Re: Overclocking an Apple ][?

[mjmahon@aol.com (Michael J. Mahon)]

Simon Williams wrote:

>Michael J. Mahon <mjmahon@aol.com> wrote:
>
>> The Apple IIc+ is such a two-domain system, and it alone
>> offers the possibility of easy overclocking of its "fast"
>> processor/cache domain.
>
>Which brings us back to the original question of what might be wrong
>with _my_ IIc+... since it seems to function perfectly well aside from
>the aforementioned lack of speed.

Hi, Simon!

This problem is sufficiently exotic that it makes me wonder
exactly how you know what speed your machine is running
at.

As you probably know, the IIc+ is a Zip Chip "workalike", and
it can be configured to run at various speeds and to slow down
for certain slots, speaker, joystick, etc. by a configuration
utility that usually runs just after ProDOS loads.

Before the utility runs, the configuration is defaulted to slow
down on some slots (for disk compatibility).  I don't know
exactly what the defaults are for the IIc+.

When the machine comes out of reset, if it finds an ESC in the
keyboard latch, it enters "synchronous mode", or 1MHz mode.
It would be bizarre if some hardware fault made it look like this
was happening all the time!

There are many other logic conditions that cause the accelerator
to slow to 1MHz for either a short time or indefinitely.  If the
timer register in the accelerator is not working, then any "short"
1MHz sequence would be extended indefinitely, producing the
symptom you describe.

Since the timer is embedded in the ASIC, if it isn't working,
replacing the ASIC may be the only solution, and those ASICs
and IIc+ machines are in one-to-one correspondence, so getting
another IIc+ may be required.  ;-(

-michael

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