Re: Apple II Interrupt card

[Todd <tmyers@autobahn.mb.ca>]

	Hi, Bill;

   The video sync and composite sync is the same thing only the terminology changed
in the different tech.ref's :) As for the heartbeat that comes over from the Unix
stuff I've been involved with (mainly Linux). The way I think this thing works is
that the sync signal goes into a counter/flipflop and pulls the IRQ line high every
30-15Hz and is reset by accessing the slot (at any of the 16 addresses @ $C0Fx for
slot 7) which pulls 'DevSelect low and resets the sync timer to low. and since slot 7
is the past in line for the IRQ you don't need to setup a pass for the IRQ line. THis
gives a simple easy circuit. The only problem to this exercise is the cost of the
board for the slot. If you can get double-sided boards made relatively cheaply then
making more of these should keep costs down.

   If you want this to run in the GS then no hardware is needed since the stock GS
has interrupts running at 60Hz and at 1Hz that you can tap into (a little more
involved than that) that is discussed in the Hardware and Firmware ref's (I believe).

   The board could be made as I earlier speculated but piggybacked over the
motherboard and hardwired into the right spots to generate a heartbeat but you would
need to make sure that _no_ other cards generate interrupts as a self-reseting
interrupt would cause havoc and eventually lockup the computer.

	Later, Todd.

Bill Garber wrote:
> 
> Todd,
> 
> Your perception of the INT MGR card's function is very good.
> The heartbeat clock as you so quaintly call it, is what the card
> provides. Judging from a quick look at the signals accessed by
> the card, I'd say the composite sync doesn't even come into play.
> This thing is basically monitoring the video sync signal and each
> cycle pulls the IRQ low and jumps to the Interrupt handling
> routine. Contiki should monitor the phase zero clock and use this
> pulse for it's timing. Feel free to correct me if I'm seeing this
> incorrectly. Now, I would think that this would be better than
> using the IOU since the clock signal drops out on disk accesses.
> I have noticed also that on a IIgs rom 3, which is not in question here,
> that my "your card" in slot 4 being my SCSI card sometimes makes
> my mouse very jerkish during disk access, so I would think that the
> use of the mouse card may not be a good idea for this clock function.
> However, when I have the SCSI in slot 7, and mouse enabled, I don't
> have this problem, so on II, II+, IIe it should work fine.
> The only problem I foresee, which has been mentioned, is that this
> card takes up a slot, and many folks have all their slots filled.
> Anyway, FWIW, there must be some pulse signal common to all
> 6502 based Apple IIs that Contiki can read for timing.
> After a quick check of the 6502 pinout, I see that IRQ is pin 4,
> Sync is pin 7, and phase zero clock is pin 37. Device select should
> not be needed as we'd be accessing the cpu directly. What do you
> think?
> 
> Bill @ GarberStreet Enterprises
> http://garberstreet.netfirms.com