🚀 go-pugleaf

In article <MWM.90May2110230@raven.pa.dec.com> mwm@raven.pa.dec.com (Mike (Real Amigas have keyboard garages) Meyer) writes:

>And now for a hardware question: how much of the A3000 hardware is
>tied to the clock rate? The cpu has to be, and I hope the memory is,
>but how about the IO hardware, etc?

Most of the motherboard logic is pretty closely tied to either 16MHz
or 25MHz, and possibly another factor or two (like ROM speed, for
instance).  Chips that need to know about clock speed have jumpers
that tell them what speed they're working with.  The memory controller,
for instance, is very tightly coupled to the clock speed.  This makes
it quite efficient, but no-adjustable.  The memory cycle changes 
between 16MHz and 25MHz systems, based on the different tradeoffs that
occur based on clock speed vs. memory speed.  Some motherboard timing
(eg, for slower things) comes from the 7MHz clock, since we know that
never changes.

The expansion bus is CPU clock speed indepent.  Zorro II cycles,
of course, always run at 7.16MHz, and within limits, Buster (the bus 
controller) syncs Zorro II to the CPU bus speeds without caring about 
the CPU bus speed (it's very likely that Buster chip speed would become 
more of an issue at higher-than-25MHz rates than actual Buster chip 
logic design).  The Zorro III bus is asynchronous and clock speed
independent, though any given Zorro III bus controller (such as Buster)
will likely use a clock or two for it's implementation of the Zorro III
cycle (Buster, in fact, uses both the 25MHz CPU clock and a second
motherboard clock that's delayed 90 degrees from this CPU clock).

>Or, from another perspective, what's the difference between the
>A3000/25 and an A3000/16 with a 25MHz '030 in it?

An A3000/25 comes with a 25MHz 68030, a 25MHz 68882, and a 50MHz
system basis clock.  An A3000/16 comes with a 16MHz 68030, a 16MHz
68881, and a 32MHz system basis clock.  That's the difference, and
why it's possible for a Coprocessor card to supply a set of clocks
for the motherboard as well as use the the motherboard supplied
clocks.

>	<mike


-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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