Re: 264 series questions

From: Nicolas Welte (
Date: 1999-10-06 09:58:50

Levente Hársfalvi wrote:
> > I know. But on the C128 it's even worse than this, because clock
> > stretching occurs on only some I/O accesses. So you never really know if
> > an I/O access at 2MHz speed takes one or two cycles.
> Huh, strange. That should involve some more clock logic than the Plus/4
> utilitizes.

Well, its all built into the VIC-IIe. It get's the 8MHz dotclock input
and generates the normal 1MHz Phi0 signal that is used to clock all I/O
chips, the variable 2MHz clock signal that drives the 8502 and the
special 4MHz clock for the Z80. It's true that there are lots of glue
logic in a C128, but it's obviously not used for the clock signals.

> One interesting thing in this subject is the SID card, designed and
> manufactured by Christian Schäffner. This has a real 8580 SID that runs
> at the constant single clock of the Plus/4. The card picks the MUX
> signal, that I should have mentioned that runs @ the constant twiee
> clock somehow, then divides it by 2 in a CMOS 4520 chip and feeds this
> clock to the SID clk input with no additional fiddling. And,
> interestingly, the SID works! I won't have expected such hack to work,
> knowing the chaotic state of the Plus/4 internal bus and timings. Even
> the readable registers of the SID can be accessed without problems.

I already wondered how a SID behaves if clocked with the strange Phi2

If the divided MUX signal and the Phi2 signal are in phase, and the SID
chip select signal is gated with Phi2, it should work without any
problems. But without gating the chip select I almost can't believe it. 

I think we will know more about the phase relation of MUX and the other
clock signals as soon as I do the oscilloscope experiments. I just
realized I'll also have to get or bring a little TV/video monitor or
otherwise I won't be able to enter programs :-( 

> Ah, and another. I recently talked with Balázs Szabó, BSZ of NST who
> also made some hardware hacks. He once designed an IEEE interface for
> his Plus/4 (I have the schematics, but unfortunately, the modified
> Kernal I do not :-( ). With his interface, according to him, his
> computer was able to communicate with his SFD-1001. He told me, he used
> a 6821 PIA without problems.

Then probably a 6522 (probably only with some tweaking like in the C64)
and 6526 should also work, but perhaps with broken timers. But on the
other hand a similar hack like for the SID would perhaps work?

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