Calculate 8 Bit Checksum on 22 Byte Array?

Using Arduino Programming Questions
1

I'm trying to calculate a simple 8 bit checksum on a 22 byte array sent across the serial line.

The structure for the communication is as follows:

Byte[0] = 255
Byte [1] - [20] = Data (values 0-254)
Byte [21] = Checksum

On the receiver, it states the checksum is calculated as as the sum of all bytes, however i'm calculating different values then the receiver is calculating. I thought perhaps it could be because of byte overflow?

Can anyone point out what i am doing wrong?

byte chk = 0;
  for ( int i=0; i<length; i++)
    chk += data[i];
2

You didn't show us where length is defined or how it gets its value.

3

Show us your values and what the receiver calculates.

4

Is 'length' the length of the entire message, including the checksum?

5

If you are receiving an Intel-hex formatted file, then calculate the CHKSUM in the following way:

Given below a real example of an Intel-hex frame received over USART Port on 30/6/2005:

:10C10000003E9032002116101E110133C10ACD19D4

: 10 C100 00 003E9032002116101E110133C10ACD19 D4
(a) (b) (c) (d) (e) <---------------------------------------> (f)

(a) indicates that a new data frame to begin.

(b) indicates number of information bytes contained in field (e) = 10h = 16 byte

(c) indicates the starting location of RAM space into which the data byte to be stored

(d) indicates (if 00) not the end-of-file; more frames are to arrive.

(e) indicates information (data) bytes contained in the current frame,

(f) indicated CHKSUM. It is calculated in the following way:
all data bytes from and including field (b) to field (e) are added. The carry is discarded. Two's complement of the remaining 8-bit is taken as CHECKSUM, and it is transmitted as the last byte of the frame.

//----------------------------------------------------------------------------------------------------------------
(1) The frame contains 22 bytes data including all fields.

(2) Field-(a) and Field-(f) will not be taken into account to calculate CHKSUM.

(3) Your codes:

byte chk = 0;
for ( int i=0; i<length; i++)   //length = 19 (22-02 = 20 - 1)
    chk += data[i];    

//sum=10+C1+00+00+00+3E+90+32+00+21+16+10+1E+11+01+33+C1+0A+CD+19 = 042C
//discard accumulated carries; we get 2C = 0010 1100
//take 2's complemet code : 1101 0011 + 1 = D4 

//----take the 2's complement of chk; you will get D4-----

chk = (chk^0xFF);  //chk = D3
chk +=chk+01;      //chk = D4
6