What is the precision on timers?

RE: What is the precision on timers?

Jeffery Bahr — Thu, 12 Jul 2018 16:54:36 GMT

Just getting back to this. Thanks for the answer and code!

RE: What is the precision on timers?

ovrebekk — Wed, 13 Dec 2017 09:12:52 GMT

I thought triggering the task should work if you did it before enabling the SoftDevice, but using the driver is probably a better idea ;)
Good to see you figured it out :)

RE: What is the precision on timers?

Nguyen Hoan Hoang — Wed, 13 Dec 2017 03:08:41 GMT

You can also try my timer driver here. Precision in usec even with low freq timer. Driver code in src. example in exemples folder.

RE: What is the precision on timers?

Jeffery Bahr — Tue, 12 Dec 2017 21:39:54 GMT

Although adding NRF_CLOCK->TASKS_HFCLKSTART = 1 improved the accuracy of the timer in the Nordic sample app (without softdevice), it caused our app -- which uses soft device S130 -- to fail. We embedded exactly the same timer code (using nrf_drv_timer_xxx calls), and they work OK, except we find again that the timer is again slightly off from 5000 microseconds, depending upon the board we're using.

We poked around and eventually added

#include "nrf_drv_clock.h"

and then

	nrf_drv_clock_init();
	nrf_drv_clock_hfclk_request(NULL);

before calling app_timer_create() for the one-second timer we use in the app.

That worked! The timer is now correct.

Thanks in advance for your help.

RE: What is the precision on timers?

ovrebekk — Tue, 12 Dec 2017 18:01:47 GMT

Hi Jeffery

The accuracy of the timers is set by the clock source, which by default is the internal 16MHz RC oscillator. This oscillator has an accuracy of around +/- 5%, and as such your timer accuracy will be the same.

To get better accuracy for your timers you should start the external 16MHz crystal, which will have an accuracy of 50 ppm or better (depending on the crystal used).

The simplest way to start the external 16MHz crystal is to trigger the HFCLKSTART task, like this:

NRF_CLOCK->TASKS_HFCLKSTART = 1;

Best regards
Torbjørn