help with xbee and propeller

Comments

• Franklin Posts: 4,747

  2011-06-22 08:52 edited 2011-06-22 08:52

  I would start by drawing up a flowchart of what you want the program to do then break it down into sections and code them one at a time. If you could explain what it is you are having trouble with that might help.
• johnfos Posts: 59

  2011-06-23 03:54 edited 2011-06-23 03:54

  Since you're only using two XBees, the simplest way to do it would be to configure them to send to each other in transparent mode. That way, they act as a simple two-way serial channel. Whatever data you write at one end comes out at the other, and you don't need any special C code at all.
  
  The default configuration is almost all you need: just then set each XBee's destination address to be the source address of the other, and it should work.
• Duane Degn Posts: 10,588

  2011-06-23 09:34 edited 2011-06-23 09:34

  I've noticed there are two different ways of sending information from Prop to Prop. One way is to have an end of transmission character. The other is to state the size of the transmission somewhere in the preamble.
  
  I've used the first method myself. The problem with an end of transmission character is you need to make sure none of the data includes such a character.
  
  In order to make sure the data doesn't include the end of transmission character you need to convert numeric data to ASCII characters. Recently I've decide to encode my data as ASCII hexadecimal character. This method requires two characters be transmitted for every byte of data.
  
  Here's some of the methods I use to convert the data.
  
  This first method lists the variables to add to the tx buffer.
  

  PUB SendData(localController)         
    if localController == Header#_NoController
      Tbug(string(13, "No Controller Found"))
      Tbug(string(13, "Robot is in autonomous mode."))
      Tbug(string(13, "Ready to receive data."))
      return                
    setCount++  '  I add a count byte to each tx so I know if all txs get through.
    if setCount == 13 or setCount == 0  ' I use 13 for my end of tx character.  
                                        ' I also exclude 0 in case I'm using strings.  
                                        ' I probably don't need to exclude it.  
                                        ' The rx side know not to expect these numbers.
      setCount++
    nordicTxBuffer[4] := setCount
    bytefill(@nordicTxBuffer + 7, 0, payloadSize - 8)  ' clear the tx buffer
    nordicTxBuffer[7] := localController      ' additional information include in my preable
    if localController == Header#_WiiNunchuck
      Nunchuck.readNunchuck
      Hex(jx, 2, @nordicTxBuffer + 8)   ' add word variables to tx buffer
      Hex(jy, 2, @nordicTxBuffer + 10)
      Hex(bc, 2, @nordicTxBuffer + 12)
      Hex(bz, 2, @nordicTxBuffer + 14)
      Hex(axRaw, 4, @nordicTxBuffer + 16)   ' add long variables to tx buffer
      Hex(ayRaw, 4, @nordicTxBuffer + 20)
      Hex(azRaw, 4, @nordicTxBuffer + 24)
      nordicTxBuffer[28] := 13
    elseif localController == Header#_PlayStation2
      PsData1 := PS2.get_Data1  
      LoadHex(PsData1, @nordicTxBuffer + 8)
      PsData2 := PS2.get_Data2 
      LoadHex(PsData2, @nordicTxBuffer + 16)
      nordicTxBuffer[24] := 13
    else
      Tbug(string(13, "Error, No Controller Found"))
      Tbug(string(13, "Something has gone wrong."))
      Tbug(string(13, "The program needs to be fixed."))
    Tbug(string(13, "Before TransmitString"))
    SafeBug(@nordicTxBuffer, payloadSize)
   
    Nordic.TransmitString(@nordicTxBuffer)
  

  
  Below is a method just to add longs which are always eight characters long.
  

  PUB LoadHex(valueToChop, localPtr)
   
    hex(valueToChop, 8, localPtr)
  

  
  The above methods called this next method which does the actual conversion.
  

  PUB hex(value, digits, localPtr)
    value <<= (8 - digits) << 2
    repeat digits
      byte[localPtr++] := lookupz((value <-= 4) & $F : "0".."9", "A".."F")
   
  

  
  The above adds the appropriate characters to the tx buffer.
  
  Here's code to retreive the information from the ASCII characters.
  

  PUB FromAsciiHex(digits, localPtr) | localCharacter  
    repeat digits
      localCharacter := byte[localPtr++]
      if localCharacter => "0" and localCharacter =< "9"
        localCharacter -= "0"
      elseif localCharacter => "A" and localCharacter =< "F"
        localCharacter -= "A"
        localCharacter += $0A
      else
        localCharacter := 0
      result <<= 4
      result += localCharacter
   
  

  
  The maximum value for digits in the above method is eight since there are eight ASCII characters per long and the above method returns a long.
  
  I'm not using XBees in this particular project but the information applies to any Prop to Prop transmission.
  
  Hopefully some of this will be useful.
  
  Duane