Re: 6809 / 6702 puzzle

Interesting...

I notice that the initial value of $D6 is one of our "hit" values.

My take on the SuperPET schematics is that most of the address lines are used in decoding a valid address for access to the 6702 chip. The 
least significant address lines (BA1 and BA0) are not used (which is where the lower 0..3) comes from but all of the remaining buffered 
address lines are used in the address decoding. A15-A12 are used to decode page 'E'. Address line A11 appears to be an active high enable 
to the '138. A10 down to A8 are the A, B and C lines to the 1 of 8 decoder ('138) with a value of 7 ('111') being used to derive an active 
low enable to the 6702. The remaining address lines (A7 down to A2) appear to also be wired to active high and active low chip enable 
lines.

My understanding therefore is that the 6702 is addressed when the address lines are 1110 1111 1110 00XX or $EFE0 through $EFE3. I would 
assume that any other reads will access non-existent devices and will potentially return garbage (unless there are some data bus 
terminating resistors somewhere which would provide a known default data bus value).

By looking at the schematics I am also led to believe that the 6702 should be accessible from the 6502 processor (as the 6809 and 6502 
appear to be "next door to each other" in the schematic diagram and share the address and data buses). This means that you should be able 
to boot the SuperPET into 6502 BASIC and use good old PEEK and POKE to access the 6702 without any interference from the BASIC 
interpreter.

Dave

> Message Received: Apr 21 2012, 09:01 AM
> From: "Rhialto" <rhialto@falu.nl>
> To: cbm-hackers@musoftware.de
> Cc: davee.roberts@fsmail.net
> Subject: Re: 6809 / 6702 puzzle
> 
> Ruud has just brought his SuperPET here for research.
> 
> Preliminary observations:
> 
> - Some keys on the keyboard don't work. This makes typing commands a bit
>   difficult. Can one clean the keyboard by taking it apart or is this
>   likely to be more difficult?
>   I do have some other 8032-type PETs whose keyboard I could swap in.
> 
> - Inside there is the single-board version of the SuperPET expansion.
>   No separate 6702 board.
> 
> - Initial value of EFE0-EFE3: D6.
> 
> - From (a remark on) the schematic at zimmers.net
>   (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/SuperPET/324055.gif)
>   it appeared that the 6702 might be addressable on all addresses EFex
>   (where e is any even digit and x is any digit). This doesn't appear to
>   be true, but the EFex and EFox (o for any odd digit) do read out
>   differently.  EFex seems to be FF (except for EFE0-EFE3) EFox seems to
>   change on some occasions (possibly when EFE0 changes).
> 
> - Reading from EFE0 doesn't change the previous value; the state of the
>   dongle doesn't change on just reads. So far, it seems to change only
>   when you write in a *different* value, that also isn't 00.
> 
> -Olaf.
> -- 
> ___ Olaf 'Rhialto' Seibert  -- There's no point being grown-up if you 
> \X/ rhialto/at/xs4all.nl    -- can't be childish sometimes. -The 4th Doctor
> 

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