samd/adc_dac: Implememt adc.read_timed() and dac.write_timed().

[robert-hh]

@dpgeorge As requested, this is the separate PR for ADC-timed and DAC-timed. Below
is a short documentation.

ADC Constructor:

ADC(Pin, *, average=16, bits=12, vref=3, callback=None)

The callback keyword option is used for timed ADC sampling. The callback is executed when all data has been sampled.

ADC Methods:

adc.read_u16()

Read single value from the Analog Pin using the bits and average setting from the constructors.

adc.read_timed(data, freq)

Read adc values into the data buffer at a supplied frequency. The buffer must be pre-allocated. Values are stored as 16 bit quantities in the binary range given by the bits option. If bits=12, the value range is 0-4095.
The voltage range is defined by the vref option.
If in the constructor a callback was defined, it will be called after all data has been read. Alternatively, the method busy() can be used to tell, if the capture has finished.

adc.busy()

busy() returns True while the data acquisition using read_timed() is ongoing, False otherwise.

adc.deinit()

Deinitialize as ADC object and release the resources used by it, especially the ADC channel and the timer used for read_timed().

DAC Constructor

DAC(id, *, vref=3, callback=None)

The resolution of the DAC is 12 bit for SAMD51 and 10 bit for SAMD21. SAMD21 devices have 1 DAC channel at GPIO PA02, accepting only 0 as id. SAMD51 devices have 2 DAC channels at GPIO PA02 and PA05 with values 0 and 1 for the id.

DAC Methods

dac.write(value)

Write a single value to the selected DAC output. The value range is 0-1023 for SAMD21 and 0-4095 for SAMD51. The voltage range depends on the vref setting.

dac.write_timed(data, freq [, count=1])

The call to dac_timed() allows to output a series of analogue values at a given rate. data must be a buffer with 16 bit values in the range of the DAC (10 bit of 12 bit). freq may have a range of 1Hz to ~200kHz for SAMD21 and 1 Hz to ~500kHz for SAMD51. The optional argument count specifies,
how often data output will be repeated. The range is 1 - 2**32. If count == 0, the data output will be repeated until stopped by a call to deinit(). If the data has been output count times, a callback will
be called, if given.

dac.busy()

Tells, whether a dac.write_timed() activity is ongoing. It returns True if yes, False otherwise.

dac.deinit()

Deinitialize the DAC and release the resources used by it, especially the DMA channel and the Timer. On most SAMD21 boards, there is just one timer available for dac.write_timed() and adc.read_timed_into(). So they cannot run both at the same time, and releasing the timer may be important. The DAC driver consumes a substantial amount of current. Calling deinit()
will reduce that as well.

[robert-hh]

Just a conflict resolved after rebase.

[robert-hh]

@dpgeorge You mentioned about this PR that you would have to consider a proper API for that ADC/DAC timed feature. I guess you did not have time and it got lost under all other requests you have. The actual PR's API is close to that of the STM32 port.

[robert-hh]

Re-based and Re-tested. Still fine.

[github-actions]

Code size report:

   bare-arm:    +0 +0.000% 
minimal x86:    +0 +0.000% 
   unix x64:    +0 +0.000% standard
      stm32:    +0 +0.000% PYBV10
     mimxrt:    +0 +0.000% TEENSY40
        rp2:    +0 +0.000% RPI_PICO
       samd: +2396 +0.903% ADAFRUIT_ITSYBITSY_M4_EXPRESS[incl +36(data) +304(bss)]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.39%. Comparing base (c1a6b95) to head (5efb48e).

Additional details and impacted files

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  Files         161      161           
  Lines       21204    21204           
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  Hits        20864    20864           
  Misses        340      340           

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