Re: Apple II mistery card...

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

I earlier wrote:

>"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.

The Mockingboard schematic is at:

http://apple2.org.za/gswv/a2zine/Docs/MockingboardSchematic.jpg

and it shows a method for delaying Phase 1 slightly using three
inverters, a couple of resistors, and a capacitor.

-michael

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