Using the CRC calculator

kt88seamp

I am creating my own program to transmit and receive data with the Parallax RF module. All I need is to add a CRC check (this is where I am in a snafu), and I can finish the rest. I downloaded and tried Beau's RF demo and it includes a CRC check. I am writing my own handshake algorithm. I looked at Beau's code and found working with the CRC object is a little tricky. For my code I need to know how to (see my program comments) using Beau's CRC:

Properly calculate and send the CRC.

Receive the CRC and validate it.

My program does successfully send data using straight RS-232. All I need to do now is packetize the data and implement the CRC and all should work fine.

Thanks,
kt88seamp [noparse]:)[/noparse]

CannibalRobotics

See my post on CRC for CrystalFontz - it has the packetizer and it's in spin. If you use it on both ends of your system, you should be fine.

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Post Edited (CannibalRobotics) : 4/29/2010 9:51:35 PM GMT

kt88seamp

I found your post.

http://forums.parallax.com/showthread.php?p=902959

Not all the code is there. Where is the portion that receives the data and validates the CRC?

CannibalRobotics

OK,
This is the transmit peice - it sends a 16bit·CRC byte pair at the end of the packet, reversed and inverted. You don't have to go to that trouble - it's device specific.
The receive component would work similarly in that you would initialize the '_crc'·to $FFFF, cycle each new byte arrival through the updatecrc routine and when you have counted enough bytes - look to see if your calculated·CRC matches the transmitted CRC at the end of your arrived packet.
Let me know how it goes.
cheers
Jim-

Pub SendText(Line,ptr) | _crc,crch,crcl,i,size,PacketSize
' This code has some overhead for clarity and readability
' Line: is a device specific command and just inserted into Packet 0
' ptr: is a string pointer for an ASCII string to be sent as the payload
' the packet structure is: <command><PayloadSize><packet[noparse][[/noparse]0]...packet[noparse][[/noparse]packetsize]><!crcL><!crcH>
                                 
    PacketSize := 17            ' initialize packer size - inclued 0
    size := strsize(ptr)        ' get length of string              
    _crc := $FFFF               ' initialize _crc
    ' Start process of building 18 packet byte <<i = 0 to 17>>
    ' Build packet 0
    if Line == 1                ' if line number 1 then CF command is 6
      Packet[noparse][[/noparse]0] := 7
    if Line == 2
      Packet[noparse][[/noparse]0] := 8            ' if line number 2 then CF command is 7 
    ' Build packet 1
    Packet[noparse][[/noparse]1] := PacketSize - 1 ' Calculate remaining data payload size for packet 1
    ' Build packet 2 - 17
    
    repeat i from 2 to PacketSize  ' set next PacketSize bytes to string byte values or spaces
      if i < (2 + size)
        Packet[noparse][[/noparse]i] := byte[noparse][[/noparse]ptr + i - 2]
      else
        Packet[noparse][[/noparse]i] := 32            'dec 32 is the 'space' charactor for empty text  
    ' Packet now built
    ' so loop through &  calculate _crc, send byte by byte
    
    Repeat i from 0 to PacketSize       'Send front of packet 
      _crc := UpdateCRC(Packet[noparse][[/noparse]i],_crc) ' Calculate crc in the loop
      TSc.tx(Packet[noparse][[/noparse]i])                 ' send the byte
    ' Finished sending packet here so wrap up by sending crc low and high
    '   Watch the MSB/LSB order and inversion here - can be tricky
        
    crcl  :=  !(_crc & $00FF)      ' Bottom 8 bits into crc low & invert      
    crch  :=  !(_crc >> 8)         ' Top 8 bits into crc High & invert 
    
    TSc.tx(crcl)                   ' complete transmission with last two bytes
    TSc.tx(crch)
 
Pri UpdateCRC(data,crc):newcrc|i,icrc 
' CRC calculation engine
' SPIN adapted from CrystalFontz generic 'c' algorythm
' data: 'new byte' or next byte in packet
' crc:  running crc mantained in calling pub/pri
 
  Repeat i from 0 to 7
    if ((crc ^ data) & $01)
      crc := crc >> 1
      crc := crc ^ $8408
    else
      crc := crc >> 1
    data := data >> 1
      
  return crc & $FFFF

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kt88seamp

I modified your CRC transmit algorithm so it transmits data bytes instead of strings, as I will not be transmitting ASCII strings in my application. Will the CRC calculate OK with the modifications I made?

CannibalRobotics

Yep, looks like it will work.

Be careful on the byte count - it's easy to get it confused.

J-

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