On Tue, 22 Sep 1998, Dave Huang wrote:

> On Tue, 22 Sep 1998, Dr. Bill Studenmund wrote:
> > Nope. Though if you had an external clock at an odd rate, we could use it.
> > For instance, that's how MIDI devices work - the supply the serial port
> > w/ a 1 MHz clock, and we get the 31250 or so baud rate from it.
> 
> Hmm, any reason we couldn't support odd rates? There's a formula for
> calculating the SCC's baud rate counter thing given the input clock
> frequency, baud rate, and divisor... some baud rates would be
> unattainable, but I'd think we could still get thousands of them.

The clock rate we're given is the reason. :-)

With the 3686400-ish Hz clock we're fed, a BRG divisor of 0 (the lowest
you can get), and the serial chip in /16 mode, the fastest baud rate we
can get is 3686400/4/16 = 57600. The BRG always divides the input clock by
2, and then divides by (2+BRG constant). So /4 is the best we can get out
of the BRG.

We do in fact support a LOT of odd rates, just not very fast ones. :-)

Take care,

Bill