Parsing H4/H5 humidity calibration data

[AmedeeBulle]

According to the datasheet, dig_H4 and dig_H5 are signed integers on 12 bits

The following code handles these as pure 16bits signed, which will never be negative...

https://github.com/finitespace/BME280/blob/7a211f03aa3ac5567b14e2cbc12ac21e185b6e0b/src/BME280.cpp#L277-L278

The datasheet is ambiguous as it refers to these 12 bits values as signed short, but the BoschSensortec reference code definitely handles the values as 12 bits signed integer -- the (int8_t)cast on the msb together with a multiplication instead of a shift ensure the sign is propagated properly to the int16_t

[Bolukan]

formula: x = (x >> 11) == 0 ? x : -1 ^ 0xFFF | x;

PS: Checked all my BME280's (4), but all calibrated with small positive dig_H4 and dig_H5.

[finitespace]

This code was taken directly from the Bosch example code.

Have you seen adverse affects due to it?

[AmedeeBulle]

I am not sure which Bosch example code you are referring to, but in all the Bosch code I have seen, dig_H4 and dig_H5 are taken as signed integers on 12 bits.

On the sensors I have, dig_H4 and dig_H5 are positive so it doesn't make a difference, but it would if they were negative.

[Bolukan]

Don't they cast it to signed? dig_H4_msb = (int16_t)(int8_t)reg_data[3] * 16;

[AmedeeBulle]

Yes they do, and this is what I am proposing in #103