SAADC Offset Calibration not working?

RE: SAADC Offset Calibration not working?

aeozyalcin — Tue, 11 Aug 2020 07:24:47 GMT

Thank you for the suggestion. It seems that my issue was calling nrf_drv_saadc_buffer_convert before calling the calibrate function. It looks like after calling nrf_drv_saadc_buffer_convert, SAADC expects me to start a conversion instead of calibration.

RE: SAADC Offset Calibration not working?

haakonsh — Mon, 10 Aug 2020 08:49:44 GMT

You can try to wait for the nrfx_saadc_is_busy flag with a 'while' loop.
while(!nrfx_saadc_is_busy ())
{
// Calibrate offset
}

I also suggest that you calibrate the offset before the first time you call nrf_drv_saadc_buffer_convert (in your init function).

A third option is to use the new SAADC v2 driver in nRF5 SDK7.

RE: SAADC Offset Calibration not working?

aeozyalcin — Sun, 09 Aug 2020 00:55:48 GMT

Thanks for the suggestions. I tried a few things based on your last comment:

I tried calling abort after initializing SAADC, and then calling calibration afterwards. However, calibration call still returns NRFX_ERROR_BUSY when called after the abort. I even tried adding some delays in between and no luck. I also tried calling nrfx_saadc_buffer_convert between the abort and the calibration call, but no go here either. Didn't make a difference.
Calling abort does not create an NRF_DRV_SAADC_EVT_DONE event, since there was already no conversion taking place when I called it. I am calling abort right after initializing the SAADC so I wouldn't expect there to be conversions taking place anyway.
When I omit the calibration call altogether, and just call abort after initializing SAADC, and then try to kick off a conversion 1 second after calling abort, nrf_drv_saadc_sample returns NRFX_ERROR_INVALID_STATE. This happens regardless of whether I call nrfx_saadc_buffer_convert between the abort and the nrf_drv_saadc_sample functions. Somehow, using the abort function puts the SAADC in a bad state that I can't seem to recover from.
Finally, everything works fine if I just give up on calibration. I can init the SAADC every minute, then call nrfx_saadc_buffer_convert, and then deinit SAADC and go back to sleep without any issues. It's when I try to do calibration and abort functions when things go wrong.

Could I ask that you take another look at my SAADC initialization function I shared in my original post, and let me know if you see anything wrong that I am doing?

RE: SAADC Offset Calibration not working?

haakonsh — Thu, 06 Aug 2020 11:00:27 GMT

Stop the SAADC before you start a calibration. Use nrfx_saadc_abort. After the calibration is complete you need to set-up the buffers before you start sampling again, with nrfx_saadc_buffer_convert.
If you take one sample with 32x oversampling your can count the number of times the NRF_DRV_SAADC_EVT_DONE has fired.

RE: SAADC Offset Calibration not working?

aeozyalcin — Thu, 06 Aug 2020 05:10:33 GMT

Thanks for the reply and the suggestions!

1. The call to nrf_drv_saadc_calibrate_offset is returning NRFX_ERROR_BUSY as you may have suspected, which explains why there is no SAADC event triggering the SAADC callback function. I even tried waiting until the function returns a success (like what I have below), but I don't think it ever returns success. My program gets stuck in the while loop.

while(nrf_drv_saadc_calibrate_offset() != NRF_SUCCESS);

You can see that I call the calibrate function right after initializing the SAADC. Is there an SAADC call in my init function that doesn't block, but needs time to finish that I am missing?

2. I am not sure I understand your reply here. Let me rephrase my question: Does SAADC write the value to my supplied buffer only after it has finished taking all the "oversample" samples, and averaged them?

Thanks

RE: SAADC Offset Calibration not working?

haakonsh — Tue, 04 Aug 2020 13:12:49 GMT

I suggest you catch the error codes that our API's use, specifically the one that nrf_drv_saadc_calibrate_offset returns.
I don't know the definition of a conversion vs sample with regards to oversampling. I guess you can write a test and see whether it is true or not.