I don't think you'd want to do that. 127 deg C really isn't that terribly hot and -128C is extremely cold. If it is necessary to retain use of an 8 bit variable then the standard solution is to just offset the temperature stored by -40. That way you can read -40 to 215C which is a pretty reasonable range. Unfortunately, I fear such a change would necessitate many other changes in code. It might be better to expand to a signed 16 bit integer.