Feat: multi-sample averaging and 50Hz mains filtering

[eirinnm]

This PR implements two features from the datasheet:

• On-chip multi-sample averaging (1,2,4,8,16 samples). Default is 1 (no averaging). Using on-chip averaging is much faster than retrieving individual samples and averaging them in user code.
• 50Hz mains frequency notch filter. Default is off (will filter 60Hz instead). Missing feature: 50/60Hz noise filtering #18

[ladyada]

hi please create functions to set/get the freq filter (none, 50, 60?) and oversampling rather than setting once in init :)

[eirinnm]

Thanks for the feedback! I've created functions set_sampling and select_mains_filtering so that these options can be changed after init. For most users they don't have to be called, because the chip has sane defaults (single sampling and 60Hz filter).

[caternuson]

Code logic looks generally OK. But instead of actual function functions, CP lib style is to use Python property feature:
https://docs.circuitpython.org/en/latest/docs/design_guide.html#getters-setters

Look at the temperature_thresholds property in this library for an example.

With that form, the following syntax is how they would get used in user code to set:

thermocouple.averaging = 16
thermocouple.noise_rejection = True

and to query (get):

print("Averaging =", thermocouple.averaging)
print("Noise Rejection =", thermocouple.noise_rejection)

In the above example code snippets, averaging and noise_rejection are the suggested property names, similar to temperature_thresholds.

[eirinnm]

Yes I see. Thanks for the advice! I've implemented it using property getter/setters now.

[dhalbert]

One spelling fix.

[caternuson]

LGTM. Nice job switching to property approach!

Functionally tested, without a TC connected, on Itsy M0:

Adafruit CircuitPython 7.3.2 on 2022-07-20; Adafruit ItsyBitsy M0 Express with samd21g18
>>> import board
>>> import adafruit_max31856
>>> import digitalio
>>> cs = digitalio.DigitalInOut(board.D10)
>>> cs.switch_to_output()
>>> max = adafruit_max31856.MAX31856(board.SPI(), cs)
>>> max.reference_temperature
24.375
>>> max.averaging
1
>>> max.averaging = 16
>>> max.averaging
16
>>> max.averaging = 42
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "adafruit_max31856.py", line 190, in averaging
ValueError: Num_samples must be one of 1,2,4,8,16
>>> max.noise_rejection
60
>>> max.noise_rejection = 50
>>> max.noise_rejection
50
>>> max.noise_rejection = 23
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "adafruit_max31856.py", line 218, in noise_rejection
ValueError: Frequency must be 50 or 60
>>>  

[eirinnm]

Thanks all, I've enjoyed contributing to this library.

[FoamyGuy]

Looks good to me, verified the open change requested was completed.

Thanks for the improvement @eirinnm