[nRF52] Best practise for ultra low power SAADC 4 channel conversion

[Mateusz Gancarczyk]

What is the best practise for multichannel SAADC conversion in ultra low power devices? For example I have to measured 4 channel in interval 250ms. Below what I try:

1. I'm set RTC for interupt 250ms. In RTC handler I initialized SAADC channel 0 and start conversion. In SAADC end of conversion handler i uintialized SAADC, get result from channel 0 and initialized for channel 1 and start conversion. Same for channels 3 and 4 but in channel 4 I'm not start new conversion but Im set RTC for 333ms Handler. I try it and I get strange result.

If I have only one channel I've get current consuption:

avarage current is around 10uA ( it's ok for my aplication ). but if I enabled conversion for 4 channel, one by one ( as I descripted above ) I've got something like that:

Current is 2 times higher! I don't know why density of the current spikes is in that level.

Mayby I should use PPI? i read this topic: devzone.nordicsemi.com/.../

or maybe different method?

[Jørgen Holmefjord]

Hi,

It seems there is some issues with using scan mode when the low-power mode in the SAADC driver is enabled. I will look into this.

I ported the method used in the saadc_low_power example from SDK 11, and added this to SDK 13 example:

main.c

This gives me an average current draw of just over 4 uA.

Best regards,

Jørgen

[vchhabra]

Hello Jorgen,

Was this issue resolved?

I am having the same issue with a nRF52840 using SDK 15.2 and S140 v6.1.

I get ~140uA on 256Hz with single channel but it goes up to 740uA with 2 channels and 760uA with 4 channels.

It seems like something is triggered like the HFCLK causing the baseline current to go high.

[Jørgen Holmefjord]

I'm not sure which issue you are referring. I have tested my above example with SDK 15.3.0, and I'm seeing ~3.6 µA average current with 4 Hz sample rate. With 256 Hz, current increase to ~63 µA average, both on PCA10056 (nRF52840 DK) with 4 channels enabled.

I have attached the project: saadc_low_power_multichannel_sdk15_3_0.zip

[vchhabra]

Hello Jorgen,

Let me elaborate a little, the code you provided me meets the specs i need but I am trying to not init and un-init the SAADC every time I have to take a sample.

I believe that once the channels are configured the Easy-DMA never goes to sleep even in low power mode giving a much larger current average than I would like. I wanted to use the non blocking call to store like 1000 values at a stretch with the SAADC connected to the RTC TICK via PPI but the higher current read makes that impossible.

I could not find anything related to this in the errata, could you please tell me if there is any way of doing that?