Path: utzoo!attcan!uunet!samsung!sdd.hp.com!hplabs!nsc!pyramid!athertn!paul
From: paul@athertn.Atherton.COM (Paul Sander)
Newsgroups: comp.sys.apple2
Subject: Re: Hardware Project for II+
Summary: See Don Lancaster's book
Message-ID: <24786@athertn.Atherton.COM>
Date: 5 Jun 90 04:53:09 GMT
References: <13128.infoapple.net@pro-generic>
Reply-To: paul@Atherton.COM (Paul Sander)
Organization: Atherton Technology, Sunnyvale, CA
Lines: 26

In article <13128.infoapple.net@pro-generic> sb@pro-generic.cts.com (Stephen Brown) writes:
>I have a couple of suggestions: First you can connect a resistor up to one of
>the joystick (PDL) inputs. There are four, and usually two are used. This will
>give you a fairly precise timer.  Alternatively, in Jim Sather's UNDERSTANDIN
>G THE APPLE II, there is a small (2 IC) circuit to create an interrupt 416
>cycles before the start of the screen display.
>
>Don't forget the clock jitter of 140nS if your loop is not written in a
>multiple of 65 cycles.

Back around 1982, Don Lancaster wrote a book entitled "Enhancing your Apple II",
where he devotes a chapter or two to synchronizing with the vertical blanking,
with a one-wire mod.

I also understand that some folks reading this newsgroup have some way of
storing particular values in the unused locations scattered about the text
and hires pages, and then watch some unused location in the motherboard's
I/O page for those particular values.  It seems that these values will get
latched someplace where a program can see them during the retrace intervals.
All of this is undocumented by Apple, of course, but I seem to recall quite
a bit of discussion about this around two years ago.  Can someone elaborate
on this technique, and is it still used?
-- 
Paul Sander        (408) 734-9822  | Wisdom is beyond price, my son.  Be
paul@Atherton.COM                  | grateful that you have it.
{decwrl,pyramid,sun}!athertn!paul  | -- Imperius, in "Ladyhawk"