' BASIC ROUTINE TO CALCULATE CRC16-CCITT
' WAGNER LIPNHARSKI - NOV/1999 - HTTP://WWW.USTR.NET
'
' A = 16 bits counter - CRC16 CCITT - Accumulator result.
' A should be preset to ZERO at the start.
' C = 8 Bits processing byte, value from zero to 255.
' After processing all "C" bytes, A value should be
' converted to Hex and have bytes inverted,
'
'         A$  = "0000" + HEX$(A)
'         A$  = RIGHT$(A$,4)
'         A$  = RIGHT$(A$,2)+LEFT$(A$,2)
'         A1  = VAL(LEFT$(A$,2))
'         A2  = VAL(RIGHT$(A$,2))
'         A1$ = CHR$(A1)
'         A2$ = CHR$(A2)
'
' Transmit A1$, then A2$, as the CRC bytes.
'
' For testing purposes, processing 10 bytes values
' from 00h to 09h, crc to be transmitted is 04h 42h


' The following is the demo example with 10 bytes
A = 0
DATA 0,1,2,3,4,5,6,7,8,9
FOR N = 1 TO 10
    READ C
    GOSUB CALCCRC
    NEXT N
' Here reordening LSB byte is transmitted first.
A$ = HEX$(A)
A$ = RIGHT$("0000" + A$, 4)
A$ = RIGHT$(A$, 2) + LEFT$(A$, 2)
PRINT "FINAL CRC: "; LEFT$(A$, 2); " "; RIGHT$(A$, 2)
END


' The following is the CRC16-CCITT calculation
CALCCRC:  
FOR V = 1 TO 8: ' 8 bits in C to process
    CY = (A MOD 2) + (C MOD 2): ' If = 1 carry bits different
    A = INT(A / 2): ' Shift Right 16 bits A
    C = INT(C / 2): ' Shift Right 8 bits C
    IF CY = 1 THEN A = A XOR 40961: ' Apply Algoritm
    NEXT V
RETURN