Add analogReference modes for the Arduino 33 BLE

[slokhorst]

Found at https://github.com/arduino/ArduinoCore-mbed/blob/master/variants/ARDUINO_NANO33BLE/pins_arduino.h#L13

Fixes #755

[per1234]

Thanks so much for this important contribution @slokhorst. There is one change I require. Once that's done, I'll get this merged ASAP.

[slokhorst]

Apparently analogReference support was just backported today to the "old" ArduinoCore-nRF528x-mbedos in arduino/ArduinoCore-nRF528x-mbedos#81, making it available in both platforms. I'm not sure when that will actually be available in a released version though.

[BallscrewBob]

Should these changes not also be mentioned in the reference section of the website as there has been a couple if mentions about the lack of documentation there by at least two users.

https://forum.arduino.cc/index.php?topic=684533.new#new

It would make sense to document those changes for users to be aware of them.

[slokhorst]

@BallscrewBob agreed. It took me a lot of searching to find out that there were two different platforms that support the 33 BLE.

It would be nice if there would be some kind of public roadmap for the two platforms. The only thing I could find was arduino/ArduinoCore-nRF528x-mbedos#81, which states:

This should be the last maintenance release of this core; since this core is based on mbed 5.x branch all functions marked for deprecation are still there; arduino/ArduinoCore-mbed, on the other hand, contains all the new stuff and should replace this core as soon as considered stable.

@facchinm do you know more? If/when is ArduinoCore-mbed expected to replace ArduinoCore-nRF528x-mbedos? What feature differences are there between the two platforms? Is there already documentation available on this that we overlooked?

[per1234]

Apparently analogReference support was just backported today to the "old" ArduinoCore-nRF528x-mbedos in arduino/ArduinoCore-nRF528x-mbedos#81, making it available in both platforms. I'm not sure when that will actually be available in a released version though.

Thanks for the clarification @slokhorst. We shouldn't mention that in the reference now, because as you say it hasn't yet been released and so isn't available to the readers of the Arduino Language Reference. When/if there is a release of the "Arduino nRF5248x Boards (Mbed OS)" platform, I would welcome a PR to update this information accordingly.

Should these changes not also be mentioned in the reference section of the website

@BallscrewBob this repository contains the content of the Arduino Language Reference:
https://www.arduino.cc/reference/en/

Arduino set up this really awesome system that makes it so that, after a pull request is merged in this repository, those changes will be published to the Arduino Language Reference via a completely automated system. This means we should soon see the content @slokhorst contributed here also added to the analogReference() reference page without any additional human effort required.