Re: 264 series questions

From: Nicolas Welte (
Date: 1999-10-07 17:20:40

Hello Ruud, wrote:
> Just FYI, I hope you remembered that I succeeded in replacing the 6510 of the
> C64 and the 6502 of several CBMs/PETs/VIC20 with a 65SC816.

Yes, I remember this. But there are different problems when replacing a
8500/01/02 with a plain 6502: the bus can be tristated with AEC, on-chip
I/O port and finally the strange GATE IN signal on the 8501. This seems
to be the only challenge here, since there are plans to replace a 6510
already, in the 2MHz project from 64'er. I don't like some details
there, but the general idea is usable. I can even ignore the I/O port
for the first experiments, because the Plus/4 doesn't really need it for
memory management like the C64. Only the cassette and serial port are
connected there. So it is mostly a task of tristating the bus (with '244
and '245 buffers) and maybe gating the R/W signal.

> This uP hasn't a
> PHI2 either. I created this signal by delaying the PH0 signals thru two 74F04
> gates. I know by experience that this delay is essential because a Atom Acorn
> refused to operate at all without this delay. But to be honest, I never tried
> the circuit on a C= without this delay.

Phi2 is only available from the processor in the C64, in the C128 it is
generated by the VIC-IIe and in the Plus/4 it even comes out of the PLA.
So this is just opposite to what you did: I will use a processor that
does output Phi2 to replace a processor that does _not_ output Phi2. So
I will just leave it alone and unused, since the system already has it's
own Phi2.

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