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From: sb@pro-generic.cts.com (Stephen Brown)
Newsgroups: comp.sys.apple2
Subject: Re: Hardware Project for II+
Message-ID: <13128.infoapple.net@pro-generic>
Date: 3 Jun 90 08:32:13 GMT
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Organization: The Internet
Lines: 24

In-Reply-To: message from mdavis@pro-sol.cts.com

In this message, Morgan Davis says:
 
>If you've ever tried to write routines on the II+ that require precise
>timing, you know you're in for trouble. Especially now with a proliferation
>of faster CPU's and speed-up cards that work on the Apple II+, the problem
>is even worse. On every other Apple, you can read the vertical blanking
>signal (wich cycles every 60th second) and sychronize timing loops to that.

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.

Hope this helps.  Stephen Brown (Willowdale, Ontario, CANADA)

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