XBEE Packet Calculation

xbee.c

#include <stdio.h>
#include <stdint.h>

#define CRC_LOCATION 13
#define DATA_START 3

int main(void) {
	// your code goes here
	uint8_t packet[] = {0x7E, 0x00, 0x0A, 0x01, 0x01, 0x50, 0x01, 0x00, 0x48, 0x65, 0x6C, 0x6C, 0x6F, 0X00};
	
	printf ("CRC: 0x%.2X\n", packet[CRC_LOCATION]);
	
	// Check if the CRC is valid
	uint16_t checksum = 0x00;
	for (int i = 3; i < sizeof(packet); i++) {
	    printf ("byte %d: 0x%.2X\n", i, packet[i]);
	    checksum += packet[i];
	}
	
	checksum &= 0x00FF;
	printf ("Checksum value: 0x%.2X\n", checksum);
	
	printf ("Checksum is %s.\n", (0xFF == checksum) ? "valid" : "invalid");
	
	// Calculate the correct checksum
	checksum = 0x00;
	packet[CRC_LOCATION] = 0x00;
	for (int i = 3; i < sizeof(packet); i++) {
	    printf ("byte %d: 0x%.2X\n", i, packet[i]);
	    checksum += packet[i];
	}
	checksum &= 0x00FF;
	checksum = 0xFF - checksum;
	
	printf ("Correct Checksum: 0x%.2X\n", checksum);
	return 0;
}