SAADC pins switch

[bart]

Hi,

I use the SAADC to sample 2 pins. I use the nrf_drv_saadc, and a timer to call the sample task.

I was trying out several gains for SAADC and I noticed that for gains 1 and 2 (4 not tested), the output buffer switched around the results of the 2 pins.

Can anyone think of a reason for this to happen?

Note: starting at gain 1, the input gets clipped because the input signal goes above 0.6V

Config:

NRF_SAADC_RESOLUTION_12BIT
NRF_SAADC_OVERSAMPLE_DISABLED
NRF_SAADC_RESISTOR_DISABLED
NRF_SAADC_REFERENCE_INTERNAL
NRF_SAADC_ACQTIME_10US
NRF_SAADC_MODE_SINGLE_ENDED

[bart]

Update

Answered here.

Update

Looks like it's fixed by defining NRF52_PAN_28, even though i'm on a PCA10040, which doesn't have that PAN listed in the docs as far as i know.

Note: The first sample in the batch is sometimes still incorrect.

Old answer

I may have found the bug: I used the pin number also as channel number, so in my case i initialized channel 1 and 2. So it was probably luck that it worked at all. nrf_drv_saadc_channel_init(pinNum, &channelConfig); should've been nrf_drv_saadc_channel_init(channelNum, &channelConfig);

[bart]

Btw: can it be that the SAMPLE task is called before the previous sample was done? I'm sampling with an interval of 200μs, and since it's pretty much random when it happens, i suspect a timing issue.