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From: dbk@teroach.UUCP (Dave Kinzer)
Newsgroups: comp.sys.amiga.tech
Subject: CIAB Timer Questions
Summary: Where does clock come from?
Keywords: CIAB, Timer, Frequency
Message-ID: <11118@mcdphx.phx.mcd.mot.com>
Date: 20 Jun 89 05:25:36 GMT
Sender: listen@mcdphx.phx.mcd.mot.com
Reply-To: dbk@teroach.UUCP (Dave Kinzer)
Followup-To: comp.sys.amiga.tech
Distribution: na
Organization: Motorola Microcomputer Division, Tempe, Az.
Lines: 42


   First let me thank Karl Lehnbauer, Paul Higgenbottom, et.al. for
the CIAB timer code.  I have substantially modified the code for my
own application, but it was a great starting point, saving me days
of time.  8^)

   I am trying to determine the exact timing of the counts but I cannot
get what I see in the documentation to agree with what was in the code.

   Problem: The code given appears to inherit whatever was in the
"inmode" bits of control register B (bits 6 and 5).  This might not
be a good assumption, since a previous task could set these to whatever
it wanted while it had the hardware.

  Assuming (go ahead, correct me 8^) ) that both bits are zero (the
only thing sensible to me), we are counting 02 clocks.  Of course, the
schematic does not have an 02 clock pin on the device, but us old 6800
hackers know that that is the E pin.  The E pin is generated by the
68000, and will be 1/8 the clock frequency.  Now, the analysis proceeds
like this.

   NTSC system crystal frequency:      28.63636 MHz
   7M clock = above/4 (68000 clock):    7.15909 MHz
   E clock = above/8  (02 pin):       894.88625 KHz

   Yeilding an E clock period of:     1.11746 uS


   Unfortunately, all the documentation for the code says the CIAB timer
runs at 1.396825 uS/clock.


So, HELP!!!    Where have I gone wrong?


   I would also like to make this work correctly for PAL machines, so I 
would like to know the exact frequency of the PAL machine's clock.


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