PAUL James wrote:
> And that's exactly the point.  If this works and it takes longer to see
> an error, 
> then it follows that the timing is the culprit, and means can be taken
> to correct that problem. 
>
> Jim
>
> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of Jinx
> Sent: Friday, January 12, 2007 5:00 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Very odd problem with RS232...
>
>
>   
>> You may be getting out of sync with the PIC and the PC if the
>> frequency is off by more than a few percent.   Try lowering the
>> baud rate on both the PC And your PIC board, and see if that makes any
>>     
>
>   
>> difference
>>     
>
> Lowering the baud rate will just make errors take longer to happen.
> If it is a timing problem, the PIC clock needs to be changed to match
> the baud rate from the PC
>   
One can "measure" the RC PIC's baud rate by timing the width of the 
start bit. This is VERY easy to
do if a software UART is being used.

I simply set the T0 interrupt to a short interval and count the total 
number of interrupts received. If the
count is too high, the PC-generated start bit is perceived to be too 
slow (so the receiving PIC clock is
running too FAST, so OSCTUNE must be increased); if the count is too 
low, the PC-generated start
bit is perceived to be too fast (so the receiving PIC clock is running 
too SLOW, so OSCTUNE must
be decreased).

Be sure to wait for a few mS while the new OSCTUNE value takes effect, 
otherwise you will simply
bounce up and down endlessly. Normally you will discover the best value 
within 5 tries. I write my
serial interfaces to allow the RC-driven PIC to measure the start bits 
of several bytes before I reply
to any of the packets. This works well to synchronize two RC-driven 
PICs, too, but ONLY ONE
should be allowed to correct itself.

--Bob


> Sam, are you going through an RS232 i/f (eg MAX232) ?
>
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