TIMER1 compare not triggered for CC=0x00

[Nick]

I am using a TIMER1 compare event to generate an interrupt. Generally the code all works fine, but I have noticed some strange behaviour when loading the CC register with 0x00. If the timer is allowed to run freely and overflow, then it is possible to generate a compare event when the timer rolls-over to 0. No problem there. But then I tried to limit the maximum timer count using;

NRF_TIMER1->SHORTS |= TIMER_SHORTS_COMPARE2_CLEAR_Enabled << TIMER_SHORTS_COMPARE2_CLEAR_Pos;
NRF_TIMER1->CC[2] = T1_TICKS_MAX;

My understanding is that this should cause the timer to reset to zero when it hits the value in CC2 (T1_TICKS_MAX). However, when I do this I no longer get any compare events generated for CC=0x00. Any other CC value less than or equal to T1_TICKS_MAX works fine.

This seems a bit unexpected to me. Is this normal behaviour? Or do I perhaps have a bug in my code somewhere?

[Susheel Nuguru]

I am not sure what you are trying to achive here. Why do you need SHORT to clear timer in this case? What is the value of T1_TICKS_MAX? if it is 0XFFFF Then you do not need that short, it will overflow and the next counter value will be zero anyway.

If you enable clear_short at 0XFFFF, i think the short logic conflicts with the event logic at the same HFCLK cycle and looks like the event is missed. In my opinion, this is expected to be unpredictable in this case.

[Nick]

Thanks for the suggestion to change the timer max count to 0xFD (or 0xFFFD in 16-bit mode). I think this would solve the problem for that particular compare channel (CC1). However, I am also using other compare channels (eg CC2) at the same time for different tasks. In my code the TIMER1 resources are shared by two different independent modules. If I change the max timer roll over value it will inadvertently affect both COMPARE events - not just the one that required the compare event at 0x00. I don't think this solution is going to work for me.

I am actually using 16-bit mode, not 8-bit mode. I may have made some typos when giving register values. Where I said 0xFE I actually meant 0xFFFE. 0xFF should have been 0xFFFF, etc. Sorry for the confusion.