Check nRF52 ADC VREF

[microbuilder]

The nRF52 ADC can be configured to use an internal +/-0.6V ref or external VDD: http://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.nrf52832.ps.v1.1%2Fsaadc.html&cp=2_2_0_36_7&anchor=concept_kxz_4pp_qr

Make sure we can easily switch between references with the ADC code on the Arduino side, and check that the change actually works.

• Add a reasonably complete ADC example sketch where the user case set:
  • ADC Resolution (via analogReadResolution(14))
  • ADC reference (via analogReference)
• Add a helper function to wiring_analog_nRF52.c to set the Gain???
• Add a helper function to wiring_analog_nRF52.c to set the acquisition time?

Also ... set the default ADC parameters to the following:

• Internal Reference (+/-0.6V)
• Default gain 1/6 (0..3.6V range)
• 10-bit output
• 3us acquisition time

Current defaults aren't useful since the gain is hard coded to '1':

NRF_SAADC->CH[0].CONFIG = ((SAADC_CH_CONFIG_RESP_Bypass   << SAADC_CH_CONFIG_RESP_Pos)   & SAADC_CH_CONFIG_RESP_Msk)
                            | ((SAADC_CH_CONFIG_RESP_Bypass   << SAADC_CH_CONFIG_RESN_Pos)   & SAADC_CH_CONFIG_RESN_Msk)
                            | ((SAADC_CH_CONFIG_GAIN_Gain1    << SAADC_CH_CONFIG_GAIN_Pos)   & SAADC_CH_CONFIG_GAIN_Msk)
                            | ((saadcReference                << SAADC_CH_CONFIG_REFSEL_Pos) & SAADC_CH_CONFIG_REFSEL_Msk)
                            | ((SAADC_CH_CONFIG_TACQ_3us      << SAADC_CH_CONFIG_TACQ_Pos)   & SAADC_CH_CONFIG_TACQ_Msk)
                            | ((SAADC_CH_CONFIG_MODE_SE       << SAADC_CH_CONFIG_MODE_Pos)   & SAADC_CH_CONFIG_MODE_Msk);

See also #14

[microbuilder]

Defaults updates via a9e0508

[hathach]

I don't know much about these analog stuffs, there is also a commit from sandeep repo for this matter sandeepmistry/arduino-nRF5@c98a190

and I made a merge update from original repo df80f2e

Please revise to make sure I does not make any stupid mistakes :)

[microbuilder]

I've already fixed this, but thanks for the reference to Sandeep's changed. I might change my own code to stay compatible with that.

[microbuilder]

Don't worry about the helper for the VBAT either, I'll take care of that.

[microbuilder]

Fixed this with c834c6b and also set the default to 1/6 and internal which is what we'll use as a default, but this keeps it mostly consistent with Sandeep's code as well.