RS232 7-bits communications

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Hi all,
RS232 working well with bits=8, however I need bits=7. I set the PC to use 7 bits, I use the bits=7 in #use rs232 but get garbage. I am missing something obvious before I get the oscilloscope out!
Keep well.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

The obvious thing is, to my opinion, 7-Bit has to be supported by the chip. PIC processors don't.

That means, 7-Bit data has to be received as 8-Bit data. This can only work, if the sender is using either parity or two stop bits (or leaves a gap of 1 Bit length between characters). If this prerequisites are met, some mask action may be necessary, but it can work at least.

Ttelmah · Guest

Yes.
A search here will find quite a few posts about this.
The 'key' is that if you want parity (say 7,E,1), you can send '8,N,1', from the PIC, and generate your own parity bit into the eighth bit, or set the eighth bit, to just generate an extra stop (nothing will normally 'mind' having an extra stop bit). The compiler has at times had various attempts a doing this for you, but with various degrees of success, so 'DIY', is probably safer...

Best Wishes

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Thanks, 7bits Even parity and 1 stop is what is required...there must be a better way than:

for (i=0;i<7;i++)
if (bit_test(txbyte,i)) parity++;
if (parity) bit_set(txbyte,7);

but I can't think of one at the moment! Any cheats for parity calculation?

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

If speed of parity calculations is an issue, use a table. It will mainly require 128 bytes of program memory and contains the precalculated data with parity for any 7 bit combination.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

There is a parity calulator routine here:
http://www.ccsinfo.com/forum/viewtopic.php?t=21363&start=1

The ASM code comes from Piclist:
http://www.piclist.com/techref/microchip/math/bit/parity.htm

Will Reeve · Joined: 30 Oct 2003 Posts: 209 Location: Norfolk, England

Thanks, neat code.
One last question. I use printf quite a bit to pipe stuff out of the SCI.
I seem to remember there is a way to direct this through the 7bit parity function and then on to a putc? Is this possible?

Ttelmah · Guest

Yes.
Just have a parity function like:
void putc_parity(int8 val)

and call printf, with:

printf(putc_parity,"what I want to send");

The characters will each be sent in turn to your putc, and this can call the 'syste' version.

Best Wishes