Re: Apple II mistery card...

[mjmahon@aol.com (Michael J. Mahon)]

"a" replied:

>Michael  wrote
>> There is typically a small RC delay network on Phase 2 to interface
>> a 6522 to the Apple bus.  A gate is usually used to buffer the delayed
>> (and rounded) signal before driving the 6522.
>>
>> -michael
>>
>
>Do you have an idea how much time delay is necessary?
>I've been trying to work that out from the 6502 datasheets and the 6522
>datasheets.  If the two chips were connected directly together it seems
>that such a delay wouldn't be necessary because the phase-2 output
>of the 6502 would be used.  However, it appears that
>the apple II system uses phase-0 as phase-2 was intended to be used and
>that phase-1 on the apple board is directly out of phase with phase-0.
>Whereas, the 6502 datasheets illustrates phase-0 as being used only
>to set the 6502 clock frequency and that the phase-1 and phase-2 outputs
>from the 6502 should be used for system timing.  I'm sure there's a story
>about why Woz decided to do it this way.  Any how, two things would help:
>1) good idea on how much delay is necessary. (6502 data sheet doesn't say
>    how much the phase-1,phase-2 outputs lag the phase-0 input) Is it
>possible
>    that no delay at all would work?
>2) a schematic of a working card that was in production or that someone
>    homebuilt that they have confidence in.  (I'm scared of frying things)
>
>I've studied the apple serial card schematic that uses the MC6850 for
>the serial line driver and its chip select input doesn't need any delay from
>the phase-0 signal from the motherboard.  That got me thinking that I
>was worried about nothing and should just try no delay.

It is entirely possible that no delay will work, for example, with a
faster 6522.  The only issue is achieving the setup and hold times
for data read from and written to the chip.  Those will be spec'ed
directly or indirectly relative to Phase 2.

A design that uses a 6522 as a parallel interface to a sound chip
is the Mockingboard, and the schematic of the Mockinboard is
available (somewhere) on the web--perhaps even linked from the
CSA2 FAQ.

-michael

Check out parallel computing for 8-bit Apples on my
Home page:  http://members.aol.com/MJMahon/