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From: alfter@uns-helios.nevada.edu (SCOTT ALFTER)
Newsgroups: comp.sys.apple2
Subject: Re: CPU speed detection
Message-ID: <2429@unsvax.NEVADA.EDU>
Date: 7 Dec 90 19:30:35 GMT
References: <6129@crash.cts.com>
Sender: news@unsvax.NEVADA.EDU
Organization: University of Nevada System Computing Services
Lines: 36

In article <6129@crash.cts.com> mdavis@pro-sol.cts.com (Morgan Davis) writes:
>Fortunately, reading $C019 in the manner given above works correctly on
>both the IIe and IIGS.  Since the IIc doesn't have a polled VBL signal,
>you'd have to write a VBL interrupt handler which did your tick counter
>incrementing.  On the II+, I designed a method by which a VBL-like signal
>can be mapped to a different hardware location ($C060).  The article
>describing this scheme was posted a few months ago.  Let me know if you
>want it.

All this talk about getting consistent timing at different clock rates
brought up a problem with some telecomm routines I was writing.  I
needed some way to time a hardware BRK signal.  Presently the routine
just uses a tightly-coded loop, but the timing is only valid at 1 MHz.
I figure that on a 60 Hz system, counting 14 ticks of the VBL will
count 233.333333... ms, which is very close to the 233 ms that a
hardware BRK is supposed to last.

A problem crops up with 50 Hz systems (not that I have one, but
someone who might want to use my program might have one).  The time
taken by one tick is longer, so instead of getting something close to
233 ms, 14 ticks will last 280 ms.  Is the timing of a hardware BRK
absolutely critical, or can you have pretty much any length so long as
it's at least 233 ms?

Also, does anybody have a clue how to go about getting a tick counter
running on a IIc?  The IIe/IIGS solution has already been spelled
out--just tack on a few extra bytes to count 14 ticks.  Something was
mentioned about having to use interrupts to count ticks on the
IIc--sounds yucky since it would involve installing and detaching an
interrupt daemon every time you want to send a BRK. :-(

-----------------------------------------------------------------------------
Scott Alfter                             _/_
                                        / v \ Apple II:
Internet: alfter@uns-helios.nevada.edu (    ( the power to be your best!
   GEnie: S.ALFTER                      \_^_/