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PICList Thread
'PIC 16C74 Serial Xmit Problems'
1995\12\20@230902 by Brad Morrow

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Hi John;

I am currently debugging the transmit portion of my firmware for the 16C74.  I
can not make it work without send bad characters every so often.  I have done
several RS-232 implementations in the past, but this one is the most difficult
one to get working properly.

Is it possible you could shed some light on my problem?  I have been in contact
with Microchip, but am getting no where fast.

Regards,

Brad Morrow

--
-
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Brad Morrow                                      Advanced Systems Division
Product Design                                  e-mail: spam_OUTmorrowTakeThisOuTspamasd.sgi.com
Silicon Graphics, Inc.                      voice-mail:      (415)390-1311
2011 N. Shoreline Blvd.                            fax:      (415)961-9075
Mountain View, CA 94039-7311
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1995\12\21@082545 by Mark A. Corio

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Are you using serial transmit interrupts?  If you are you must be very
careful to properly save and restore the state in the interrupt service
routine.  We had similar problems before and that was our cause.  We also
went to a polled transmit routine and used the interrupts only on the receive
of a character as our application was not required to do anything else during
the transmission time.

Another thing to consider...are you using proper level shifting circuits to
get the +/-5 to +/- 12 volts required for RS-232 (as much discussed
recently)?  If not you may be on the hairy edge of your receiver's noise
margins.

Good Luck!!

Mark A. Corio
Rochester MicroSystems, Inc.
200 Buell Road, Suite 9
Rochester, NY  14624
Tel:  (716) 328-5850
Fax:  (716) 328-1144
e-mail:  .....McorioKILLspamspam@spam@aol.com

***** Designing Electronics For Research & Industry *****

1995\12\21@134023 by Glyn Davies 'Gryn'

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>
> Hi John;
>
> I am currently debugging the transmit portion of my firmware for the 16C74.  I
> can not make it work without send bad characters every so often.  I have done
> several RS-232 implementations in the past, but this one is the most difficult
> one to get working properly.
>
> Is it possible you could shed some light on my problem?  I have been in
contact
> with Microchip, but am getting no where fast.
>
> Regards,
>
> Brad Morrow
>

Sounds like you are over running whatever you are sending to.  I've been playing
with a 74 @20Mhz, & 9600 and have overrun lots of kit !

One bet might be to get it to talk to something with a 16550, see what's going
on, or use one of those serial snooping boxes if you have one available.

Alternatively, try slowing down the baud rate, I'm about to implement an
Xon/Xoff for the 74, (when I've finished my Xmas exams :)

Glyn

--

Glyn D / http://www.cs.man.ac.uk/~daviess / Computer Science 3rd year

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