C128 and 8MHz Z80 (revisited)

Hello

With a simple daughterboard, a GAL and a 74LS74 the Z80 in C128 can run at
dot clock frequency (almost 8MHz) during its 1MHz phase of the system
clock, effectively doubling the speed.

Basic benchmarks written in Turbo Pascal showed me that indeed it is about
twice faster.
CP/M is visibly snappier, to the point of being usable.

There was a message here back in 2002 about a PCB for C128 that boosted Z80
to 8MHz
https://www.softwolves.com/arkiv/cbm-hackers/7/7361.html
Unfortunately that device was not fully reverse-engineered and it seems
that it was never mentioned again (nor here nor anywhere else). Even linked
photo is not available anymore.

Once I saw it, it bothered me like a note that solution exists, scribbled
on a margin of a math book.

Here is a schematic of my circuit for that:

https://github.com/ytmytm/c128-z80-8mhz/blob/main/z80-dotclock-gal-and-latch/plots/z80-dot-gal.pdf

There is no PCB design yet because so far I only used a development board,
where I missed the fact that /WR or /RESET will be needed.
Right now I have a daughterboard with GAL+74LS74 plugged into a
daughterboard with Z80 plugged into Z80 socket in C128.

Here is the PLD file for GAL programming with the whole logic:

https://github.com/ytmytm/c128-z80-8mhz/blob/main/z80-dotclock-gal-and-latch/gal/z80-dot.PLD

It is written for 22V10 because that's what I had at hand, but 16V8 is
enough for that job. For 16V8 just renumber the output pins.

I put the rest of my notes on Github in the project respository, including
Kicad files:

https://github.com/ytmytm/c128-z80-8mhz

I didn't try to overclock the original Z80B in my C128, I replaced it some
time ago by a Z84C0020 that uses much less energy.

This circuit works fine for me. During tests it was stable for several
hours, busy with drawing fractals and calculating pi using Monte-Carlo
method. The CPU doesn't even get warm to the touch. Disk I/O worked fine,
as well as REU ram disk M: (emulated by UII+).

I also tried using the 16MHz clock signal from VDC (pin 2). The C128 would
start, but CP/M won't boot. However all my simple tests passed. I suspect
that memory access is an issue and some of the memory writes fail.
But in principle 16MHz clock seems possible.

There are some things I was not able to figure out yet:

1) How can we keep it simple but run at full speed all the time and stall
only on memory and I/O access? It's not clear if the device mentioned in
2002 was really working at 8MHz all the time or just during Z80 turn.

2) Since I already used ATF22V10 which has asynchronous reset for
registers, it should be possible to get rid of 74LS74 with a different
assignment of pin 1 (GAL CLK input). I wasn't able to do it. I'm probably
doing something wrong - messing up high/low vs active/inactive or my CUPL
skills are lacking.

3) Is it possible to go faster than DOT CLOCK? Even at 16MHz (8 ticks
between bus access) it seems that CPU would spend more time being busy
rather than waiting for the bus. (No need to bother if point (1) can be
done).

ytm