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From: rsbx@cbmvax.UUCP (Raymond S. Brand)
Newsgroups: comp.sys.amiga.tech
Subject: Re: CIAB Timer Questions
Message-ID: <7100@cbmvax.UUCP>
Date: 20 Jun 89 15:08:35 GMT
References: <11118@mcdphx.phx.mcd.mot.com>
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 45

in article <11118@mcdphx.phx.mcd.mot.com>, dbk@teroach.UUCP (Dave Kinzer) says:
> Keywords: CIAB, Timer, Frequency
> Summary: Where does clock come from?
> 

[...]

> 
>   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
                    ^^^^^
                   this should be 1/10

> 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.
>

The E clock from a 68000 runs at 1/10 the rate of the processor clock and
this is what the 02 clock on the 8520s is. On an NTSC Amiga, the EClock is
715909Hz, and on a PAL Amiga it is 709379Hz.

These clock rates are also part of the bus spec, so they shouldn't change
when new machines are introduced.

WARNING: If you have a genlock in you system, the master clock (28MHz) is
supplied by the genlock. This means that the EClock will be 1/40 of that
clock rate.

----------------------------------------------------------------------
Raymond S. Brand		Commodore-Amiga
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