crc16 ccitt calculation

[hottoh]

ccitt crc16 algoritham

the polynomial for this algorithm is X**16+X**12+X**5+1


D = current data byte that is input to the algorithm

CRCLSB,CRCMSB=data byte.least significant and most significant CRC accumulator
bytes

x>>n means x is shifted n bits to right or is the same as x divided by 2 to the power n

x<<n means x is shifted n bits to left or is the same as x multiplied by 2 to the power n



======================algoritham=======================================

at beginning,
CRCLSB = 0FFH(octal 377)(decimal 255)
CRCMSB = 0FFH(octal 377)(decimal 255)
then for each data byte in a block,
X=D XOR CRCMSB
x=x xor (X>>4)
CRCMSB=CRCLSB XOR(X>>3)XOR(X<<4)
CRCLSB=X XOR(X<<5)
and at end,
CRCLSB=CRCLSB XOR 0FFH
CRCMSB=CRCMSB XOR 0FFH

======================this is vb6 program for above algoritham=======================================



Public Function CRC16A(Buffer() As Byte) As Long

Dim i As Long
Dim Temp As Long
Dim crc As Long
Dim j As Integer
crc = &HFFFF&
For i = LBound(Buffer) To UBound(Buffer)
Temp = Buffer(i) * &H100&
crc = crc Xor Temp
For j = 0 To 255
If (crc And &H8000&) Then
crc = ((crc * 2) Xor &H1021&) And &HFFFF&
Else
crc = (crc * 2) And &HFFFF&
End If
Next j
Next i
CRC16A = crc And &HFFFF

End Function


Private Sub Command1_Click()

Dim aBuf() As Byte

Dim crc As Long
Dim p As String

p = Text1.Text
aBuf = StrConv(p, vbFromUnicode)
crc = CRC16A(aBuf)
Text2.Text = Hex(crc)

End Sub


===============could you help to get correct answer=====

 

[Thunder2781]

Hi,

I'm searching for a normal CRC16 code does somebody have a working version?

 

[Thunder2781]

I have found this code but I cann't get it work in VB6
I hope somebody could help me out

'This Function is used to format output from hex calculation
'Into a format of iwidth string. This is sometimes neccesary to return also
the
'Hight order byte zero.
Public Function FmtHex(ByVal i As Long, Optional iwidth As Long = 0) As
String
If iwidth &lt;= 0 Then
FmtHex = Hex(i)
Else
FmtHex = Right(String(iwidth, &quot;0&quot &amp; Hex(i), iwidth)
End If
End Function



Option Explicit
Global gl_crc16 As Long


Public Function DoCrc16(ByVal dat As Long) As Long
'This function calculates the crc16 of a long byte
'make sure that the global value gl_crc16 is 0 before starting
dat = (dat Xor (gl_crc16 And 255)) And 255
gl_crc16 = (gl_crc16 And 65280) / 256
If (odd_parity(dat) = 1) Then gl_crc16 = gl_crc16 Xor 49153
dat = (dat * 64) And 65535
gl_crc16 = gl_crc16 Xor dat
dat = (dat * 2) And 65535
gl_crc16 = gl_crc16 Xor dat
gl_crc16 = gl_crc16
DoCrc16 = gl_crc16
End Function
Private Function odd_parity(ByVal dat As Long) As Long
Dim n As Long
Dim i As Long

n = 0
For i = 0 To 7
n = n + dat And 1
dat = (dat And 65534) / 2
Next i
odd_parity = n And 1
End Function

 

[chiph]

What line is the compiler complaining about?

Chip H.


____________________________________________________________________
If you want to get the best response to a question, please read FAQ222-2244 first

 

[johnwm]

Just convert the HTML bits back to standard:
For instance replace:
If iwidth &lt;= 0 Then
with
If iwidth <= 0 Then

and replace:
FmtHex = Right(String(iwidth, &quot;0&quot &amp; Hex(i), iwidth)
with
FmtHex = Right(String(iwidth, "0") & Hex(i), iwidth)

________________________________________________________________
If you want to get the best response to a question, please check out FAQ222-2244 first.
'If we're supposed to work in Hex, why have we only got A fingers?'
Drive a Steam Roller

 

[Thunder2781]

I found some good working code at:

http://www.di-mgt.com.au/src/basCRC16.txt

So this solved my problem.

But thanks for your help.