Mbed-os: [API] Request API change configure ADC sampling time

[LMESTM]

Description

There have been 2 recent fixes where we needed to increase sampling time of ADCs #4326 and #4641,

This raises the question of optionally configuring this sampling time from the application, for instance
as MIN / MAX / DEFAULT or MEDIUM. That would require a modification of the AnalogIn.h API, whether in a new constructor variant or by adding a new method.

[LMESTM]

Adding more comments and suggestions from @ohagendorf regarding ADC:

Maybe two features or three would be nice to have:

setting sampling time
setting resolution
But curently for a fast conversion there is another huge problem. We analysed the adc read function:

float read() {
    lock();
    float ret = analogin_read(&_adc);
    unlock();
    return ret;
}

and lock()+unlock() need much more time than analogin_read(). I had to look into the project documentation for the exact numbers but it was something around 1.5µs for the conversion and 5...6µs for calling lock()+unlock() (Nucleo_F446RE).
One idea would be to add another read function with a callback under timer control or use continuous mode. In this case a fast equidistant conversion corresponding to the maximum conversion rate of the hardware would be possible. Or in combination with the above function to set the sampling time with a slower timing.
[Mirrored to Jira]

[adbridge]

Internal Jira reference: https://jira.arm.com/browse/IOTHAL-343

[farrenv]

I had a similar issue working with the Nordic nrf52840 dev kit. I had to step into the analogin function until i reached analogin_api.c. From here i had to modify the following from 10 microseconds to 40. This immediately improved the results with a battery monitoring circuit that we are using. It would be nice to not have to modify the OS to get this functionality.

https://github.com/ARMmbed/mbed-os/blob/a477354292a3c6b6478e6e5833a4a5fc90ccb432/targets/TARGET_NORDIC/TARGET_NRF5x/TARGET_NRF52/analogin_api.c#L90

[cmonr]

@elm3 ?

[elm3]

The request to extend the ADC interface is in the backlog of the appropriate team. I have just pinged them asking if there are plans that I can share.

[ciarmcom]

Thank you for raising this issue. Please note we have updated our policies and
now only defects should be raised directly in GitHub. Going forward questions and
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This GitHub issue will now be closed.