C64 + 6809

Date: 2008-09-17 11:38:25

Hallo allemaal,
This morning I had a weird idea: replace the 6510 by a 6809. Using Eagle
I checked if replacing the 6510 was possible and I found no problem. But
checking the data sheets I saw that the 6809 needs a crystal or external
clock four times the bus clock. OK, end of idea I thought. But then I
got the idea to have a look at the schematics of the SuperPET. My very
first thought was that the SP has a 16 MHz clock on the board and it is
easy to derive 1 MHz from it. But then I remembered that the 6809 board
is connected with the 8032 board through the socket of the 6502. And
here is no 16 Mhz available (I thought, but checking the SCH of the
all-in board I saw that 16 MHz is provided through the flatcable but
isn't used as so).
What completely baffled me was the fact that the outputs Q and E are
forced to become inputs and fed with a signal derived from CLK0. So IMHO
theoretically one could use the circuit from the first 6809 board
(324037-1.gif) to replace the 6510 with the 6809.
Some questions:
- did anyone notice this, IMHO weird, use of the Q and E outputs before?
- how is this possible, I mean, two outputs connected together? 
- are there 6809 experts amongst us?
- beside the well known "because it can be done", is there another
reason we benefit from a 6809 in a C64? (I was thinking about the FLEX

    / __|__
   / /  |_/     Groetjes, Ruud
   \ \__|_\
    \___|       URL: Ruud.C64.org


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