Question

I have a number that is “significant byte”, it may be 0 or 255.

Which means 0 or -1.

How to convert 255 to -1 in one time.

I have a function that doesn’t works for me:

acc->x = ((raw_data[1]) << 8) | raw_data[0];

Answer 1

Assuming that every 8th bit set to 1 means negative (254 == -2) then a widening conversion from signed types should do:

int n = (signed char)somebyte;

so

unsigned char rawdate[2] = ...;
int msbyte = (signed char)rawdata[1];
acc->x = (msbyte << 8) | (raw_data[0] & 0xFF);