ds18b20 and CRC check

[iw2fvo]

Good day to all of you in the forum.
I wrote a picbasic code that allows me to read the temperature from about nine DS18B20 sensors. I just read the temperature from the 18b20 but I do not read the crc and I do not check it for data integrity.
The programs works well and I get valid data from all the connected sensors.
I would like to implement CRC check in my program and I need some help.
> What data should I read from the sensor and how could I compute the CRC and compare it for integrity data check ?
Thanks in advance for any help.
Regards,
Ambrogio

[aerostar]

Have a look at post 17 on this thread http://www.picbasic.co.uk/forum/showthread.php?t=7533

[iw2fvo]

OK, thanks.
Is the code attached to post #18 the final one ?
Thanks
Ambrogio

[iw2fvo]

Hi,
I just used the crc part of the code indicated by you: it is working well.
Thanks
regards,
Ambrogio

[Scampy]

I've tried to incorporate the code (modified to suit my variables) in post #17 so that it will display "NC" on an LCD if the sensor is faulty or not present... but it doesn't work.

This is the CRC check subs - the only change is the TempT variable insted of DQ

GetCRC:    
for x = 0 to 7                  ; Get CRC for each of the 8 bytes
    DataByte = TempT[x]                ; Assign array pointer using value of x
    gosub CalcCRC                   ; Get CRC value
    next x                          ; Repeat until all bytes are done
    if TempT[8] <> CRCCalc then        ; Do the CRC values match?
        CRCError = 1                ; Set flag indicating a problem with this sensor
          else                        
        CRCError = 0                ; Set flag indicating no problem with this sensor
    endif                           
    CRCCalc = 0                     ; Reset CRC calculation variable
    return

;--------------------- CRC Bit Calcuation Method -------------------------------

CalcCRC:
    for i = 0 to 7                      ; Do for all 8 bits in DataByte
    DataBit = CRCCalc.0 ^ DataByte.0    ; XOR bit0 of DataByte and CRC
    DataByte = DataByte >> 1            ; Position DataByte for next bit test
    if DataBit = 0 then Shift           ; If test bit not set, just shift CRCCalc
    CRCCalc = CRCCalc ^ $18             ; If set, account for EXOR feedback

shift:
    CRCCalc = CRCCalc >> 1              ; Shift right the CRCCalc byte
    CRCCalc.7 = DataBit                 ; CRC bit 0 to bit bucket
    next i                              ; Data bit rotates into CRC bit 7
    return

I then have this as the main part of my program loop

gosub GetCRC                    ; Calculate the CRC for comparison

  OWOUT TEMPIN,1,[$CC,$44]
    HIGH TEMPIN
    PAUSE 1000
    OWOUT TEMPIN,1,[$CC,$BE]   
    OWIN TEMPIN,0,[TempT.LowByte,TEMPT.HighByte]
    TempC = (TempT*/1600)
    if CRCError = 1 then 
        LCDout $FE,$80+5,"N/C     "
    endif
    if CRCError = 0 then
    LCDout $FE,$80,"Temp ",DEC (TempC/100),".",#TempC dig 1,"C" 
    endif

With teh DS18B20 connected I get a true temperature displayed on the screen, but If I remove the DS18B20 from the inpt pin the LCD displays 163.7c on the display (presumably it's the maximum value for a word variable and the the formulat applied to convert it to a temperature) rather than "NC"

Any suggestions ?

EDIT:

OK I thought about it a bit more and realised I needed to read the sensor first and then do the check and then apply the if then condition.... but now all I get is "NC" on the screen even when the sensor is connected !

[richard]

OWIN TEMPIN,0,[TempT.LowByte,TEMPT.HighByte]

what about the other 5 data bytes that you need for the crc calc , and the crc actually byte sent by the ds18b20 to check against ?

hint , if you feed the final crc byte sent into the crc routine then if there are no errors the result is always zero