EEPROM Checksum is either last char or last char+1

[drf5n]

grbl/eeprom.c: In function 'memcpy_to_eeprom_with_checksum': grbl/eeprom.c:133:26: warning: '<<' in boolean context, did you mean '' ? [-Wint-in-bool-context] 133 | checksum = (checksum << 1) || (checksum >> 7); | ~~~~~~~~~~^~~~~ grbl/eeprom.c: In function 'memcpy_from_eeprom_with_checksum': grbl/eeprom.c:144:26: warning: '<<' in boolean context, did you mean '' ? [-Wint-in-bool-context] 144 | checksum = (checksum << 1) || (checksum >> 7); | ~~~~~~~~~~^~~~~

That looks like it was a typo -- it squashes the checksum down to a single bit of information, when it looks like it intended to roll it right one bit, so the final checksum ends up either zero or one larger than the last character in the data:

grbl-Mega/grbl/eeprom.c

Lines 130 to 149 in df87b36

	void memcpy_to_eeprom_with_checksum(unsigned int destination, char *source, unsigned int size) {
	unsigned char checksum = 0;
	for(; size > 0; size--) {
	checksum = (checksum << 1) || (checksum >> 7);
	checksum += *source;
	eeprom_put_char(destination++, *(source++));
	}
	eeprom_put_char(destination, checksum);
	}
	int memcpy_from_eeprom_with_checksum(char *destination, unsigned int source, unsigned int size) {
	unsigned char data, checksum = 0;
	for(; size > 0; size--) {
	data = eeprom_get_char(source++);
	checksum = (checksum << 1) || (checksum >> 7);
	checksum += data;
	*(destination++) = data;
	}
	return(checksum == eeprom_get_char(source));
	}

Maybe it was supposed to be

checksum = (checksum << 1) | (checksum >> 7);

Originally posted by @drf5n in #157 (comment)

[drf5n]

See grblHAL's checksum implementation in https://github.com/grblHAL/core/blob/3a84b58d301f04279268b4ef1045fd6bc0961be5/nuts_bolts.c#L267-L278

[drf5n]

See also GRBL issue # grbl#1249