Decrease the timer resolution of SDK 14

RE: Decrease the timer resolution of SDK 14

Atul Bansal — Thu, 21 Jun 2018 23:05:49 GMT

Want to send the timestamp values in the advertisement packets and I don't want to have large timer values as the packet sizes are very small. About half a second resolution would have been pretty good. But, I guess I will try to work with 125 ms.

RE: Decrease the timer resolution of SDK 14

superpak — Thu, 21 Jun 2018 22:55:29 GMT

Yes, I believe so. What is the reasoning for having a 500 ms resolution? Just curious!

RE: Decrease the timer resolution of SDK 14

Atul Bansal — Thu, 21 Jun 2018 22:46:21 GMT

Cool. Got it! But 500 ms resolution is impossible then? 125 ms is the minimum we could get?

RE: Decrease the timer resolution of SDK 14

superpak — Thu, 21 Jun 2018 22:37:15 GMT

The app_timer library sets the prescaler for you. Look at the rtc1_init(..) routine in app_timer.c

The APP_TIMER_CONFIG_RTC_FREQUENCY is the prescaler value for app_timer library.

static void rtc1_init(uint32_t prescaler)
{
    NRF_RTC1->PRESCALER = prescaler;
    NVIC_SetPriority(RTC1_IRQn, RTC1_IRQ_PRI);
}

This is called by app_timer_init(...)

ret_code_t app_timer_init(void)
{
.....

rtc1_init(APP_TIMER_CONFIG_RTC_FREQUENCY);

....
}

RE: Decrease the timer resolution of SDK 14

Atul Bansal — Thu, 21 Jun 2018 21:37:59 GMT

The counter resolution of 30.517 us is the highest resolution right, not the lowest one.

Is there any way in which I can get a resolution of 500 ms? And even if I make do with the 125 ms resolution, where should I change the PRESCALER register? I didn't see any such command in the documentation of SDK 14. I could change the APP_TIMER_CONFIG_RTC_FREQUENCY macro, but in the description, there are some values provided to us, and I suppose, only those can be used as the possible values of the macro. I am attaching the code from the sdk_config file.

// <o> APP_TIMER_CONFIG_RTC_FREQUENCY  - Configure RTC prescaler.
 
// <0=> 32768 Hz 
// <1=> 16384 Hz 
// <3=> 8192 Hz 
// <7=> 4096 Hz 
// <15=> 2048 Hz 
// <31=> 1024 Hz 

#ifndef APP_TIMER_CONFIG_RTC_FREQUENCY
#define APP_TIMER_CONFIG_RTC_FREQUENCY 31        
#endif

RE: Decrease the timer resolution of SDK 14

superpak — Thu, 21 Jun 2018 21:28:01 GMT

16383 will not work as the prescaler is only 12 bits!

RE: Decrease the timer resolution of SDK 14

superpak — Thu, 21 Jun 2018 21:26:43 GMT

The RTC peripheral has a 12 bit prescaler. So the max value for prescaler will be (2^12) - 1 = 4095. This will give you a counter resolution of 125 ms with an overflow of 582.542 hours.

If you want the l̶o̶w̶e̶s̶t̶ highest counter resolution, you can set the prescaler (APP_TIMER_CONFIG_RTC_FREQUENCY) to 0. This will give you a counter resolution of 30.517 us and an overflow of 512 seconds.

RE: Decrease the timer resolution of SDK 14

Atul Bansal — Thu, 21 Jun 2018 16:59:01 GMT

Hi Bjorn

I knew it already that I have to change the PRESCALER to get the resolution I want. But, I am working with SDk 14.2 and I found that they have removed the PRESCALER argument from the APP_TIMER_TICKS function which was threre in SDK 13. The details can be found here.

Thus, I need to know the code or any command which will change the PRESCALER value. As far as I searched, I can only find that I need to change the APP_TIMER_CONFIG_RTC_FREQUENCY macro present in sdk_config.h file and that too it's maximum value was 31. If I change it to 16383, my device stops advertising.

Atul

RE: Decrease the timer resolution of SDK 14

Bjørn Kvaale — Thu, 21 Jun 2018 09:00:15 GMT

Hi Atul,

I believe this link will be helpful for you. Since you want a resolution of 500 ms, this is the same as 1/.5 s = 2 Hz. Therefore f_RTC = 2Hz.

Solving for the equation gives you a prescaler value of:

PRESCALER = round(32768 Hz/ 2 Hz) - 1 = 16383.