High Sample Rate with ADC and SoftDevice

[Chris]

I am trying to sample the ADC every 2ms. I notice lots of slowness with BLE advertising and typically can't connect to device over BLE when sampling. I am using PPI, configuring, and starting before the softdevice is enabled.

1. Should nRF51822 be capable of reading ADC every 1ms to 2ms reliably?

2. Should I move all the PPI configuration to after the softdevice is enabled and be using the sd_ppi functions?

Thanks!

Edit: Headline, format, added tags.

[Stefan Birnir Sverrisson]

Hi Chris

The application timers use the RTC1 timer in the background. When using application timers, you need to share your timing with other periodic tasks that are also using application timers. Also, the application timers are setting software interrupts that may introduce latency. For real time sampling it would be best to use TIMER1 or TIMER2 directly with PPI channels to trigger the ADC sampling. You could also use RTC1 directly if you are not enabling the application timers. RTC0 and TIMER0 are blocked by the softdevice.

It is chosen in this example to use the external 16MHz crystal for the ADC sampling because that will guarantee that the accuracy of the ADC will be within specification. You can use the internal 16MHz RC clock but then the accuracy is no longer guaranteed. The accuracy of the internal RC is much lower than of the crystal. However, when using the external crystal, current consumption will be higher since it takes longer to start up the crystal than the RC, see tables 16 and 18 in the nRF51822 PS v1.3. In short, use the external crystal for maximum accuracy, use the internal RC for lowest current consumption.