load the capture/compare register with a value that is smaller than the counter value. Checking for the counter before loading the capture compare register would introduce a race condition.

What would be the best / idomatic way to prevent a race condition here? Best without stopping the timer, disabling interrupts and so on.... How about this:

NRF_TIMER0->EVENTS_COMPARE[ 0 ] = 0;
NRF_TIMER0->CC[ 0 ]             = target_time;
NRF_TIMER0->TASKS_CAPTURE[ 1 ]  = 1;

/* manually triggering event for timer beeing already behind target time */
NRF_TIMER0->EVENTS_COMPARE[ 0 ] = NRF_TIMER0->EVENTS_COMPARE[ 0 ] || NRF_TIMER0->CC[ 1 ] >= NRF_TIMER0->CC[ 0 ];
NRF_TIMER0->INTENSET            = TIMER_INTENSET_COMPARE0_Msk;

would that work? Does someone of you have a better approache to cope with possible race conditions here?

Best regards, Torsten

[Øyvind Karlsen]

Hi,

Have a look at how this is handled in the pwm-library on github. The idea is that you will only update the compare value if you're within a safe margin.

Things become harder if it is critical that you never hit the 600 mark if you're trying to reset. Here you might save yourself some headache changing around your states instead.

Best regards,

Øyvind

[Øyvind Karlsen]

This should work. Be careful about what priorities you give to the different things you need to do though, since you will still generate an interrupt if the cc has higher priority than the function you want to use to unconditionally reset it.