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

[bjorn-spockeli]

How large is the buffer that you have allocated? It would be nice if you could share the initialization as well as the callback code for the SAADC driver.

[bart]

I fear that would be a bit too much code for here, but here's a link to the file. Currently, I have two buffers of nrf_saadc_value_t[150]

[bjorn-spockeli]

@bart: Sorry for responding so late. I have only seen the order of the measurements being switched when the number of samples each time the sample task is called divided by the buffer-size is un-even, i.e. the last sample does not fit in the buffer and is placed in the second buffer.

[bart]

Heh, yeah, I thought of something like that too, but then i should see it switching every X samples. In my case, it switched only when i changed the gain (and recompiled etc).

[bjorn-spockeli]

Hi Bart, sorry for revisiting this so late, again. I can't see any reason to why you should see this issue if you've not set a un-even buffer size. However, I took a look at the code you linked to and I just saw that its code for the nRF51 ADC and not the nRF52s SAADC.