--------------------------------------------------
From: "Isaac Marino Bavaresco" <isaacbavaresco@yahoo.com.br>
Sent: Friday, September 17, 2010 1:06 PM
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Subject: Re: [EE] CRC for Maxim temp sensor DS18B20

> Em 17/9/2010 03:49, Ruben J=F6nsson escreveu:
>>> Here is some example data from the sensor:
>>> 0x82 0x01 0x4B 0x46 0x7F 0xFF 0x0E 0x10
>>> CRC calculated by sensor (sent with data as 9th byte): 0x70
>>> CRC calculated by spreadsheet: 0x92
>>>
>> Here are my C routines to calculate and check the CRC for 1-wire devices=
:
>>
>> byte OW_CRC(byte *p,int numBytes)
>> // Return the CRC value for numBytes number of bytes in an array
>> // Note that if the CRC is included as the last byte the result
>> // should be 0 if the CRC is good.
>> {
>>   int byteIndex;
>>   int bitCount;
>>   byte CRC=3D0;
>>   byte nextByte;
>>   bool b;
>>   for(byteIndex=3D0;byteIndex<numBytes;byteIndex++){
>>     nextByte=3Dp[byteIndex];
>>     for(bitCount=3D0;bitCount<8;bitCount++){
>>       b=3D(CRC ^ nextByte)&1;
>>       CRC>>=3D1;
>>       if (b){
>>         CRC^=3D0x8c;
>>       }
>>       nextByte>>=3D1;
>>     }
>>   }
>>   return CRC;
>> }
>>
>> bool OW_IsIDValid(OW_ID *pID)
>> // Return true if the one wire ID has a good CRC. 8th byte is CRC.
>> {
>> return (OW_CRC((byte*)pID,8)=3D=3D0);
>> }
>>
>> if (OW_CRC(scratchpad,9)=3D=3D0){  // Valid CRC for nine bytes as read f=
rom
>>                                // scratchpad where the 9th byte is the=20
>> CRC.
>>
>> /Ruben
>>
>
>
> For bit-banged serial communications (as 1-Wire), I personally prefer to
> do the CRC calculations one bit at a time as they arrive, it is much=20
> faster.
>
> The code that does the real calculation takes just a couple of lines and
> you don't need any loops, pointer dereferences, etc.


That sounds like a good way of doing things - with this project speed is no=
t=20
really an issue though (one reading every 15 mins, PIC sleeps in between) s=
o=20
I'll probably leave it as is, plus there is not much time left to get it=20
finished.
=20


--=20
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