What does the Nordic device actually do when oversampling is selected for the ADC

[robin]

Hi

I used your example to try out the ADC.

If I have 10 or 12 bit ADC selected and I am collecting 4 samples as per the example and I select 8x oversampling I assume this effectively 32x oversampling if I add the 4 samples and divide by 4.

What I want to know is what does selecting 8x oversampling actually do

Do you take 8 samples and then provide 1 result ?  If so then I could simply take 8 samples and do the same.

Or does your HW do something different ?

I am trying to fully understand how your ADC works as we need this for our application.

Robin

[Håkon Alseth]

Hi Robin,

 

 

If I have 10 or 12 bit ADC selected and I am collecting 4 samples as per the example and I select 8x oversampling I assume this effectively 32x oversampling if I add the 4 samples and divide by 4.

What I want to know is what does selecting 8x oversampling actually do

Do you take 8 samples and then provide 1 result ?  If so then I could simply take 8 samples and do the same.

Or does your HW do something different ?

Yes, in your case, the hardware will do 32x samples and provide 4x samples to the application when it's done.

 

The SAADC peripheral, when configured with 8x oversampling, will then take 8 samples for every 1 sample that is transferred to the application.

When oversampling is enabled, you get a EVENTS_DONE for every sample that is taken, and a EVENTS_RESULTDONE when the oversample process is finished and the result is transferred to RAM.

If BURST is not enabled, you then have to trigger the TASKS_SAMPLE 8 times in the case you have oversampling=8x, as described in the PS:

http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/saadc.html?cp=2_1_0_36_4_2#saadc_operationmodes_oversample

 

Kind regards,

Håkon

[robin]

Hi

thanks for the reply

I think I understand this but just to clarify can you answer the following

I am using  nRF5_SDK_15.2.0_9412b96\examples\peripheral\saadc

example as the base for this, with some modification.

Sampling event is setup for 1ms
With the SAMPLES_IN_BUFFER set to 1 only
Acquisition time 10us
ADC 10 bit
Oversample 8x

SO with the above settings I think the hw/sw will perform like this
Every 1ms it will collect 8 samples but the result sample will be 8 samples divided by 8, is this correct ?

How will the samples be collected ?
Is the acquisition window opened 8 x 10us periods and if so what is the time between each acquisition  window opening?

Also if I change the SAMPLES_IN_BUFFER from 1 to 4 for example
Does this mean 4 result samples will be collected once every 1ms or does it mean there will be 4 result samples available after 4 ms.
If all 4 result samples are collected once every 1 ms following on from previous question does that
mean there will be 8 x 4 10us acquisition periods. IE the acquisition window opens 8 times for each result sample, meaning it opens and closes 32 times.

Thanks Robin

[Håkon Alseth]

Hi Robin,

 

 

robin said:

Sampling event is setup for 1ms
With the SAMPLES_IN_BUFFER set to 1 only
Acquisition time 10us
ADC 10 bit
Oversample 8x

SO with the above settings I think the hw/sw will perform like this
Every 1ms it will collect 8 samples but the result sample will be 8 samples divided by 8, is this correct ?

Yes, that is correct.

If you set 8x oversampling, and 10 acq time, it will take 8 * 10 us to finish. The finished sample will then be already averaged for you.

robin said:

How will the samples be collected ?
Is the acquisition window opened 8 x 10us periods and if so what is the time between each acquisition  window opening?

It will be 8 * 10 us for this scenario.

robin said:

Also if I change the SAMPLES_IN_BUFFER from 1 to 4 for example
Does this mean 4 result samples will be collected once every 1ms or does it mean there will be 4 result samples available after 4 ms.
If all 4 result samples are collected once every 1 ms following on from previous question does that
mean there will be 8 x 4 10us acquisition periods. IE the acquisition window opens 8 times for each result sample, meaning it opens and closes 32 times.

 This will mean that all 4 will be available after 1 ms, resulting in 8 x 4 *10 us period.

 

Kind regards,

Håkon