crc.bas

Rs232串口通信专题范例,Vusual Basic,Mscomm,PCOMMPRO

Attribute VB_Name = "CRC"
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'用来计算CRC的值
'BCRC是所要计算的进位值(16或32)，如16位元CRC时，其值即为16
'GP是衍生多项式，以16进位表示较佳
'BDF是欲求出CRC的原始数值
'传回值即是计算後的CRC值
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Function CalCRC(BCRC As Integer, GP As Long, BDF As Long) As Long
  Dim C1&, C2&
  Dim A1%, PreC1&
  Dim LeftValue%, A1Shift%
  If BDF = 0 Then
    CalCRC = 0
    Exit Function
  End If
  A1 = (Len(Hex(BDF)) - 1) * 4  '求出第一次的参考偏移位元值
  C1 = BDF * 2 ^ BCRC           '将被求值先乘上位元值
  LeftValue = Val("&H" & Mid(Hex(BDF), 1, 1))   '取出最左边的值
  If LeftValue >= 1 Then A1Shift = 1    '只有一位数，权值为1
  If LeftValue >= 2 Then A1Shift = 2    '只有二位数，权值为2
  If LeftValue >= 4 Then A1Shift = 3    '只有叁位数，权值为3
  If LeftValue >= 8 Then A1Shift = 4    '四位数，权值为4
  A1 = A1 + (A1Shift - 1)            '第一次的偏移值
  C2 = GP * 2 ^ A1          '第一次的运算值
  Do
   PreC1 = C1               '记录先前值
   C1 = C1 Xor C2           '二进位长除法之馀数
   Do
     A1 = A1 - 1  '''
     If C1 > PreC1 Then     '若馀数大於衍生多项式，则予以降幂
       C1 = PreC1 Xor (GP * 2 ^ A1)
     Else
       Exit Do
     End If
   Loop
   C2 = GP * 2 ^ A1         '下一次的运算值
  Loop Until C1 <= GP       '直到馀数小於衍生多项式
  CalCRC = C1
End Function