Hi Olin,

Well, I went through your analysis and I agree, you are right, I
should have said +/-5% instead of 10%. However, if one side is
guaranteed to be a crystal-based UART and has much less than 1% error
(i.e., exact divisor), then it should work.

Looking at PIC datasheets, the 16F88 and older PICs have +/-5% INTRC
osc tolerance over V and T range. Some of the 18F series, though, have
+/-2% over the full commercial temp and V range.

Sean


On Mon, Jan 5, 2009 at 8:52 AM, Olin Lathrop <olin_piclist@embedinc.com> wrote:
> Sean Breheny wrote:
>> A
>> standard 8 bit serial transmission with 1 start and 1 stop bit can
>> tolerate about 10% total error.
>
> Not even close.  Do the math.
>
> The 0 time reference is the leading edge of the start bit.  The center of
> the last data bit is therefore 8.5 bit times later.  That should be within
> 1/4 bit for good reliability.  0.25 / 8.5 = 2.9%.  The guaranteed to fail
> drift of the last bit is 1/2 bit time, which is 5.9% clock error between
> receiver and transmitter.  Then add the fact that most receivers sample the
> incoming line at 16x the baud rate.  That means the receiver could be off by
> 1/16 of a bit time in measuring the 0 reference time for the character.  So
> the guaranteed to fail clock error becomes (1/2 - 1/16) / 8.5 = 5.15%.
> Again, you want to not exceed about half that in a real system, so a total
> of 2.5% error between transmitter and receiver is a good rule of thumb.
>
> Note that some of the newer PICs have internal oscillators that are good to
> this level, except maybe at extremes of voltage and temperature.  Keep in
> mind however that this eats up the entire error budget at one end, so such
> systems rely on the other end being derived from a baud rate crystal.  This
> is not something you'd generally want to do in a commercial product, but
> it's OK for one off hobby use when you know the PIC will only be
> communicating with your PC.
>
>
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