> At 4MHz for Fosc, according to the formula in the data sheet, for 38400 
> baud
> you need a divisor of 0.627 for BRGH= 0, and 5.5104 for BRGH=1. Now at
> BRGH=1, using 5 as the divisor (it can only be an integer) you end up with
> an actual baud rate of 41666.6, which is an error of 8.5%. Using 6 as a
> divisor (again nearest integer the other side) gives a baud rate of 
> 35714.2,
> which is an error of -6.99%. Now anything over about 4% is likely to
> introduce errors(allowing for small errors at the other end), so these
> equate to what you see.

You can get zero baud rate error by choosing a crystal whose frequency is an 
exact multiple of

    64 * Bit rate

There are many standard crystals that meet this criteria.

For example, at 115,200 bits per second, you could use a 7.3728 MHz crystal.

To stay under 4MHz, you could use 57,600 bits per second and use a 3.68640 
MHz crystal.

The highest crystal frequency you can use under 20MHz for 115,200 bits per 
second is 18.432 MHz.


Bob Ammerman
RAm Systems


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