RTC Time Keeping using app_timer with SD132

[aeozyalcin]

There are a few threads on here about this already, but I wasn't find the answer to the exact question I had.

I am using the nRF52832 with SD132 and SDK15.2 on a custom board with an external 32Khz XTAL. I am trying to implement a real time clock to keep time in FW. For this, I decided to use the app_timer module, since the SoftDevice already initializes the LF CLK source as the external LF XTAL. Going through my SDK_config file, I confirmed that the LF CLK source is correctly selected as LF XTAL, and the RTC prescaler is at 32768, which should give ~30uS resolution. Using the app timer, I just set up a repeated app timer with the "APP_TIMER_MODE_REPEATED" option, and APP_TIMER_TICKS(1000) to fire my handler that increments a variable that keeps epoch time in a uint32_t variable.

On my setup, I am seeing about 8 seconds of drift over ~12 hours, which is pretty excessive. My board is custom, and I don't know the quality or the accuracy PPM spec of the 32Khz XTAL on there, but I wanted to confirm that what I did in FW was sound. Does this sound like a good way of implementing an "accurate" (as accurate as it can be with the bottleneck being XTAL accuracy) RTC on a nRF52 device using a SoftDevice?

Thanks!

[Einar Thorsrud]

Hi,

Using the app timer like this makes sense, and is what I would recomend. You will get the same accuracy as the 32 kHz clock source, which is your external crystal in this case. So if you need better accuracy, you need to use a more accurate crystal.