Maarten Hofman wrote:
>> USART is great, great when you have time for do something else than the
>> real
>> thing. But the whole thing depends on timing, number of instructions etc
>> which is not very nice I know -- sometimes modifing only one instruction
>> takes as long as an hour. But that's my problem, I wanted to put it into
>> the
>> smallest PIC :-) -- 10F2xx, no interrupts, only 1 8bit timer without
>> knowing
>> that is overflowed etc.
>>     
>
>
> I would be a bit scared doing a serial interface on a PIC using its internal
> oscillator. In the past I've had issues of drift (not sure where the drift
> came from, but i guess it could be temperature or just the phase of the
> moon) where I had to readjust the speed of USART to compensate for a change
> in the oscillator frequency. Of course, you can calibrate the 10F200, and
> its oscillator might be more accurate than the one in the 16F628A and the
> 16F688, so it might be less of an issue. Also, you are coding it yourself,
> so your base accuracy could be higher.
>
> Greetings,
> Maarten Hofman.
>   
I experimented with the PIC12C672 timer for UART purposes. What I found 
was that the timer was
faster at first then slowed down, on a cycle that seemed to be about 
f/20. What that meant was that at
19.2kb it wasn't useable, but at 1200 it worked OK, because the cycle 
was smoothed out.

But there is a better way to do it; use Manchester coding, which is not 
dependent on precise timing. I
can send and receive this type of transmission at 2x or 1/2x of the 
intended speed and I have no problem
reading the data. I now use this for all non-UART serial schemes.

--Bob
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