Re: How do I add firmware to a card?

["Alex Freed" <alexf@mirrow.com>]

"Tristan Mumford" <xtristan.xmumford@xgmail.xcom> wrote in message 
news:45a07cb2$0$38175$c30e37c6@lon-reader.news.telstra.net...
>
> The uC is attached directly to the apple bus via it's PSP and a few extra
> lines like a couple of address lines maybe?
>

Gentlemen, before you get carried away please try to model the timing with a 
simulator
or even on a piece of paper.

When designing the PseudoDisk I originally thought it could be possible to 
directly connect
the AVR to the Apple bus. The AVR runs at 16 MHz and executes (for the most 
part) an
instruction per clock cycle. So on the surface it is fast enough. Now 
consider that you have
to respond to an external signal - for example /DEVSEL. Interrupts take too 
many cycles.
A busy loop will not respond predictably - it will work sometimes and will 
not work during
some cycles. The read data to 6502 should appear within 150 nS from /DEVSEL 
assertion.
Can't be guaranteed.

Even an ARM uC running at 60 MHz will often fail to react this fast. The 
reasonable solutions
are either using RDY signal to make 6502 wait till the uC is ready or to use 
glue logic. I used
a CPLD for this purpose.

-Alex.