CRC16 CCITT - A plea for help ...

[SwingeyP]

Hello. I am trying to write a CRC16 in OSHON

I was given this code in C (I don't understand c syntax)

Original c code

unsigned short crc16(unsigned char *buff, int len){
  unsigned short crc = 0xFFFF;
 
  while(len){
    crc ^= *buff++;
    for(int i = 0; i < 8; i++){
      if(crc & 1)
        crc = (crc >> 1) ^ 0xA001;
      else
        crc = crc >> 1;
      }
    len--;
    }
 
  return crc;
  }

I have come up with this but it's not working can anyone see what i'm doing wrong please?

Dim ch As Byte
Dim crc As Word
Dim bt As Byte
Dim tmp As Byte
Dim polynomial As Word

polynomial = 0x1021

Hseropen 9600


ch = "M"


'-----------------------------------------
'This would be a function
'-----------------------------------------
crc = 0xffff
'Hserout "ch = ", #ch, CrLf

crc = crc Xor ch  'XOR crc with ch -
'hserout "initial XOR = " #crc, CrLf

For bt = 0 To 8
	tmp = crc And 1  'if(crc & 1)
	'Hserout "tmp = ", #tmp, CrLf
	If tmp = 1 Then  'is this right?
		crc = ShiftRight(crc, 1)  'shift to the right
		'Hserout "After Shift = ", #crc, CrLf
		crc = crc Xor polynomial
		'Hserout "After XOR = ", #crc, CrLf, CrLf
		
	Else
		crc = ShiftRight(crc, 1)
		'Hserout "Just Shift crc = ", #crc, CrLf, CrLf
	Endif
Next bt

Hserout "CRC = ", #crc, CrLf  'M = 7899


End

I have gone through copying the bits on paper and the code seems to do what it should but the checksum for M according to this site - https://www.lammertbies.nl/comm/info/crc-calculation.html
should be 0x7899.

My RTTY project can not continue without this CRC :-(

Regards - Paul

 

[SwingeyP]

CRACKED IT ....

Dim data As Word
Dim crc As Word
Dim bt As Byte
Dim tmp As Word
Dim polynomial As Word

polynomial = 0x1021  '0x1021 -

Hseropen 9600
data = 0x4d  'M 77 1001101

'-----------------------------------------
'This would be a function
'-----------------------------------------
crc = 0xffff

data = ShiftLeft(data, 8)  'shift the data left 8
crc = crc Xor data  'XOR the data with the CRC
For bt = 0 To 7
	tmp = crc And 0x8000  'and the CRC with 0x8000
	If tmp <> 0 Then  'if this vale <> 0 then will be either 8000 or 0
		crc = ShiftLeft(crc, 1)  'shift the CRC left 1
		crc = crc Xor polynomial  'XOR the CRC with polynomial
	Else
		crc = ShiftLeft(crc, 1)  'else just shift the crc left 1
	Endif
Next bt

Hserout "CRC = ", #crc, CrLf  'M = 0x7899 -  or maybe 0x55f5 -

End

 

[allado]

Thanks !!!...good Job