RE: New small sized MOS8701 replacement

The question is why would you want to do that? The clock chips that Passi,
my self, and others are using is specifically made for this purpose and
works much better than the 8701.

I have been doing stability testing the last few evenings and have found
that the IC525 clock chip will run without glitching in most cases where the
8701 will not. I set up a MOSFET, driven by a signal generator, to short out
a series resistor so I can vary the supply voltage rapidly. I can vary the
supply by ~1V and the '525 goes on merrily and the 8701 falls over.

I was trying to recreate the failure case that all the C64 Reloaded boards
have with 5V loads on the user port causing clock instability. The 8701
replacement on the MK1 and the built-in clock on the newer version stumbles,
but you can plug a real 8701 into the Mk1 and it works fine. Not having an
actual MK1 board to test on I'm trying to recreate it. 

At this time, it looks like it is probably related to the layout/jumpers of
the two crystals or some strange interaction with the built-in switching
supplies for the VIC and SID. Some '525 based replacements that have a
built-in crystal will work on the MK1 so I'm leaning toward some goofy thing
in the trace routing around the crystals. 

On a real C64 I can't induce a failure and on the 8701 testing rig I made I
also cannot induce a failure.

For the 8701 replacement I designed I was after a simple drop-in
replacement. You have a dead 8701, you pop the replacement in, and it works.
No jumpers, not additional crystal needed, etc. 

It was interesting that Passi, my self and Appy Paul all designed '525 based
replacements at about the same time and all took a different route to get it
to fit in a small area.

Jeff Birt

-----Original Message-----
From: tokafondo <tokafondo_at_gmail.com> 
Sent: Thursday, July 30, 2020 5:59 AM
To: cbm-hackers_at_musoftware.de
Subject: Re: New small sized MOS8701 replacement

One question for you guys:

What if instead of creating a new 8701, a small board integrating the old
style of dividing the clock could be made?

In the earlier revisions of the C64, the schematics show a bunch of 74LSxx
chips doing the clock division.

Could that be replicated in a FPGA or CPLD module, instead of using this
small chip?



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