Calculate and verify checksums
To test data integrity, the device calculates and verifies a checksum on non-escaped data.
To calculate the checksum of an API frame:
- Add all bytes of the packet, except the start delimiter 0x7E and the length (the second and third bytes).
- Keep only the lowest 8 bits from the result.
- Subtract this quantity from 0xFF.
To verify the checksum of an API frame:
- Add all bytes including the checksum; do not include the delimiter and length.
- If the checksum is correct, the last two digits on the far right of the sum equal 0xFF.
Example
Consider the following sample data packet: 7E 00 0A 01 01 50 01 00 48 65 6C 6C 6F B8+
Byte(s) | Description |
---|---|
7E | Start delimeter |
00 0A | Length bytes |
01 | API identifier |
01 | API frame ID |
50 01 | Destination address low |
00 | Option byte |
48 65 6C 6C 6F | Data packet |
B8 | Checksum |
To calculate the check sum you add all bytes of the packet, excluding the frame delimiter 7E and the length (the second and third bytes):
7E 00 0A 01 01 50 01 00 48 65 6C 6C 6F B8
Add these hex bytes:
01 + 01 + 50 + 01 + 00 + 48 + 65 + 6C + 6C + 6F = 247
If an API data packet is composed with an incorrect checksum, the XBee/XBee-PRO Zigbee RF Module will consider the packet invalid and will ignore the data.
To verify the check sum of an API packet add all bytes including the checksum (do not include the delimiter and length) and if correct, the last two far right digits of the sum will equal FF.
01 + 01 + 50 + 01 + 00 + 48 + 65 + 6C + 6C + 6F + B8 = 2FF