[RESOLVED] crc 16 ccitt 8408

[sergelac]

i need a function to calculate the CRC16 from a string, not a byte array

this is the function from the documentation of the device, but its in C

word reentrant update_crc16 (word oldcrc, byte d)
{
    word crc, carry, b;
    byte i;

    crc = oldcrc;
    for (i = 0; i < 8; ++i)
    {
        b = (d & (0x01 << i))?0x0001:0x0000;
        carry = (crc & 0x0001) ^ b;
        crc >>= 1;
        if(carry)crc ^= 0x8408;
    }

    return crc;
}

// you must initialize the initial crc with 0x8408

i already have a equivalent function to do bit-shift in VB (<< >>=)

but i dont understand that line :
b = (d & (0x01 << i))?0x0001:0x0000;
and the word "reentrant"

[WoF]

What you see there is a conditional expression, best replaced by vb's IIf()
It is an expression with an implicit If/Then/Else.
Let's translate:
b = (d & (0x01 << i))?0x0001:0x0000;

'As If Then Else:
If d And (1 * 2^i) Then b = 1 Else b = 0
'As IIf:
b = IIf(d And (1 * 2^i), 1, 0)

The (0x01 << i) cn be simplified to 2^i in the end. Same as 1 * 2^i

Reentrant means, that the function must be written so that it can call itself.

[sergelac]

thanks, its working

Private Function CRC16(ByVal crc As Integer, d As Byte) As Integer
  Dim carry As Integer, i As Byte
  For i = 0 To 7
    carry = (crc And 1) Xor IIf(d And (2 ^ i), 1, 0)
    crc = (crc And &HFFFF&) \ 2
    If carry <> 0 Then crc = crc Xor &H8408
  Next i
  CRC16 = crc
End Function

Public Function CalcCrc16(ByVal buf As String) As String
  Dim crc As Integer, t As Integer
  crc = &H8408
  For t = 1 To Len(buf)
    crc = CRC16(crc, Asc(Mid$(buf, t, 1)))
  Next t
  CalcCrc16 = Chr$(HiByte(crc)) & Chr$(LoByte(crc))
End Function