Sensirion CRC

Three different routines to calculate the Sensirion CRC as used in their temperature/humidity sensors.

Code:

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Sensirion CRC
'Choose one of the following CRC routines.  The table based one is the fastest
'but requires a large table in memory.  The assembly language one is slower but
'doesn't require much memory.  The PBP one is the slowest. 
'The assembly language one was written with the PIC18 instruction set.  If used
'on earlier PICs the rotate instruction will need to be changed accordingly.

'Make sure to initialize SHTCRC before calling the routine for the first time.
'Place the data to be crc'd in SHTRxCRC and call the routine.  CRC will be
'accumulated in SHTCRC unless reset externally from the routine.

'PIC18F252

SHTCRC          var byte    'accumulative CRC, usually initialize to zero before
                            'calling CRC routine for first time - used by
                            'all routines
SHTRxCRC        var byte    'data to be added to CRC total - used by all
                            'routines
SHTCRCBitNo    var byte    'internal to Sht_CRC_Basic, Sht_CRC_AL
SHTTestByte    var byte    'internal to Sht_CRC_AL
SHTCRCTestBit  var bit    'internal to Sht_CRC_Basic, Sht_CRC_AL

Sht_CRC_Basic:
' This code confirmed operational 12/30/03
' PBP version should work with all PIC series
    For shtCRCbitno = 0 to 7        'Do for all 8 bits
        Shtcrctestbit = shtcrc.7 ^ shtrxcrc.7 'XOR msb
        shtrxcrc = shtrxcrc << 1    'Position received byte for next test
        IF shtcrctestbit = 0 Then Sht_No_XOR 'If tested bits same, skip
        shtcrc = shtcrc ^ $18      'Else, invert bits 4,5
Sht_No_XOR:
        ShtCRC = shtcrc << 1        'Shift the CRC byte
        ShtCrc.0 = shtcrctestbit    'And set its bit 0 to the xor'd value
                                    'after shift this would be bit 8 position
        Next shtCRCbitno
    Return


Sht_CRC_Table:
' This code confirmed operational 12/29/03
' For PIC18 series, table may be too long for previous PIC series
    Shtrxcrc = shtrxcrc ^ shtcrc
    Lookup shtrxcrc, [0,49,98,83,196,245,166,151,185,136,219,234,125,76,31,_
    46,67,114,33,16,135,182,229,212,250,203,152,169,62,15,92,109,134,183,_
    228,213,66,115,32,17,63,14,93,108,251,202,253,168,197,244,167,150,1,_
    48,99,82,124,77,30,47,184,137,218,235,61,12,95,110,249,200,155,170,132,_
    181,230,215,64,113,34,19,126,79,28,45,186,139,216,233,199,246,165,148,_
    3,50,97,80,187,138,217,232,127,78,29,44,2,51,96,81,198,247,164,149,248,_
    201,154,171,60,13,94,111,65,112,35,18,133,180,231,214,122,75,24,41,190,_
    143,220,237,195,242,161,144,7,54,101,84,57,8,91,106,253,204,159,174,128,_
    177,226,211,68,117,38,23,252,205,158,175,56,9,90,107,69,116,39,22,129,_
    176,227,210,191,142,221,236,123,74,25,40,6,55,100,85,194,243,160,145,_
    71,118,37,20,131,178,225,208,254,207,156,173,58,11,88,105,4,53,102,87,_
    192,241,162,147,189,140,223,238,121,72,27,42,193,240,163,146,5,52,103,_
    86,120,73,26,43,188,141,222,239,130,179,224,209,70,119,36,21,59,10,89,_
    104,255,206,157,172], shtcrc
    Return
   
' ********** Subroutine Sht_CRC_AL **********
' This code confirmed operational 12/31/03
' Uses PIC18 Instruction Set
ASM
_Sht_CRC_AL                ; SHTxx CRC (a reflected CRC from Dallas 1wire)
    MOVLW  8
    MOVWF  _ShtCRCBitNo    ; Do this for all 8 bits of CRCByte
CRCLOOP
    MOVF    _ShtCRC,0      ; Put CRC into W
    XORWF  _ShtRxCRC,0    ; XOR CRC with CRCData
    MOVWF  _ShtTestByte    ; Put results in TestByte
    RLCF    _ShtTestByte,1  ; Shift TestByte.7 into Carry bit
    BTFSS  STATUS,C        ; If Carry = 1, do polynomial math
    GOTO    CRCSHIFT        ; Otherwise just shift CRC and CRCByte
    MOVF    _ShtCRC,0      ; Get latest copy of CRC
    XORLW  0x18            ; Polynomial math
    MOVWF  _ShtCRC        ; Keep a copy of results
CRCSHIFT
    RLCF    _ShtCRC,1      ; Rotate CRC into carry
    RLCF    _ShtRxCRC,1    ; Rotate CRCData, carry shifts in but not important
    DECFSZ  _ShtCRCBitNo,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

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This is the code I use for calculating the CRC from the Dallas 18B20 temperature sensor. The only difference I can notice is the locations of the << and >> in the two code. Can you give me an example of how your code is used cause I'm still a little fuzzy on what data does where. I know the CRC value byte is the third byte but not sure what bytes are used to calculate the CRC to compare to this value.

Code:

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------------------------ Calculate the CRC from Byte 9 ------------------------

GetCRC:
    for x = 0 to 7
    databyte = dq[x]
    gosub CalcCRC
    next x
    if dq[8] <> crccalc then
        sensorerror = 1
        else
        sensorerror = 0
    endif
    crccalc = 0               
    return

'--------------------- CRC Bit Calcuation Method -------------------------------

CalcCRC:
    for i = 0 to 7
    databit = crccalc.0 ^ databyte.0
    databyte = databyte >> 1
    if databit = 0 then Shift
    crccalc = crccalc ^ $18

shift:
    crccalc = crccalc >> 1
    crccalc.7 = databit
    next i
    return

---

Code:

---

'------------------ Read SHT71 Temperature / Humidity Sensor -------------------

ReadSHT71:
    gosub Initialize
    Sensorcmd = ReadTemp
    gosub readsensor
    if result => 4000 then
        tempC = result - 4000
        SignC = 160
        else
        tempC = 4000 - result
        SignC = "-"
    endif
    gosub ConvertToKelvin   
    gosub ConvertToFahrenheit
    gosub Initialize
    Sensorcmd = ReadHumidity
    gosub readsensor

'--------------------- Calculate Relative Humidity -----------------------------

    w = result * 405
    w = div32 100
    x = result * result
    x = div32 1000
    x = x * 28
    x = div32 100
    RHLinear = w - x - 400

'---------- Calculate Relative Humidity with Temperature Compensation ----------

    w = (8 * result + 1000) / 10
    if SignC = 160 then
        if tempC > 2500 then
            x = (tempC - 2500) * w
            x = div32 100
            RHTempComp = RHLinear + x / 100
            else
            x = (2500 - temp) * w
            x = div32 100
            RHTempComp = RHLinear - x / 100
        endif
        else
        x = (2500 + tempC) * w
        x = div32 10000
        RHTempComp = RHLinear - x
    endif
    sleep 1
    return

'---------------------------- Initialize the Sensor ----------------------------

Initialize:
    high DataPin
    low clk
    for i = 1 to 10
        high clk
        pause 1
        low clk
        pause 1
    next i
    call TransferStart
    return

'---------------------------- Get Data from Sensor -----------------------------

ReadSensor:
    gosub TransferStart
    gosub WaitSensor
    shiftout DataPin,clk,1,[Sensorcmd\8]        ' Send command byte
    input DataPin                              ' Wait for acknowledge
    low clk
    while DataPin = 1
    wend
    pulsout clk,10                              ' Send acknowledge
    while DataPin = 0
    wend
    while DataPin = 1                          ' Wait for conversion to complete
    wend
    low clk
    shiftin DataPin,clk,0,[result.byte1\8]      ' Get the first byte, 8 bits
    low DataPin
    pulsout clk,10                              ' Send acknowledge
    shiftin DataPin,clk,0,[result.byte0\8]      ' Get the second byte, 8 bits
    low DataPin
    pulsout clk,10                              ' Send acknowledge
    shiftin DataPin,clk,0,[crc\8]              ' Get third byte, 8 bits, CRC
    high DataPin
    pulsout clk,10                              ' Send acknowledge
    input DataPin                              ' End of Transmission
    input clk
    return

'---------------------------- Start Transfer -----------------------------------

TransferStart:
    high clk
    pause 1
    low DataPin
    pause 1
    low clk
    pause 1
    high clk
    pause 1
    high DataPin
    pause 1
    low clk
    return

'---------------------------- Wait for Sensor ----------------------------------

WaitSensor:
    result = 4096

Loop:
    result = result - 1
    if DataPin && result.bit11 then Loop
    return
   
'-------------------------- Convert to Kelvin ----------------------------------

ConvertToKelvin:
        if signC = "-" then
            tempk = 27315 - temp
        else
            tempk = temp + 27315
          endif
          return
         
ConvertToFahrenheit:
    if SignC = 160 then
        SignF = 160
        dummy = TempC * 18
        TempF = div32 10
        TempF = TempF + 3200
        else
        TempF = (tempc + 5000) * 900
        TempF = DIV32 500
        if TempC => 1778 and SignC = "-" then
            TempF = TempF - 12200
            SignF = "-"
            else                               
            TempF = 12200 - TempF
            SignF = 160
            endif   
    endif
        return

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There isn't much difference between the Sensirion and 1wire CRC calculations. The Sensirion tests Bit 7 of the data and does a left shift for the next test bit whereas the 1wire version tests Bit 0 and does a right shift. They both use the same polynomial.

If you haven't downloaded Sensirion's CRC application note, you need to do that. In there it states the CRC covers the whole transmission (command and response bytes) without the acknowledge bits.

The CRC is initialized to the low 4 bits of the STATUS register. If you have not changed the STATUS register it defaults to zero meaning you should initialize SHTCRC to zero. If you have changed the status register then you need to read the application note carefully to see how to initialize SHTCRC (it has to be reversed).

Then send your command byte and the two received bytes to the CRC routine via variable SHTRxCRC. After receiving the CRC byte from the device it must be reversed. In PBP you can do this: Reversed_CRC = Received_CRC REV 8. Now compare Reversed_CRC and SHTCRC. They should be equal if the transmission was OK.

That's it....

Thank you very much for all your help Tom. Without your assistance I would still be out in the dark. Getting more familiar with this CRC checking thing as I go. This is my final code to do the CRC check. I simply compare CRC to CRCCalc and look for the match.

Code:

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'--------- Calculate the CRC from Command Byte and Received Bytes --------------

GetSHTCRC:
    CRCCalc = 0               
    DataByte = SensorCmd
    gosub CalcSHTCRC
    DataByte = SensorData.byte1
    gosub CalcSHTCRC
    databyte = SensorData.Byte0
    gosub CalcSHTCRC
    CRC = CRC REV 8
    return

'--------------------- CRC Bit Calcuation Method -------------------------------

CalcSHTCRC:
    for i = 0 to 7
    DataBit = CRCCcalc.7 ^ DataByte.7
    DataByte = DataByte << 1
    if DataBit = 0 then ShiftSHT
    CRCCcalc = CRCCcalc ^ $18

ShiftSHT:
    CRCCcalc = CRCCcalc << 1
    CRCCcalc.0 = DataBit
    next i
    return

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