Bug in packetFrequencyError

[Nick507]

In function LoRaClass::packetFrequencyError()
there is a wrong conversion: freqError -= 524288;
it must be freqError -= 1048576;
because 1<<20 = 1048576

[torntrousers]

Hello @Nick507, I added that packetFrequencyError function, back in 2018, so i don't remember the code super well! I think it might have been derived from code from a Semtech library. Could you give any more explanation on how you found the problem or why you think its a bug?

[Nick507]

Hello Anthony, according to sx127x datasheet, frequency error value is 20 bit two's complement value (4.1.5. Frequency Error Indication). The algorithm of converting negative value from two's complement form is to substract 2^N from that value, where N is number of bits, which is 20 in this case.
https://en.wikipedia.org/wiki/Two%27s_complement
524288 is the 2^19.
The simple example:
1111 - is -1 for 4bit two's complement value (according to wiki)
most significant bit is set, it means that this is negative value, and need to convert it: 15-16=-1, where
15=1111 (our value)
16=2^4=1<<4