Re: VBL interrupts (was: Ok, anyone started Contiki 4 AII yet?)

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

On Tue, 18 Mar 2003 10:40:07 -0500, Yves McDonald
<""yves.mcdonald\"@NO SPAM sympatico.ca> wrote:

>>Some good news for //e, 2c users!  From a quick look in my //e Technical 
>>book (Apple/Addison-Wesley edition), there is an interrupt associated 
>>with Vertical Blanking (every 16ms) and OS interrupt support with ProDOS 8.

No, the //e has only a pollable I/O location, not an interrupt.

>>The bad news: sorry for ][+ users but there are no built-in interrupt 
>>sources, altough a clever hardware hack exists to get vertical retrace 
>>interrupts (REF: Jim Sather, Understanding the Apple II, p.3-19 to 
>>3-26).  I guess Apple ][ users will have to provide their own  timer 
>>interrupt services.

Not surprising.  The VBL signal is certainly available, and is only at
most one inversion away from driving IRQ.  There's probably even a
spare open-collector inverter on the main board...

But from what was said earlier in the original thread, what is needed
is an (approximate) real time, not timer interrupts.  So a clock card
could do the job just fine.

Since it was also stated (by Adam) that it is called only within the
idle loop, then a software timing loop to count elapsed time should
be perfectly adequate, bypassing all this talk of interrupts and real
time.  ;-)

-michael

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