This may prove to be of use:
http://weblog.cs.uiowa.edu/294s05-project/uart/slides/deciphering-serial-packets.pdf
Also, you may not be handling "escaped" values correctly.
Bytes with the value of 125 and 126 are modified in the stream
as described in the above document.
MS
Angela Darie wrote:
> /
> / Hy!
> I have some problems in understanding the CRC algorithm within Iris
> mote. So I'm using a Matlab code to calculate the CRC but when I compare
> with the received CRC I don't see any match beetwen the signals.
> My packet format is synch byte || Packet Type ||/ // /Sequence number
> ||/ /Message address ||/ /Message address ||/ /Message Type/ ||// /Group
> ID ||/ /Packet size/ /||/ /Data/ || /Data /|| /CRC /|| /CRC / || /synch
> byte/ /.
>
> So, for example I receive at a time instance the following packet:
>
> ans =Val= 126
>
> ans =Val= 66
>
> ans =Val= 125
>
> ans =Val= 94
>
> ans =Val= 0
>
> ans =Val= 4
>
> ans =Val= 125
>
> ans =Val= 93
>
> ans =Val= 4
>
> ans =Val= 94
>
> ans =Val= 28
>
> ans =Val= 11
>
> ans =Val= 0
>
> ans =Val= 186
>
> ans =Val= 10
>
> ans =Val= 126
>
>
> The red spot is the payload and the next 2 bytes are the CRC! The next
> code I use to compare the CRC data received with the one calculated in
> matlab(with r value)
>
> File CRC16.m
> %**************************************************************************
> % This script calculates the 16-bit ITU-T CRC, as described in 7.2.1.9
> % IEEE 802.15.4-2006 std. (ZigBee).
> %
> % Author: Everton Leandro Alves
> % Date: 06/19/2008
> %
> % The generator polynomial is G(x)=x^16+x^12+x^5+1. From the given
> % explanation the steps are:
> % 1 - Remainder register 'r' is initialized to 0;
> % 2 - The message is shifted into the divider (b0 first);
> % 3 - Operations are done in the order: a) XORs, b) left shift of r
> % register and c) r3 and r10 update.
> % 4 - The r register is appended to the message.
> %
> %**************************************************************************
>
> clear all
> clc
>
> msg = [66;125;94;0;4;125;93;4;94;28;11;0];
> msg = de2bi(msg,8);
>
> msg = reshape(msg,1,8*length(msg));
>
> r=zeros(1,16); % Remainder register initialization
>
> for c3=1:length(msg)
>
> s1=bitxor(msg(c3),r(1)); % XOR between r0 and the message bit
> s2=bitxor(s1,r(12)); % XOR r11
> s3=bitxor(s1,r(5)); % XOR r4
>
> r=[r(2:16) s1]; % Left shift of r, and r15 update
>
> r(11)=s2; % r10 update
> r(4)=s3; % r3 update
> end
>
> msg_FCS=[msg r]; % Message + FCS field
> display('Message + FCS field:')
> msg_FCS
>
> When I calculate the CRC, I use everything except the two frame synch
> bytes, and the two CRC bytes. Does anyone have any clue where I'm going
> wrong?
> /
>
> /
>
>
> ------------------------------------------------------------------------
>
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