Couldn't change sampling frequency when using Timer + PPI with ADC.

[Alisha]

Hi,

I just create simple example based on nRF51 ADC examples, using SDK12.3.0 with softdevice S130 v2.

I used PPI + Timer to get sample from ADC. I used TIMER2 to trigger ADC sampling task, but changing ADC_SAMPLE_RATE doesn't affect rate of sampling.

I also read the following question but couldn't understand the solution:

https://devzone.nordicsemi.com/f/nordic-q-a/19639/adc-ppi-timer-on-the-nrf51822-polling-too-fast

What is the maximum possible delay between two consecutive timer event?

Regards,

[Alisha]

It seems I found the problem, the timer bit width is 16 bit and I try to set the ADC_SAMPLE_RATE to 10 second. Also the frequency of timer is 31.25 kHz, with simple computation the maximum possible delay is 2^16 / 31250 = 2.09 seconds. Am I right?

[MartinBL]

That sounds right. Maybe you can look into using the RTC instead. You can still use PPI and you should be able to achieve 0.1 Hz sampling. 

[Alisha]

Thanks for your reply. Would you please explain how should I set timer setting for 0.1 Hz sampling with TIMER2 peripheral, I don't want to use RTC timer?

[MartinBL]

As you said yourself, you can only reach a sampling period of 2.09 seconds (~0.48 Hz). You are not able to scale the TIMER frequency below 31.25 kHz and the counter register is too small. You either need to use timer interrupts (trigger interrupt every 2 sec e.g. and count to 5) or the RTC. 

[Alisha]

Thanks for you reply, I use the same approach currently.