drivers/onewire/ds18x20.py: Mask power-on reset value 0550h

[amotl]

According to the specification of the DS18B20 sensor [1] (Page 6),

The power-on reset value of the temperature register is +85°C.

This commit prevents propagating this accidentally as a valid reading when starting the conversion has been missed.

[1] https://datasheets.maximintegrated.com/en/ds/DS18B20.pdf

[amotl]

Dear Damien,

this synchronizes the DS18B20 driver with a pending pull request to its pure-Python variant [1,2], which is the only thing which can be used by people unable to use the native variant yet, e.g. on Pycom MicroPython.

With kind regards,
Andreas.

cc @robert-hh

[1] robert-hh/Onewire_DS18X20#7
[2] robert-hh/Onewire_DS18X20#8

[amotl]

Dear Damien,

a little bit off-topic to this, we also wanted to let you know that we just have been able to unlock native 1-Wire/DS18X20-support for Pycom MicroPython through pycom/pycom-micropython-sigfox#356.

Thankfully, Pycom upgraded to MicroPython 1.11 the other day so the patch was rather small.

Thanks again for your hard work on Genuine MicroPython and keep up the spirit.

With kind regards,
Andreas.

[amotl]

Also trying to add this to @robert-hh's pure-Python driver at https://github.com/robert-hh/Onewire_DS18X20, he commented on robert-hh/Onewire_DS18X20#8:

I intentionally did not implement this test. 85°C is a valid result, which must not be suppressed by the driver. Checking must be done by the calling code, which knows the context.

He is absolutely right, so I will retract this PR. Sorry for the noise.

---

A member of our community showed us [1] how the corresponding ethersex driver uses a semaphore for propagating the conversion error. This way, it the condition can be implemented within the driver, but the caller may decide about its outcome. Thanks @weef!

• Define semaphore:
  https://github.com/ethersex/ethersex/blob/master/hardware/onewire/onewire.h#L265-L266
• Set semaphore:
  https://github.com/ethersex/ethersex/blob/master/hardware/onewire/onewire.c#L813-L820
• Evaluate semaphore:
  https://github.com/ethersex/ethersex/blob/master/control6/examples/mqtt.src#L160-L166

[1] https://community.hiveeyes.org/t/untersuchung-und-verbesserung-des-timings-bei-der-ansteuerung-der-ds18b20-sensoren-unter-micropython/2309/103