At 02:11 PM 6/22/04 -0400, you wrote:
>that gives a baudrate with a bit of error, the errors vary from 0% to several
>percent. How much can I get away with?

Anthony,

First check the archives - this one has been around forever.

Suppose you have one start bit, 8 data bits and one stop bit. Ten bits to
send before the system can re-sync.  This is asynchronous comms, right?
Different uarts sample differently but for the sake of the discussion,
assume one that samples at 50% bit time. Thus, you must accumulate less
than 50% of a bit time of error after 10 bits.

So if you are 5% off from perfect baud rate, after 10 bits go by, you will
be right at half a bit off and the last bit will likely be sampled wrong.
If you are running 2.5% fast and the receiver is 2.5% slow, same thing.
Some uarts sample differently and will be more or less tolerant than this.

1% error 'should' work anywhere. 2% is starting to eat up the error budget.
If there will be more than this, please tell us what product it goes in so
we can avoid purchasing it.

Many clever ways have been thought up to deal with this.  One method lets
the PC start comms with some sync characters.  You measure these and bit
bang a software uart padding as required to get acceptable bit times.  Lots
easier to just get the right crystal...

Tom

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