Understand checksum value

[MatteoRM]

Hi,

i’ve an UM7 and i’m trying to implement it on my system.I’m using Binary Packet and i’ve a question about checksum.

“The two checksum bytes consist of the unsigned 16-bit sum of all preceding bytes in the packet, including the packet header. ”

I don’t understand it.So,i have to sum the value of pt,address and databytes(PT+ADDRESS+DATABYTES) and check if it is equal to checksum1 or checksum 0(or checksum 0 appended to checksum1?)?

Thanks!

[till]

hi matteo,

Please see UM7 Datasheet page 37:

uint16_t computed_checksum = 's' + 'n' + 'p' + packet_data->PT + packet_data->Address;
for( index = 0; index < data_length; index++ ) {
  packet->data[index] = rx_data[packet_index + 5 + index];
  computed_checksum += packet->data[index];
}
uint16_t received_checksum = (rx_data[packet_index + 5 + data_length] << 8);
received_checksum |= rx_data[packet_index + 6 + data_length];

checksum1 and checksum0 are just the two bytes making up the 16-bit short integer. you take a 16-bit integer initialized to 0 and then add the value of each byte in the packet to calculate the checksum.

the received checksum consists of two bytes which you have to combine to a 16-bit integer.

regards,

-till

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