power consumption increases 1.1mA after first ADC sample

[Allen]

Hello,

I am experiencing a power issue after the first ADC sample is triggered. Sampling 2 ADC channels using PPI and TIMER as the triggering mechanism. Here is a capture using PPK2.

It is alright to have a spike in power but in this case, once the power increases it never decreases.

Is there a way to fix this?

TIA

[Jørgen Holmefjord]

Hi, 

The increase in current comes from the EasyDMA being enabled and drawing current once the SAADC START task is triggered.

In order to reduce the current, you can only have a buffer size equal to the number of active channels enabled, and not use the double-buffering feature.

There is a low-power mode in the SAADC driver that you can try to enable, but we have previously seen some issues with this together with enabling multiple channels. If you are using LP mode, it is important that you use the function nrf_drv_saadc_sample_task_get() when setting up the PPI channels, is this will trigger the START task over PPI and not the SAMPLE task. The SAMPLE task will be triggered in the IRQ handler when the STARTED event is generated.

The post provides another approach where the SAADC driver is uninit and re-initialized between each sampling, which should work with multiple channels.

Best regards,
Jørgen

[Jørgen Holmefjord]

The SPIM peripheral will consume the same amount of current when it is transferring data, but this is normally limited to a shorter period of time, as you start a transfer that finishes when all data have been transferred. The difference to the SAADC peripheral is that the SAADC is started when you configure the buffers, but the sampling/actual transfer of data to RAM happens when you trigger the SAMPLE task. Since the SAADC peripheral does not know when the sampling will happen, it is ready to transfer data to RAM at any time. Ideally, the EasyDMA access should only have been enabled after sample task has happened and result is transferred to RAM, but this is not how the design works.

You will see similar current draw if you enable UARTE in RX mode, as this needs to be ready to transfer received data to RAM at any given time.