Implementing 64-bit Epoch Time Using Timer

RE: Implementing 64-bit Epoch Time Using Timer

ovrebekk — Thu, 27 May 2021 07:39:32 GMT

Hi Darren

Yes, you shouldn't have to disable the compare event itself, just temporarily disable the interrupt.

nrf_timer_int_disable/enable should work fine, or you could disable the timer interrupt globally.

Best regards
Torbjørn

RE: Implementing 64-bit Epoch Time Using Timer

WestCoastDaz — Wed, 26 May 2021 02:57:47 GMT

ovrebekk

I have some followup questions regarding the suggested implementation. You sequence mentions disabling the timer interrupt and then enabling the timer interrupt once I have gotten the number of milliseconds using the capture.

I took this to mean disabling the CC[0] interrupt and then enable the CC[0] interrupt. The nrfx_timer.c implementation

nrfx_timer_compare_int_enable

calls

nrf_timer_event_clear

which clears the timer counter which will cause time to be lost every time the epoch is read, no?

Should I just call

nrf_timer_int_enable

directly instead?

Thanks,

RE: Implementing 64-bit Epoch Time Using Timer

ovrebekk — Tue, 18 Aug 2020 09:11:57 GMT

Hi Darren

Yes, dividing by 1000 should work fine.

You can also reduce the speed of the timer to 125kHz by using a PRESCALER value of 7, and divide by 125 to get the milliseconds, but the end result is the same.

Best regards
Torbjørn

RE: Implementing 64-bit Epoch Time Using Timer

WestCoastDaz — Mon, 17 Aug 2020 20:38:16 GMT

ovrebekk

Thank you for taking the time to answer. Sorry I couldn't reply until now as I was away on vacation without much access to good internet.

I understand your implementation. I don't need the microseconds so it looks like the best option to get the milliseconds is to divide the result of the compare by 1000?

Thanks,

Darren

RE: Implementing 64-bit Epoch Time Using Timer

ovrebekk — Wed, 12 Aug 2020 14:39:20 GMT

Hi Darren

As you probably know the CC registers can be used either as compare registers, capture registers or both.

My point is just that if you use say CC[0] to generate an interrupt after 1 second you should use a separate register, such as CC[1], to capture the current timer value whenever you want to read the current time.

Best regards
Torbjørn

RE: Implementing 64-bit Epoch Time Using Timer

WestCoastDaz — Wed, 12 Aug 2020 03:31:17 GMT

ovrebekk

Thanks for taking time to answer. I like this idea as it reduces overall load on the system. Can you elaborate on point 2? I'm not sure I follow on the meaning of "capture the current timer value to a separate CC register"

Thanks,

Darren

RE: Implementing 64-bit Epoch Time Using Timer

ovrebekk — Tue, 11 Aug 2020 08:10:51 GMT

Hi Darren

For efficiency reasons I would suggest using a longer interval before you update your variable, and do a timer capture to get the 'lower bits' of the timestamp.

As an example if you configure the timer to run at a 1MHz frequency (prescaler = 4) you can reload it every second, and get the milliseconds (and microseconds if needed) by doing a capture on the timer.

To get the current timer value you could do the following:

- Disable timer interrupt
- Capture the current timer value to a separate CC register
- Add the captured value to your variable counting the seconds, and return this as the current time
- Enable timer interrupt

Then in the interrupt you essentially need to increment the second variable, and increment the capture register by a million to schedule the next interrupt.

Best regards
Torbjørn