old MAC's

From: Eric Smith <eric_at_brouhaha.com>
Date: Tue Oct 24 11:18:45 2000

"Bill Layer" <b.layer_at_vikingelectronics.com> wrote:
> Yes, the 68040 is a drop-in replacement for the 68LC040. There is
> information out there about how to overclock that CPU as well. It is
> rumored that the Canadian 605s ran at 33/66MHz. AFAIK, the 68LC040 in your
> Quadra 605 actually runs at 50MHz internally, but on a 25MHz external bus.

No. The 68040 (all variants including the LC and EC) require a double-speed
clock input, but the internal pipeline does NOT execute at the double
speed. A 68040 with 33 and 66 MHz clock inputs actually runs at 33 MHz.
Motorola documentation is very clear on this. As soon as Motorola
atarted sending out "advanced information" data sheets it became apparent
that this would cause a lot of confusion.

Think of it more like a 2 MHz 8080 that requires a two-phase clock.
It does some internal things on one phase and some on the other. The
68040 is similar but derives the phases internally from a double-speed
clock.

And before someone tries to "correct" me, yes, I do know that the 68040
is CMOS, and does not use traditional two-phase NMOS logic structures
like the 8080. The reason it needs multiple phases is simply due to
the way the pipeline is structured.

More modern high-end microprocessors have internal PLLs or DLLs to
synthesize the various necessary internal timing signals so that there
is only one clock input, and usually that clock input is at a much
lower frequency than the pipeline.
Received on Tue Oct 24 2000 - 11:18:45 BST