Date   : Mon, 16 Mar 1992 15:11:29 GMT
From   : krfiny!jeffj@uunet.uu.net (J. Jonas)
Subject: Re: Z80 chrystal upgrade ???

In article <DDVKWAG@irie.ais.org> gravitar@ais.org (Kevin Robert Spears)
writes:
>Is it really that easy to increase the clock speed, just change the
>timebase crystal?  Even assuming all the ICs could handle the increase in
>speed, wouldn't any of the firmware be time-critical andd need to be
>re-written?

I'd be very careful to run diagnostics for several hours before
trusting the machine at a higher speed.
(diagnostics that test all CPU functions, memory,
and as much of the interface chips as possible).

When I was designing a Z80 system, I knew that I had 2 cycles for
memory read/write, 1.5 for opcode fetch so I bought chips
that would just meet that timing (allowing for memory decode).
Go any faster and the memory would probably start giving some
garbage results but only for some locations.
The peripheral chips may act strange.
The Zilog I/O chips need to see the 2 byte sequence for
"RETI" (return from interrupt) to reset the interrupt daisy chain.
If they miss that sequence at the higher speeds,
the I/O system may hang.

So try it, but carefully and be aware that ANY part of the system
may react.

BIOS wait loops are possible, but I had hoped that the CTC
(counter timer circuit) was used for timing, particularly since
the Z80 was so nicely interrupt driven.

- jeffj
jeffj@synsys.uucp
synsys!jeffj@uunet.uu.net