Re: Apple II Interrupt card

[et472@FreeNet.Carleton.CA (Michael Black)]

Michael J. Mahon (mjmahon@aol.com) writes:
> Unless we know that interrupts are actually required for Contiki, this
> whole thread is just an exercise.
> 
> Adam earlier suggested that the interrupt routine is only used to
> update a "real time seconds" clock, which is only polled in the idle
> loop to determine (presumably) whether something has timed out.
> 
> If this is the case (someone with the source should be able to
> tell for sure), then a simple timing loop while idling will do the job
> with sufficient accuracy for TCP/IP.
> 
> No interrupt source would be needed to run Contiki.
> 
> If you really want to play with timer interrupts, then the method of
> tapping into VBL (available on _all_ Apple II's) and using it to pull
> an open-collector gate low on the IRQ line will do the job.  You
> may want to use a two-input gate so that an annunciator can
> enable/disable the VBL interrupt.  In any case, no slot card is
> required--this is all doable with wires and machined-pin sockets.
> 
> -michael
> 
After digging out the Apple Reference Manual, and being daunted because
despite the schematic in two different places it has to be one of
the most difficult to follow, I remembered that Don Lancaster had done
something along these lines in his "Enhancing Your Apple II, Vo 1"
(was there ever a second volume?).

Alas no interrupt, but he takes a signal off the video timing chain
and feeds it into to a "phantom SW3 input".  It's in the Field Sync
chapter.  He shows software to keep track of that input.

    Michael