SAADC low power scan mode

RE: SAADC low power scan mode

Jørgen Holmefjord — Wed, 19 Apr 2017 12:28:53 GMT

You are not allowed to set RESULT.MAXCNT=1 when multiple channels are enabled. From the documentation:

In Scan mode, the size of the Result buffer must be large enough to have room for a minimum one result from each of the enabled channels. To secure this, RESULT.MAXCNT must be specified to RESULT.MAXCNT >= "number of channels enabled".

RE: SAADC low power scan mode

FormerMember — Mon, 03 Apr 2017 12:02:29 GMT

Yes, this is what I'm looking for. For a single channel the uninit can be done by the PPI SAADC_EVENT_END -> SAADC_TASK_STOP. For multiple channels, even with RESULT.MAXCNT=1 it looks like the EasyDMA is getting activated. Is this a hardware bug? Would this be fixed in the next revision?

RE: SAADC low power scan mode

Jørgen Holmefjord — Mon, 03 Apr 2017 11:44:23 GMT

If you mean that you want to avoid the NRF_DRV_SAADC_EVT_DONE event, and still keep low power and only wakeup on channels crossing the limits, I don't think this is possible. You would not be able to uninit the SAADC between sampling times, meaning the additional EasyDMA current will be present.

RE: SAADC low power scan mode

FormerMember — Mon, 03 Apr 2017 09:35:02 GMT

Yes, I'm using it to set the limits.

RE: SAADC low power scan mode

Jørgen Holmefjord — Mon, 03 Apr 2017 08:59:32 GMT

Have you looked at the function nrf_drv_saadc_limits_set()?

RE: SAADC low power scan mode

FormerMember — Fri, 31 Mar 2017 14:09:43 GMT

I went through your code and see what you're doing. Thanks. My question still stands: Is there any low power way to have two channels sampling periodically with the processor waking up only if any of the two channels cross its limits? The question has been updated to provide some context.

RE: SAADC low power scan mode

FormerMember — Fri, 31 Mar 2017 13:44:59 GMT

Thanks Jørgen. I'll take a look and get back.

Regarding the burst mode and oversampling, I want what's described in the link that you sent. For both the channels being sampled, I would like 2^OVERSAMPLE samples to be averaged in each channel since the signal level in my application is now. This should be possible when the burst mode is enabled in both the channels, right?

RE: SAADC low power scan mode

Jørgen Holmefjord — Fri, 31 Mar 2017 09:24:13 GMT

Hi,

First, you should not use oversampling with scan mode (multiple channels enabled), due to the reason described here. Also, burst mode would not give you any benefit in this setup, and should be disabled.

I modified this example (based on SDK 11) to work with two channelse and a sampling rate of ~20 ms (make sure RTC frequency is set to 32768). With these settings, the PPK reports an average current consumption of ~30 µA. The example does use EasyDMA, but it will disable the SAADC periheral between sampling, which also turns off EasyDMA. I guess you can try to tweak it further, but at least it is better than the 6-700 you got. You can use this as reference for your own development if you want.

Note that the example is not thoroughly tested and is not an official release from Nordic.

saadc_low_power_multiple_channels.zip

Best regards,

Jørgen