CRC8
Contents |
Action
Returns the CRC8 value of a variable or array.
Syntax
Var = CRC8( source , L)
Remarks
|
Var |
The variable that is assigned with the CRC8 of variable source. |
|
Source |
The source variable or first element of the array to get the CRC8 of. |
|
L |
The number of bytes to check. |
CRC8 is used in communication protocols to check if there are no transmission errors.
The 1wire for example returns a CRC byte as the last byte from it’s ID.
The code below shows a VB function of CRC8
Function Docrc8(s As String) As Byte
Dim j As Byte
Dim k As Byte
Dim crc8 As Byte
crc8 = 0
For m = 1 To Len(s)
x = Asc(Mid(s, m, 1))
For k = 0 To 7
j = 1 And (x Xor crc8)
crc8 = Fix(crc8 / 2) And &HFF
x = Fix(x / 2) And &HFF
If j <> 0 Then
crc8 = crc8 Xor &H8C
End If
Next k
Next
Docrc8 = crc8
End Function
When you want to use a different polynome, you can override the default by defining a constant named CRC8_POLY
Const CRC8_POLY = &HAA 'use a different value
See also
CHECKSUM , CRC16, CRC16UNI , CRC32 , TCPCHECKSUM
ASM
The following routine is called from mcs.lib : _CRC8
The routine must be called with Z pointing to the data and R24 must contain the number of bytes to check.
On return, R16 contains the CRC8 value.
The used registers are : R16-R19, R25.
- X = Crc8(ar(1) , 7)
Ldi R24,$07 ; number of bytes
Ldi R30,$64 ; address of ar(1)
Ldi R31,$00 ; load constant in register
Rcall _Crc8 ; call routine
Ldi R26,$60 ; address of X
St X,R16 ; store crc8
Example
$regfile = "m48def.dat" ' specify the used micro $crystal = 8000000 ' used crystal frequency $baud = 19200 ' use baud rate $hwstack = 32 ' default use 32 for the hardware stack $swstack = 10 ' default use 10 for the SW stack $framesize = 40 ' default use 40 for the frame space Config Com1 = Dummy , Synchrone = 0 , Parity = None , Stopbits = 1 , Databits = 8 , Clockpol = 0 Dim Ar(10) As Byte Dim J As Byte Ar(1) = 1 Ar(2) = 2 Ar(3) = 3 J = Crc8(ar(1) , 3) 'calculate value which is 216 Print J End
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