FullDuplexSerial Stop Bits

Comments

• Peter Jakacki Posts: 10,193

  2007-05-26 00:30 edited 2007-05-26 00:30

  Hi Tom,
  
  Usually it's not a problem if 2 stop bits are transmitted as most applications are not trying to push the limits of a already limited communications medium. The FullDuplex object does not actually send 2 stop bits but it effectively does so because it transmits an idle bit before the start bit. The code snippet around line 252 (comments added) shows that a stop bit is appended with #$100 and then room is made for a start bit and a padding bit by shifting left 2 bits. The data is transmitted lsb first so the first bit is actually an idle bit.
  
  

                          or      txdata,#$100          'Add a stop bit after the 8 data bits
                          shl     txdata,#2               'make room for a start bit plus a padding bit
                          or      txdata,#1               'force the padding bit to an idle/stop bit 
                          mov     txbits,#11            'count 8 data + 1 stop + 1 start + 1 idle/stop
                          mov     txcnt,cnt
  
  

  
  
  
  I haven't tried it but you could try this.
  
  

                          or      txdata,#$100          'Add a stop bit after the 8 data bits
                          shl     txdata,#1               'make room for a start bit plus a padding bit
                          mov     txbits,#10            'count 8 data + 1 stop + 1 start
                          mov     txcnt,cnt
  
  

  
  
  *Peter*
• TomS Posts: 128

  2007-05-26 00:37 edited 2007-05-26 00:37

  Thanks Peter,
  
  I'll give it a try.
  
  Tom
• TomS Posts: 128

  2007-05-26 12:18 edited 2007-05-26 12:18

  It Works!
  
  Thanks Again Peter. I'm not making the change for speed but for compatability.
  
  Tom
• Mike Green Posts: 23,101

  2007-05-26 13:49 edited 2007-05-26 13:49

  From a compatibility standpoint, it shouldn't matter. To any receiver attached to the Propeller, a serial stream with 2 stop bits is indistinguishable from a serial stream with 1 stop bit and a little bit of internal delay between characters transmitted due to computation or other I/O. The side of FullDuplexSerial dealing with receiving an external serial stream only needs one stop bit, doesn't even check for that, and will accept essentially any delay between characters.
• TomS Posts: 128

  2007-05-26 20:52 edited 2007-05-26 20:52

  Thanks for the good info Mike. I'm replacing a commercial device that uses one stop bit with my propeller driven device and I want the change to my device to be totally transparent. You're right, it probably wouldn't make any difference though.
  
  Tom