Dallas CRC8 Routines

[Tom Estes]

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.

[taats]

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

[Tom Estes]

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.

[CocaColaKid]

Tom,

I finally have everything working properly. Thank you so very much for your help and understanding.

[taats]

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

so thanks anyway!!,

Taats

[CocaColaKid]

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]]

[Tom Estes]

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.