PIC Microcontroller Bit Math Method

CRC

By Charles Ader

;********************************************************************
;
; Compiled with MAPLAB 4.12.12
;
;********************************************************************

        LIST    p=16C54 ; PIC16C54 is the target processor

crc_hi  equ     00EH
crc_lo  equ     00FH
STATUS  equ     3       ; STATUS register F3
CARRY   equ     0       ; Carry bit in status register

;********************************************************************
;               CRC Test Program
;********************************************************************

        org 0h
start   clrf    crc_hi
        clrf    crc_lo

        movlw   080H    ; crc_hi:crc_lo = 00 00
        call    CRC16

        movlw   075H    ; crc_hi:crc_lo = A0 01
        call    CRC16

        movlw   08AH    ; crc_hi:crc_lo = 27 A0
        call    CRC16

        movlw   00BH    ; crc_hi:crc_lo = DF A6
        call    CRC16

        movlw   075H    ; crc_hi:crc_lo = BD 1E
        call    CRC16

        movlw   0C7H    ; crc_hi:crc_lo = EF FC
        call    CRC16

        movlw   0AAH    ; crc_hi:crc_lo = D3 AE
        call    CRC16

        movlw   075H    ; crc_hi:crc_lo = C3 D2
        call    CRC16

        movlw   0C7H    ; crc_hi:crc_lo = BA 82
        call    CRC16

        movlw   055H    ; crc_hi:crc_lo = F3 7B
        call    CRC16

        movlw   043H    ; crc_hi:crc_lo = 1C 73
        call    CRC16

        movlw   01CH    ; crc_hi:crc_lo = 14 1c
        call    CRC16

        movlw   014H    ; crc_hi:crc_lo = 00 14
        call    CRC16

        goto    start   ; crc_hi:crc_lo = 00 00

;********************************************************************
;
; CRC-16  (x^16+x^15+x^2+x^0)
; No tables, No loops, No temporary registers used.
;
; Input:  W = Data byte for CRC calculation
;         crc_hi:crc_lo 16 bit CRC register
;
; Output: crc_hi:crc_lo updated.
;
; Notes:  CARRY is trashed.
;         DIGIT CARRY is trashed.
;         ZERO is trashed.
;         W is zero on exit.
;
; 30 instructions, 31 machine cycles per byte.
;
; Copyright (C) February 8, 2000. All Right Reserved.
; Charles Ader, PO Box 940 Pleasanton, California, USA.
;
; This code started out as an example program found in
; Dallas Semiconductor Application Note 27:
;
;   Understanding and Using Cyclic Redundancy Checks
;   with Dallas Semiconductor iButton(TM) Products.
;
; The application note shows an 8051 assembly language
; routine that calculates the same CRC as the hardware
; in the DS5001/2 secure micro.
;
;********************************************************************

CRC16   xorwf   crc_lo,W        ;W = input XOR old crc_lo
        xorwf   crc_hi,W        ;Swap old crc_hi with W
        xorwf   crc_hi,F        ;
        xorwf   crc_hi,W        ;new crc_hi = input XOR old crc_lo
        movwf   crc_lo          ;new crc_lo = old crc_hi
;
; Calculate parity of crc_hi, (input XOR old crc_lo),
; an place the result in carry.
;
; Save crc_hi in W and use crc_hi as a temp
; location so we can test bits.
;
; Note: We use INCF STATUS,F to compliment
;       carry. This is safe because the XORWF
;       will clear the zero flag if any of the
;       INCF instructions are going to execute.
;
;       The INCF can be executed a maximum of
;       four times after the XORWF before the
;       bits to the left of zero in status will
;       be affected.
;
;       We use INCF at most three time here.
;
        movf    crc_hi,W        ;Save crc_hi in W
        swapf   crc_hi,F        ;Trade nibbles
        xorwf   crc_hi,F        ;XOR high half byte with low
        rrf     crc_hi,F        ;Initialize Carry
        btfsc   crc_hi,0
        incf    STATUS,F        ;Compliment carry
        btfsc   crc_hi,1
        incf    STATUS,F        ;Compliment carry
        btfsc   crc_hi,2
        incf    STATUS,F        ;Compliment carry
        movwf   crc_hi          ;Restore crc_hi from W
;
; Use the parity of crc_hi, (input XOR crc_lo),
; to complete the CRC calculation.
;
        movlw   001H
        btfsc   STATUS,CARRY    ; If carry
        xorwf   crc_lo,F        ; flip bit 0 of crc_lo
        movlw   040H
        rrf     crc_hi,F        ; shift parity into crc_hi
        btfsc   STATUS,CARRY    ; if shift out is one
        xorwf   crc_lo,F        ; flip bit 6 of crc_lo
        rlf     crc_hi,W        ; unshift crc_hi into W
        xorwf   crc_hi,F        ; combine them
        rrf     crc_hi,F        ; shift parity back into crc_hi
        movlw   080H
        btfsc   STATUS,CARRY    ; if shift out is one
        xorwf   crc_lo,F        ; flip bit 7 of crc_lo

        retlw   0

;********************************************************************
;               Power on reset
;********************************************************************

        org     01FFH
        goto    start

        end

-------------End CRC16.ASM------------------

Here is the same CRC16 routine for the x86 and 8051

----------CRC16.C for x86 start------------------------
#include <stdio.h>


/********************************************************************

  CRC-16  (x^16+x^15+x^2+x^0)
  No tables, No jumps.

  Input:  c = Data byte for CRC calculation
          usp_crc = pointer to 16 bit CRC register

  Output: CRC at pointer usp_crc is updated


  Copyright (C) February 11, 2000. All Right Reserved.
  Charles Ader, PO Box 940 Pleasanton, California, USA.

  This code started out as an example program found in
  Dallas Semiconductor Application Note 27:

    Understanding and Using Cyclic Redundancy Checks
    with Dallas Semiconductor iButton(TM) Products.

  The application note shows an 8051 assembly language
  routine that calculates the same CRC as the hardware
  in the DS5001/2 secure micro.

********************************************************************/
void crc16(unsigned short *usp_crc, unsigned char c)
{
unsigned short crc;     /* use a local temp so the compiler will */
                        /* handle all the x86 pointer crap.      */
    crc = *usp_crc;

    _asm {
        mov     al,c
        mov     cx,crc

        XOR     AL,CL
        MOV     CL,CH
        MOV     CH,AL

        LAHF
        NOT     AH
        RCR     AH,1
        RCR     AH,1
        AND     AH,1
        XOR     CL,AH

        RCR     AX,1
        SBB     AH,AH
        AND     AH,0x40
        XOR     CL,AH

        MOV     AH,AL
        XOR     AL,CH
        RCL     AH,1
        RCR     AL,1
        MOV     CH,AL
        RCR     AL,1
        AND     AL,0x80
        XOR     CL,AL

        mov     crc,cx
    }
    *usp_crc = crc;
}

/******************************************************
*         MAIN FUNCTION                               *
*******************************************************/

main()
{
unsigned int crc;
    while(1) {
        crc = 0;          /* crc = 0x0000 */
        crc16(&crc,0x80); /* crc = 0xA001 */
        crc16(&crc,0x75); /* crc = 0x27A0 */
        crc16(&crc,0x8A); /* crc = 0xDFA6 */
        crc16(&crc,0x0B); /* crc = 0xBD1E */
        crc16(&crc,0x75); /* crc = 0xEFFC */
        crc16(&crc,0xC7); /* crc = 0xD3AE */
        crc16(&crc,0xAA); /* crc = 0xC3D2 */
        crc16(&crc,0x75); /* crc = 0xBA82 */
        crc16(&crc,0xC7); /* crc = 0xF37B */
        crc16(&crc,0x55); /* crc = 0x1C73 */
        crc16(&crc,0x43); /* crc = 0x141C */
        crc16(&crc,0x1C); /* crc = 0x0014 */
        crc16(&crc,0x14); /* crc = 0x0000 */
    }
} /* end of main */
----------CRC16.C for x86 end  ------------------------

----------CRC16.A51 for 8051 start---------------------
;
; From Dallas Application note 27
;
; CRC-16  (x^16+x^15+x^2+x^0)
; No tables, No loops, No temporary registers used.
;
; This code is from Dallas Semiconductor Application Note 27:
;
;   Understanding and Using Cyclic Redundancy Checks
;   with Dallas Semiconductor iButton(TM) Products.
;
; This routine that calculates the same CRC as the hardware
; in the DS5001/2 secure micro.
;-----------------------------------------------------------------------
        NAME    CRC_test

?DT?CRC_test SEGMENT DATA
        rseg    ?DT?CRC_test
CRCH:   ds      1
CRCL:   ds      1


?CO?CRC_test    SEGMENT CODE
        RSEG    ?CO?CRC_test

        public  CRC_test

CRC_test:

        mov     CRCH,#0
        mov     CRCL,#0

        mov     A,#080H
        call    CRC16

        mov     A,#075H
        call    CRC16

        mov     A,#08AH
        call    CRC16

        mov     A,#00BH
        call    CRC16

        mov     A,#075H
        call    CRC16

        mov     A,#0C7H
        call    CRC16

        mov     A,#0AAH
        call    CRC16

        mov     A,#075H
        call    CRC16

        mov     A,#0C7H
        call    CRC16

        mov     A,#055H
        call    CRC16

        mov     A,#043H
        call    CRC16

        ljmp    CRC_test


;CALCULATE CRC16 IN PARALLEL TO THE GREATEST EXTENT PRACTICAL
;       INPUT:  BYTE TO BE INCUDED IN CRC CALCULATION IS IN ACC
;       OUTPUT: CRCH:CRCL UPDATED TO INCLUDE THE NEW BYTE
;
CRC16:
        PUSH    ACC                     ;save this in case the caller needs
it
        XRL     A,CRCL
        MOV     CRCL,CRCH               ;put the high byte of the crc in its
dest..
        MOV     CRCH,A                  ;save data xor low(crc) for later
        MOV     C,P
        JNC     CRC0
        XRL     CRCL,#001H

CRC0:
        RRC     A                       ;get the low bit in c
        JNC     CRC1
        XRL     CRCL,#040H

CRC1:
        MOV     C,ACC.7
        XRL     A,CRCH                  ;compute the results for bits P...U
        RRC     A                       ;shift them into place
        MOV     CRCH,A                  ;and save them
        JNC     CRC2
        XRL     CRCL,#080H

CRC2:
        POP     ACC                     ;and restore everything and return
        RET

        end

----------CRC16.A51 for 8051 end  ---------------------

Comments:

For compatibility PIC18, change RRF and RLF for RRCF and RLCF.
And also, INCF STATUS,F don't work, change for BTG STATUS,C
Work great now.
Put FF in CRC_LO, CRC_HI at the start to make MODBUS 16bit CRC result.+

At modern PIC INCF STATUS,F cannot be used to compliment
carry. This can be avoided and the whole code even shorted:

    xorwf Crc16 + 0, W; W = input XOR old crc_lo( writren at C, see above
    )[Hynek];
    xorwf Crc16 + 1, W; Swap old crc_hi with W;
    xorwf Crc16 + 1, F;
    xorwf Crc16 + 1, W; new crc_hi = input XOR old crc_lo;
    movwf Crc16 + 0; new crc_lo = old crc_hi;
    
    ; Calculate parity of crc_hi, ( input XOR old crc_lo ), ;
    ; an place the result in carry.;
    
    ; Save crc_hi in W and use crc_hi as a temp;
    ; location so we can test bits.;
    
    swapf Crc16 + 1, W; Save crc_hi in W[Hynek];
    xorwf Crc16 + 1, W; XOR high half byte with low[Hynek];
    btfsc W, 1; Compute parity of bits 0 - 3 at bit 0[Hynek];
    xorlw 1; b0 ^= b1[Hynek];
    btfsc W, 2; [Hynek];
    xorlw 1; b0 ^= b2[Hynek];
    btfsc W, 3; [Hynek];
    xorlw 1; b0 ^= b3[Hynek];
    andlw 1; W = parity[Hynek];
    
    ; Use the parity of crc_hi, ( input XOR crc_lo ), ;
    ; to complete the CRC calculation.;
    
    xorwf Crc16 + 0, F; flip bit 0 of crc_lo;
    rrf W, W; Carry - parity[Hynek];
    rrf Crc16 + 1, F; shift parity into crc_hi;
    movlw 0x40; [hynek];
    btfsc STATUS, Carry; if shift out is one;
    xorwf Crc16 + 0, F; flip bit 6 of crc_lo;
    rlf Crc16 + 1, W; unshift crc_hi into W;
    xorwf Crc16 + 1, F; combine them;
    rrf Crc16 + 1, F; shift parity back into crc_hi;
    movlw 0x80;
    btfsc STATUS, Carry; if shift out is one;
    xorwf Crc16 + 0, F; flip bit 7 of crc_lo;

file: /Techref/microchip/crc16ca.htm, 12KB, , updated: 2021/7/6 07:58, local time: 2024/12/21 18:49,

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