Dual channel ADC capture missing samples from one channel

RE: Dual channel ADC capture missing samples from one channel

NewtoM — Thu, 19 Nov 2020 22:15:50 GMT

Oh, both pieces of information are useful, thank you very much!

RE: Dual channel ADC capture missing samples from one channel

Jørgen Holmefjord — Wed, 18 Nov 2020 11:23:57 GMT

[quote user="newtom"]As for the SAADC; it's a pity the internal timer is not an option for multi-channel acquisition! I very much liked the idea of not bothering the CPU with single "sample task" triggers, which becomes a real overhead when you are about to use high rates with many channels. It would be cool to have the internal timer for more channels in future revisions...[/quote]

There is no need to bother the CPU in order to trigger the SAMPLE task. If you setup a TIMER or RTC peripheral to generate a COMPARE event on the desired sample rate, you can use PPI to trigger the SAMPLE task in the SAADC peripheral directly using hardware signals.

[quote user="newtom"]As for mixing the use of driver API and peripheral registers: as far as I can recall, SAADC driver did not have a function to configure internal timer. Has it changed recently?[/quote]

In fact, it has changed. We introduced a new SAADC driver API in nrfx in SDK 17.0.0, which divides the driver into a simple and an advanced mode configuration. In the advanced mode configuration, it is possible to set a internal_timer_cc parameter to enable the internal timer of the SAADC and use continuous mode. Unfortunately, there is not much documentation about this driver yet, but you can find a few examples in above link.

RE: Dual channel ADC capture missing samples from one channel

NewtoM — Wed, 18 Nov 2020 08:03:30 GMT

Dear Jørgen,

thank you very much for the quick reply! That's definitely something I am very keen on when dealing with Nordic's products!

As for the SAADC; it's a pity the internal timer is not an option for multi-channel acquisition! I very much liked the idea of not bothering the CPU with single "sample task" triggers, which becomes a real overhead when you are about to use high rates with many channels. It would be cool to have the internal timer for more channels in future revisions...

As for mixing the use of driver API and peripheral registers: as far as I can recall, SAADC driver did not have a function to configure internal timer. Has it changed recently?

Thanks again!
Tamas

RE: Dual channel ADC capture missing samples from one channel

Jørgen Holmefjord — Tue, 17 Nov 2020 14:15:11 GMT

Hi Tamas,

The internal timer can only be used with a single channel, see Continuous sampling:

"Note: Note that the internal timer can only be used when a single input channel is enabled."

You will have to use a TIMER/RTC to trigger the sample task at the desired interval in order to sample two channels. You can reference the SAADC example in the SDK, or one of the examples in this GitHub repository.

It is also not generally recommended to to mix usage of the driver APIs and writing directly to the peripheral registers.

Best regards,
Jørgen