Dallas CRC8 Routines


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  1. #1
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    Default Dallas CRC8 Routines

    Three different PBP programs to calculate the Dallas 8bit CRC.


    ' PIC16F628
    ' These routines are documented in Dallas Semiconductor Application Note 27
    ' Understanding and Using Cyclic Redundancy Checks
    ' This program contains three subroutines for calculating the Dallas 1Wire 8bit CRC.
    ' The computationally intensive short version is called CRC8. Use this one when
    ' you don't have enough PIC memory to store the table version (approx 270 words)
    ' and can afford the processing time.
    ' The table version CRC8LU is computationally fast since it works on byte at a time
    ' rather than bit at a time as CRC8 does.
    ' The Assembly Language version of the computationally intensive version is CRC8AL.
    ' Times per 8 bit CRCByte:
    ' CRC8LU 35usec
    ' CRC8AL 115usec
    ' CRC8 260usec
    DEFINE OSC 4
    @ DEVICE XT_OSC
    @ DEVICE MCLR_OFF
    @ DEVICE LVP_OFF
    CMCON = 7

    TestBit var Bit ' CRC8
    TestByte var Byte ' CRC8AL
    CRCData var Byte ' CRC8 & CRC8LU & CRC8AL
    CRC var Byte ' CRC8 & CRC8LU & CRC8AL
    I var Byte ' CRC8 & CRC8AL
    ' Test data
    CRCData = $02
    CRC = 0
    'Gosub CRC8
    'Gosub CRC8LU
    Call CRC8AL
    ' Test CRC should be $BC
    END

    ' ********** Subroutine CRC8 **********
    ' PBP Version not using lookup table
    ' Expects CRC starting value and CRCData
    ' CRC will be cumulative unless reset externally
    ' Gives ending CRC value and destroys CRCData
    ' Uses I-Byte, TestBit-Bit, CRC-Byte, CRCData-Byte
    CRC8:
    For I = 0 to 7 ' Do for all 8 bits in data byte
    TestBit = CRC.0 ^ CRCData.0 ' XOR bit0 of data byte and crc
    CRCData = CRCData >> 1 ' Position data byte for next bit test
    If TestBit = 0 then Shift ' If test bit not set, just shift CRC
    CRC = CRC ^ $18 ' If set, account for EXOR feedback
    Shift: ' Shift right the CRC byte
    CRC = CRC >> 1 ' CRC bit 0 to bit bucket
    CRC.7 = TestBit ' Test bit rotates into CRC bit 7
    Next I
    Return

    ' ********** Subroutine CRC8AL **********
    ' Assembly Language version of Dallas CRC8 calculator
    ' Expects CRC starting value and CRCData
    'CRC will be cumulative unless reset externally to routine
    ' Gives ending CRC value in CRC and destroys CRCData
    ' Uses I-Byte, TestByte-Byte, CRC-Byte, CRCData-Byte
    ASM
    _CRC8AL ; Dallas CRC8 Assembly Language PIC16F628
    MOVLW 8
    MOVWF _I ; Do this for all 8 bits of CRCByte
    CRCLOOP
    MOVF _CRC,0 ; Put CRC into W
    XORWF _CRCData,0 ; XOR CRC with CRCData
    MOVWF _TestByte ; Put results in TestByte
    RRF _TestByte,1 ; Shift TestByte.0 into Carry bit
    BTFSS STATUS,C ; If Carry = 1, account for EXOR feedback gates
    GOTO CRCSHIFT ; Otherwise just shift CRC and CRCByte
    MOVF _CRC,0 ; Get latest copy of CRC
    XORLW 0x18 ; XOR with 18Hex
    MOVWF _CRC ; Keep a copy of results
    CRCSHIFT
    RRF _CRC,1 ; Rotate CRC incorporating carry
    RRF _CRCData,1 ; Rotate CRCData, carry shifts in but not important
    DECFSZ _I,1 ; Keep track of #times through loop
    GOTO CRCLOOP ; Go to start if not looped through the 8 bits
    RETURN ; Return if all 8 bits analyzed
    ENDASM

    ' ********** Subroutine CRC8LU **********
    ' Expects CRC at some starting value with data to CRC
    ' passed in CRCData.
    ' Gives ending CRC value and destroys CRCData
    ' Uses CRCData-Byte, CRC-Byte
    CRC8LU:
    CRCData = CRCData ^ CRC
    Lookup CRCData, [0,94,188,226,97,63,221,131,194,156,126,32,163,253, 31,65,_
    157,195,33,127,252,162,64,30,95,1,227,189,62,96,13 0,220,_
    35,125,159,193,66,28,154,160,225,191,93,3,128,222, 60,98,_
    190,224,2,92,223,129,99,61,124,34,192,158,29,67,16 1,255,_
    70,24,250,164,39,121,155,197,132,218,56,102,229,18 7,89,7,_
    219,133,103,57,186,228,6,88,25,71,165,251,120,38,1 96,154,_
    101,59,217,135,4,90,184,230,167,249,29,69,198,152, 122,36,_
    248,166,68,26,153,199,37,123,58,100,134,216,91,5,2 31,185,_
    140,210,48,110,237,179,81,15,78,16,242,172,47,113, 147,205,_
    17,79,173,243,112,46,204,146,211,141,111,49,178,23 6,14,80,_
    175,241,19,77,206,144,114,44,109,51,209,143,12,82, 176,238,_
    50,108,142,208,83,13,239,177,140,174,76,18,145,207 ,45,115,_
    202,148,118,40,171,245,23,73,8,86,180,234,105,55,2 13,139,_
    87,9,235,181,54,104,138,212,149,203,41,119,244,170 ,72,22,_
    233,183,85,11,136,214,52,106,43,117,151,201,74,20, 246,168,_
    116,42,200,150,21,75,169,247,182,232,10,84,215,137 ,107], CRC
    ' PBP only handles 255 constants in the list (256 for 18Cxxx). Since table is
    ' indexed starting at zero, special case for $FF (256th element).
    If CRCData = $FF Then
    CRC = 53
    EndIF
    Return
    Tom

  2. #2

    Default

    If I used this line of code:

    owin portb.7, 0, [temp.byte0,temp.byte1,SKIP 6,crc_value]

    Would the calculated CRC be the same value as crc_value?

  3. #3
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    More than likely no.

    You don't state which and I may not be familiar with the 1 wire device you are using but normally you would not be able to skip any characters it transmits. They would all need to be received and included in the CRC calculation. You will need to read the data sheet for the 1 wire device you are using to see what all is included in the CRC total.

    To get that total feed each byte into the CRC routine, The CRC will be calculated and after all bytes that the manufacturer has included in the CRC total have been passed trough it, the CRC should be the same as the CRC received from the 1 wire device.

    Tom
    Tom

  4. #4

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    I actually working with the DS18S20 chip. From what I'm reading though it sounds like I calculate the CRC from the bytes read. Is this correct?

  5. #5
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    I am not familiar with the DS18S20 but a quick look at the data sheet shows the CRC is calculated by reading all 8 bytes of the scratchpad memory, running them through the CRC routine and comparing its output to the 9th byte read from the scratchpad which is the CRC value generated by the DS18S20. See example 3 in the datasheet.
    Tom

  6. #6

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    I think I can now safely say I'm completely lost as this is making no sense to me. Do I calculate the CRC for each bit using bit0 ^ bit1... and then XOR all the bytes one at a time or do I just XOR the bytes themselves? Please excuse my lack of understanding as this is all new to me.

  7. #7
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    You should read Dallas Application note #27 Understanding and using cyclic redundancy checks.

    You will need to select one of the three provided crc8 routines and put that in your PBP program. You call it as a subroutine. Each routine does the same thing but take different amounts of time/different amounts of programming space doing it. These routines operate on a byte at a time not a bit at a time. They do the bit calculations for you.

    Receive one byte from the device or more likely, you will have to receive all bytes that are included in the crc check and place them in appropriate variables or an array. Make the variable CRCData = the first byte then do a gosub to the name of the CRC subroutine. Do the same thing for the second byte, third, etc. After all the bytes have been passed through the CRC routine then the variable CRC will contain the crc value that should match the one sent from the device.
    Tom

  8. #8
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    Default Similar Problem

    Hi Tom,

    I read your posts too about the CRC8, but i'm confused.
    I have a divice connected to my COM1-port and I want to read the information the device sends. This device demands the CRC8-protocol is used.

    the form of communications is like this:
    =====
    functioncode(1 byte), number of data-bytes(2 bytes),data(x byte),checkbyte(1 byte)
    =====

    i'm doing my project in Visual Basic 6.0.

    the sub has this code in it right now. this doesn't work... have got any ideas?? or anyone else?


    =====

    Private Sub CRC8(ByRef data() As Variant)
    '--------------------------------------------------------------------------
    ' Function : (Private) CRC8
    ' Description : computes a 1 byte checksum using bytes 1 to 7 of the
    ' Header 8-byte array (Protocol Header)
    '
    ' Parameter : Data ()As Byte | each byte group is assumed to be High byte first, Low byte last
    ' Return Value : CRC8 as Byte | CRC value
    '--------------------------------------------------------------------------
    Dim crc As Byte
    Dim Length As Byte
    Dim Index As Byte
    Dim CRCtable1() As Variant
    crc = 0
    Length = 1
    CRCtable1 = Array( _
    &H0, &H7, &HE, &H9, &H1C, &H1B, &H12, &H15, &H38, &H3F, &H36, &H31, &H24, &H23, &H2A, &H2D, &H70, &H77, &H7E, &H79, &H6C, &H6B, &H62, &H65, &H48, &H4F, &H46, &H41, &H54, &H53, &H5A, &H5D, _
    &HE0, &HE7, &HEE, &HE9, &HFC, &HFB, &HF2, &HF5, &HD8, &HDF, &HD6, &HD1, &HC4, &HC3, &HCA, &HCD, &H90, &H97, &H9E, &H99, &H8C, &H8B, &H82, &H85, &HA8, &HAF, &HA6, &HA1, &HB4, &HB3, &HBA, &HBD, _
    &HC7, &HC0, &HC9, &HCE, &HDB, &HDC, &HD5, &HD2, &HFF, &HF8, &HF1, &HF6, &HE3, &HE4, &HED, &HEA, &HB7, &HB0, &HB9, &HBE, &HAB, &HAC, &HA5, &HA2, &H8F, &H88, &H81, &H86, &H93, &H94, &H9D, &H9A, _
    &H27, &H20, &H29, &H2E, &H3B, &H3C, &H35, &H32, &H1F, &H18, &H11, &H16, &H3, &H4, &HD, &HA, &H57, &H50, &H59, &H5E, &H4B, &H4C, &H45, &H42, &H6F, &H68, &H61, &H66, &H73, &H74, &H7D, &H7A, _
    &H89, &H8E, &H87, &H80, &H95, &H92, &H9B, &H9C, &HB1, &HB6, &HBF, &HB8, &HAD, &HAA, &HA3, &HA4, &HF9, &HFE, &HF7, &HF0, &HE5, &HE2, &HEB, &HEC, &HC1, &HC6, &HCF, &HC8, &HDD, &HDA, &HD3, &HD4, _
    &H69, &H6E, &H67, &H60, &H75, &H72, &H7B, &H7C, &H51, &H56, &H5F, &H58, &H4D, &H4A, &H43, &H44, &H19, &H1E, &H17, &H10, &H5, &H2, &HB, &HC, &H21, &H26, &H2F, &H28, &H3D, &H3A, &H33, &H34, _
    &H4E, &H49, &H40, &H47, &H52, &H55, &H5C, &H5B, &H76, &H71, &H78, &H7F, &H6A, &H6D, &H64, &H63, &H3E, &H39, &H30, &H37, &H22, &H25, &H2C, &H2B, &H6, &H1, &H8, &HF, &H1A, &H1D, &H14, &H13, _
    &HAE, &HA9, &HA0, &HA7, &HB2, &HB5, &HBC, &HBB, &H96, &H91, &H98, &H9F, &H8A, &H8D, &H84, &H83, &HDE, &HD9, &HD0, &HD7, &HC2, &HC5, &HCC, &HCB, &HE6, &HE1, &HE8, &HEF, &HFA, &HFD, &HF4, &HF3 _
    )
    Do While Length < 7
    Index = crc Xor data(Length)
    crc = CRCtable1(Index)
    Length = Length + 1
    Loop
    CHK8 = crc
    End Sub

    =====

    thanks,

    Taats

  9. #9
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    Default

    I'm not a VB person, don't even own a copy to try your routine.

    The routines I wrote were for the Dallas Semiconductor CRC8 check. Any other CRC routine may not work in the same manner. I note you changed the table entries which may be in a effort to fix this but that may not always work either. Some CRC routines calculate with a different poly or "reflected" or changed in other ways. You will need to find out the particulars of the CRC your device is sending.

    If you are trying to emulate the Dallas 1wire CRC8 then the table values you have are incorrect.
    Tom

  10. #10

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    Tom,

    I finally have everything working properly. Thank you so very much for your help and understanding.
    Last edited by CocaColaKid; - 15th August 2005 at 19:59.

  11. #11
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    taats Guest

    Default Crc8

    after a couple of hours of hard work, i got it working now...

    so thanks anyway!!,

    Taats

  12. #12

    Default

    Is there an easier way to do this?

    owin portb.7,0,[data_in[0], data_in[1], data_in[2], data_in[3], data_in[4], data_in[5], data_in[6], data_in[7], data_in[8]]

  13. #13
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    Default

    Using other 1wire devices I have been able to do the following:

    ' Read the next 32 bytes, calculate running CRC but discard the bytes
    For Index = 1 to 32
    OWIn OWPort, 0, [CRCData]
    GoSub CRC
    Next Index
    OWIn OWPort,0,[DesiredData]
    CRCData = DesiredData
    GoSub CRC

    In other words there may be enough time in between data bytes sent from the 1 wire device to calculate the CRC on a running basis. If you don't need all the data the CRC can be calculated and the data discarded, retaining only that data you need.
    Tom

  14. #14

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    I was wondering if that would actually work or not. This is my final line. Again thanks for all your help.
    Code:
        for x = 0 to 8
        owin portb.7,0,[i]
        dq[x] = i
        next x

  15. #15
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    Default Re: Dallas CRC8 Routines

    Found an error in the CRC8LU subroutine when validating CRC byte after doing a 1-wire search.
    A CRC of $F0 on a device returns an error.
    This is because of a typo in Tom's code, the value 140 highlighted in red in the quote below is incorrect.
    From the table of Example 3 in Dallas AN27, the value here should be 240.

    Norbert


    Quote Originally Posted by Tom Estes View Post
    Three different PBP programs to calculate the Dallas 8bit CRC.
    ....

    ' ********** Subroutine CRC8LU **********
    ' Expects CRC at some starting value with data to CRC
    ' passed in CRCData.
    ' Gives ending CRC value and destroys CRCData
    ' Uses CRCData-Byte, CRC-Byte
    CRC8LU:
    CRCData = CRCData ^ CRC
    Lookup CRCData, [0,94,188,226,97,63,221,131,194,156,126,32,163,253, 31,65,_
    157,195,33,127,252,162,64,30,95,1,227,189,62,96,13 0,220,_
    35,125,159,193,66,28,154,160,225,191,93,3,128,222, 60,98,_
    190,224,2,92,223,129,99,61,124,34,192,158,29,67,16 1,255,_
    70,24,250,164,39,121,155,197,132,218,56,102,229,18 7,89,7,_
    219,133,103,57,186,228,6,88,25,71,165,251,120,38,1 96,154,_
    101,59,217,135,4,90,184,230,167,249,29,69,198,152, 122,36,_
    248,166,68,26,153,199,37,123,58,100,134,216,91,5,2 31,185,_
    140,210,48,110,237,179,81,15,78,16,242,172,47,113, 147,205,_
    17,79,173,243,112,46,204,146,211,141,111,49,178,23 6,14,80,_
    175,241,19,77,206,144,114,44,109,51,209,143,12,82, 176,238,_
    50,108,142,208,83,13,239,177,140,174,76,18,145,207,45,115,_
    202,148,118,40,171,245,23,73,8,86,180,234,105,55,2 13,139,_
    87,9,235,181,54,104,138,212,149,203,41,119,244,170 ,72,22,_
    233,183,85,11,136,214,52,106,43,117,151,201,74,20, 246,168,_
    116,42,200,150,21,75,169,247,182,232,10,84,215,137 ,107], CRC
    ' PBP only handles 255 constants in the list (256 for 18Cxxx). Since table is
    ' indexed starting at zero, special case for $FF (256th element).
    If CRCData = $FF Then
    CRC = 53
    EndIF
    Return

  16. #16
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    Default Re: Dallas CRC8 Routines

    found 2 more typos for total of 3...

    line with 35,125,159,193,66,28,"154", Dallas has 254
    line with 101,59,217,135,4,90,184,230,167,249,"29", Dallas has 27
    line with 50,108,142,208,83,13,239,177,"140", but Dallas example is 240 (as noted above)

  17. #17
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    Default Re: Dallas CRC8 Routines

    Has anyone ever converted Scott Dattalo's method (1-Wire Polynomial) to PBP?

    Code:
    ;
    ;  Scott Dattalo's version (20 words, 23 cycles per byte)
    ;
    crc_8
            xorwf   CRC,F           ;
            clrw                    ;
            btfsc   CRC,0           ;
            xorlw   0x5E            ; 
            btfsc   CRC,1           ; 
            xorlw   0xBC            ; 
            btfsc   CRC,2           ; 
            xorlw   0x61            ; 
            btfsc   CRC,3           ; 
            xorlw   0xC2            ; 
            btfsc   CRC,4           ; 
            xorlw   0x9D            ; 
            btfsc   CRC,5           ;
            xorlw   0x23            ; 
            btfsc   CRC,6           ; 
            xorlw   0x46            ; 
            btfsc   CRC,7           ; 
            xorlw   0x8C            ; 
            movwf   CRC             ; 
            return                  ;
    It seems like a pretty good compromise between size and performance.

    Regards, Mike
    Last edited by Mike, K8LH; - 23rd May 2012 at 03:05.

  18. #18
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    Default Re: Dallas CRC8 Routines

    If your concern is size and speed, why not use it as is with he addition of _ before CRC variable (_CRC)?

    I think it will work just fine.

    Ioannis

  19. #19
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    Default Re: Dallas CRC8 Routines

    Quote Originally Posted by Ioannis View Post
    If your concern is size and speed, why not use it as is with he addition of _ before CRC variable (_CRC)?

    I think it will work just fine.
    Hi Ioannis:

    Since the Dattalo method wasn't mentioned in the thread, I simply wanted to pass it along, hoping it might be useful or helpful to PBP users. I'm afraid I don't know how you would implement it in PBP.

    While I've tried all of the methods discussed in this thread in the past, now days in my C programs I use bit level cumulative CRC built into my low level 1-Wire read/write bit driver. Total cost for doing it this way is 8 words of program memory, 1 byte of RAM, and 0 cycles processing overhead since the CRC code executes during a portion of the delay required to fill out the 60 usec read slot timing.

    Cheerful regards, Mike

  20. #20
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    Default Re: Dallas CRC8 Routines

    Hmm, yes I see what you mean. In Basic this does not seem an easy job to do.

    I will try it and see if it works.

    Ioannis

  21. #21
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    Default Re: Dallas CRC8 Routines

    My main concern is how to save the W register and then restore it.

    I tried

    MOVW _wsave

    but did not compile.

    Ioannis

  22. #22
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    Default Re: Dallas CRC8 Routines

    Quote Originally Posted by Ioannis View Post
    My main concern is how to save the W register and then restore it.

    I tried

    MOVW _wsave

    but did not compile.

    Ioannis
    The command would be: movwf

    Here is a quick attempt at converting the asm to PBP. Note that this is not the most efficeint/elegant conversion, but it should be a bit clearer was to what's happening.

    Code:
    crc VAR BYTE
    i_w VAR BYTE
    
    goto start                  
    
    crc_8:
       crc = i_w ^ crc        ;xorwf crc,f
       i_w = 0                   ;clrw
    
       IF crc.0 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $5E    ;xorlw 0x5e
          END IF
       IF crc.1 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $BC    ;xorlw 0xbc
          END IF
       IF crc.2 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $61    ;xorlw 0x61
          END IF
       IF crc.3 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $C2    ;xorlw 0xc2
          END IF
       IF crc.4 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $9D    ;xorlw 0x9d
          END IF
       IF crc.5 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $23    ;xorlw 0x23
          END IF
       IF crc.6 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $46    ;xorlw 0x46
          END IF
       IF crc.7 = 1 THEN     ;btfsc crc,0
          i_w = i_w ^ $8C    ;xorlw 0x8c
          END IF
    
       crc = i_w             ;movwf crc
    RETURN
    
    start:
     crc = 0                    ;clrf crc
     i_w = 1                    ;movlw 1
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                    ;clrf crc
     i_w = 2                    ;movlw 2
     GOSUB crc_8            ;call  crc_8
     
     crc = 0                    ;clrf crc
     i_w = 4                    ;movlw 4
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                   ;clrf crc
     i_w = 8                   ;movlw 8
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                   ;clrf crc
     i_w = $10                ;movlw 0x10
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                   ;clrf crc
     i_w = $20                ;movlw 0x20
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                   ;clrf crc
     i_w = $40                ;movlw 0x40
     GOSUB crc_8            ;call  crc_8
    
     crc = 0                   ;clrf crc
     i_w = $80                ;movlw 0x80
     GOSUB crc_8            ;call  crc_8
    END

  23. #23
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    Default Re: Dallas CRC8 Routines

    This is a translation for another basic compiler:
    Function crc_dattalo(crc As Byte, crc_data As Byte) As Byte
    Code:
    'dattalo's CRC-8 Function
    '(328_Tcy / 41.0_us)
        Dim i As Byte
        i = crc_data Xor crc
        i = i And 0xff
        crc = 0
        If i.0 = 1 Then
            crc = crc Xor 0x5e
        Endif
        If i.1 = 1 Then
            crc = crc Xor 0xbc
        Endif
        If i.2 = 1 Then
            crc = crc Xor 0x61
        Endif
        If i.3 = 1 Then
            crc = crc Xor 0xc2
        Endif
        If i.4 = 1 Then
            crc = crc Xor 0x9d
        Endif
        If i.5 = 1 Then
            crc = crc Xor 0x23
        Endif
        If i.6 = 1 Then
            crc = crc Xor 0x46
        Endif
        If i.7 = 1 Then
            crc = crc Xor 0x8c
        Endif
        crc_dattalo = crc
    End Function
    Some additional info and comparison between three different approaches: http://www.ccsinfo.com/forum/viewtop...015&highlight=
    Last edited by languer; - 24th May 2012 at 06:15.

  24. #24
    Join Date
    Aug 2005
    Location
    Michigan, USA
    Posts
    224

    Default Re: Dallas CRC8 Routines

    Even though I'm not a PBP developer, I've always enjoyed exchanging ideas and experiences with members here. That said, I wonder if anyone is interested in my zero overhead CRC8 method, even if it isn't a PBP example?

    In my zero overhead CRC8 method I use a portion of the 60-usec read/write 'slot' time to implement CRC8 at the bit level within the OneWire read bit routine. Here's the tail end of a read bit routine from code for an enhanced mid-range 16F device;

    Code:
    ;
    ;  bit level cumulative CRC (data bit in Carry at entry & exit)
    ;
            clrf   WREG             ;                                 |00
            rlf    WREG,W           ; save C in wreg (0x00 or 0x01)   |00
            xorwf  crc,F            ; xor b0 bits, result in 'crc'    |00
            rrf    crc,F            ; is xor result 0 (Carry = 0)?    |00
            skpnc                   ; yes, skip, else                 |00
            iorlw  0x18             ; w = x8+x5+x4+1 poly and bit 0   |00
            rrf    WREG,W           ; restore C, wreg = 0x8C or 0x00  |00
            xorwf  crc,F            ; apply the polynomial or not     |00
    ;
    ;  remainder of 60-usec read/write 'slot'
    ;
            DelayCy(46*usecs-14)    ; 46-usecs minus 14 cycles        |00
            banksel owtris          ; bank 1                          |01
            bsf     owtris,owpin    ; output hi-z '1' @ 61-usecs      |01
            banksel 0               ; bank 0                          |00
            bsf     INTCON,GIE      ; interrupts on                   |00
            return                  ; Carry = data bit                |00
    Using CRC in my DS18x20 programs is simply a matter of clearing the 'crc' variable before reading the 9 byte DS18x20 'scratchpad' or an 8 byte ROMID and verifying that the 'crc' variable equals zero afterwards. You must read all 9 'scratchpad' bytes to have a valid 'crc'. Just ignore or throw away the bytes you don't need.

    I admit I have no idea if this method could be implemented in PBP with its intrinsic OneWire commands but I suspect you gurus will find a way to use it if you think the method is worthwhile.

    Have fun. Cheerful regards, Mike (K8LH)
    Last edited by Mike, K8LH; - 8th May 2018 at 18:20.

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