Re: Adt(pro)

[David Wilson <mcs6502@gmail.com>]

On Sep 19, 10:15 pm, "John B. Matthews" <nos...@nospam.invalid>
wrote:
> Also, my table was wrong. I had incorrectly indicated that clearing the
> low nibble of the control register specified an external clock, which is
> actually specified in bit four. I think I fixed it:
>
> <http://home.woh.rr.com/jbmatthews/ssc.html#tbl3>

Bit 4 is the RECEIVER Clock Source selector. The transmitter always
uses the internal Baud rate selected by bits 3..0.

The foot note indicating that 115200 is only the case if you have a
standard 1.8432MHz crystal applies equally to the other 15 speeds. The
6551 was designed generate a range of standard baud rates only if
supplied with a 1.8432MHz clock.

My Rockwell data sheets indicate 400ns (2.5MHz) as the clocking limit
for both the 6551 and 65C51 parts.