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Re: My Mockingboard Clone

On 8/3/2010 12:30 PM, Linards Ticmanis wrote:

On 08/03/2010 03:00 PM, Polymorph wrote:


I just completed implementing my most challenging hardware project to
date - a Mockingboard clone!

I can report that the card works great! I have bought but not installed
audio amplifiers for the card, but with powered speakers you don't need
the amplifiers - it sounds great just as it is.


Looking good.

I'd recommend connecting the /RST inputs of the AY chips to system
/RESET through two diodes, so that it doesn't annoy you by blaring on
and on after a reset. It's what I did on my own clone (which looks
roughly similar to yours). The diodes are of course necessary to prevent
a system reset from occurring when the software wants to reset just the
AY chips through the VIA PB2 line.


I actually published a 1-pager in CALL-A.P.P.L.E. about that very
modification back in the early 1980s!  ;-)  This was a bug in all the
slot Mockingboards.


Also, if you do your own programming for the card and don't just want to
use it with existing games, it makes sense to connect PB6 to PB7 on each
of the VIAs (kudos to Michael Mahon, IIRC, for that idea). That way you
can link the chip's two 16-bit counters together for longer intervals.
Without it, the lowest possible IRQ frequency is about 15 times per
second. With that connection, intervals of more than one hour are possible.


Thanks!  I used this approach to build a software clock that kept
accurate time even when interrupts were disabled for seconds at a time
by disk accesses (or whatever).

It's also worth noting that a Mockingboard has two mostly unused ports
on the two 6522s (six bits on each), and four completely unused ports
on the AY-3-8910s (16 bits on each).  The AY ports are much less
versatile than the 6522's and an extra bit fiddle away (accessed through
the 6522s) but they are there, and completely unused.  It could have
supported a pretty flashy LED display!  ;-)

-michael

NadaNet 3.1 for Apple II parallel computing!
Home page:  http://home.comcast.net/~mjmahon/

"The wastebasket is our most important design
tool--and it's seriously underused."