SAADC Burst oversampling

RE: SAADC Burst oversampling

BEN_HB — Mon, 18 Nov 2019 01:15:42 GMT

Thank you.

RE: SAADC Burst oversampling

Einar Thorsrud — Fri, 15 Nov 2019 11:34:47 GMT

Hi,

I see. Then this may be related to erratum 86. You can employ the workaround or simply ignore the first sample after calibration. Note that there is no need to calibrate frequently. The SAADC has a temperature-dependent offset, but the temperature does not change significantly you can ignore this. Alternatively, you can calibrate only if the temperature has changed significantly, or by other methods.

1. We do not have official numbers on that. Essentially you should always calibrate at least once.

2. Yes, the calibration value is still valid, since this does not reset the peripheral itself.

3. I was not clear. Setting the value to 8 gives you 256 (2^8), but if you are referring to the numbers used in the define names in nrfx, then 8x is mapped to 3, which is 8 (2^3=8). Similarly, 256x is mapped to 8, which is 2^8=256. This is the highest possible value.

RE: SAADC Burst oversampling

BEN_HB — Thu, 14 Nov 2019 07:02:42 GMT

yes right.

but in my code, first trigger and second trigger sample task is not result normally.

I found the cause.

If I remove calibration func, my issue is not happened.

I executed calibration before trigger sample.

1. If calibration is not excute, How much adc value is error depend on temperature? max and min

2. if after calibration, saadc uninit and re saadc init, the calibration offset value is still remain?

after calibration, I try saadc uninit and one more init saadc, the issue is disppear.

3. if select 256x, is it right 2^256???. it too huge number. It take so long time.

thanks you,

RE: SAADC Burst oversampling

Einar Thorsrud — Wed, 13 Nov 2019 13:41:04 GMT

Hi,

[quote user=""]1. I saw document and if set oversampling is 8x means that sampled as fast as 2^8 and averaged the values? is it right?[/quote]

Yes. Oversampling 8x means 2^8=256. And since you also use burst you just need to trigger the SAMPLE task once for each oversampled sample, instead of once for every raw sample.

[quote user=""]2. 8x oversampling value is average of 256 sampled value?[/quote]

Yes.

[quote user=""][/quote]

after first nrf_drv_saadc_sample_task_get() , NRF_DRV_SAADC_EVT_DONE is not triggerd.

after second nrf_drv_saadc_sample_task_get(), NRF_DRV_SAADC_EVT_DONE is triggered but value is strange. always minus value. number is random.

I do not immediately spot a problem. But I also dont understand the comment. Calling nrf_drv_saadc_sample_task_get() multiple times should not be relevant. Do you mean nrf_drv_saadc_buffer_convert()?

In any case, you should just need to trigger the SAMPLE task once, this is the basic difference between using burst mode and not. (if using burst mode, trigger SAMPLE task once, and oversampling happens automatically. If not, trigger SAMPLE every time for as many times as you have configured oversampling.