I just have a simple question about how the Checksum algorithm works for a particular Bluetooth module (BM62). The picture above has the UART protocol explained, and it explains the checksum rule, but I am having trouble understanding how it actually works, and cannot seem to guess the checksum value as it is given in the example in the picture.
The idea seems to be that you need to come up with CHKSUM such that LENH + LENL + OPCODE + PARAM + CHKSUM has 0 in the least significant byte. So, let's do the summation in 8 bits (or modulo 256):
LENH + LENL + OPCODE + PARAM + CHKSUM = 0
CHKSUM = -(LENH + LENL + OPCODE + PARAM)
IOW, CHKSUM = -(0 + 2 + 1 + 0) = -3 = 0xFD. (Remember that all of this was done in 8 bits).
You can verify that CHKSUM satisfies the requirement (you're now doing everything in 16 bits):
0 + 2 + 1 + 0 + 0xFD = 0x100
And that has 0 in the least significant byte. If we did this in 8 bits as well, we'd get 0 instead of 0x100 and that would also be a valid way to check correctness.