Re: Ok, anyone started Contiki 4 AII yet?

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

In article <3E77D9FF.6060401@mindspring.com>, Rob <rsteinmetz@mindspring.com>
writes:

>On Mon, Mar 17, 2003 at 11:25:28PM -0600, Rob Steinmetz wrote:
>
> > David Wilson wrote:

<snip>

> > >The Apple II does have a crystal - 14.31818 MHz if I recall it 
>correctly.
> > >It is the CPU clock that is more complicated with its stretched 
>cycle 1 in
> > >65.
> > >
> > >
>
> > Thanks for the information. This did not make it to comp.sys.apple2. Do
> > you mind if I post it there?
>
>
>Sure. I must have hit reply instead of followup.
>
>On my MPF-III (a //e clone) I added a flip flop so that it could generate
>IRQ everytime the screen got to VBL - I used it to run a software clock for
>ProDOS before I built myself a hardware battery backed clock. The only 
>problem
>is that the 5.25" driver disables IRQ for long periods so your clock 
>gradually
>looses time. Here in  Australia our power is 50Hz rather than 60Hz so that
>would change the code required to count ticks slightly.

This is why no simple timer-interrupt counting routine can ever be
expected to keep accurate time on the Apple II as long as it can access
a 5.25" disk.  When I wrote a software clock, I used the 6522 chip in a
peripheral card (not standard equipment) to both generate 1Hz pulses
and to count them (in another 6522 counter register).  The effect was
a one second interrupt (which was not always serviced within a second)
and a counter which kept track of exactly how many ticks had occurred
since the clock was started.  Wraparound of the tick counter is handled
without additional logic.

-michael

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