Modbus 8051 Code (ASM)

[DwigB]

I have done some exhausted search for ASM code for implementing MODBUS CRC calculation. I finally gave up and developed this simple code that works.

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Code ASM - [expand]
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; The registrars where the CRC results will reside have to be set to 0xFFFF. 
 
CRC16_Init:                             ; Reset CRC Registar
        MOV D_CRC16_Hi, #0FFh
        MOV D_CRC16_Lo,#0FFh
        RET

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Once this is done the string of bytes to be transmitted are processed by calling the routine below.

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Code ASM - [expand]
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CRC16_AddCalc:                     ; Calc CRC
        PUSH R_0                       ; Save Reg 0
        PUSH Acc                       ; Save Acc
        MOV     R0,#08h                 ; 8 Bits
        XRL     D_CRC16_Lo,A         ; Lo ^=Data
L?P1:
        CLR     C                         ; Clear Carry
        MOV     A,D_CRC16_Hi        ; D_CRC << 1
        RRC     A                         ; Shift Left
        MOV     D_CRC16_Hi,A        ; Store Back
        MOV     A,D_CRC16_Lo        ; Get Hi Byte
        RRC     A                         ; Shift Left
        MOV     D_CRC16_Lo,A        ; Store Back
        JNC     L?P2                      ; Skip if Bit 15 wasn't set
        XRL     D_CRC16_Hi,#0A0h   ; XOR in Polynomial High
        XRL     D_CRC16_Lo,#01h     ; XOR in Polynomial Lo
L?P2:
        DJNZ    R0,L?P1
        POP     Acc
        POP     R_0
        RET

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