Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!wuarchive!psuvax1!rutgers!cbmvax!daveh
From: daveh@cbmvax.commodore.com (Dave Haynie)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: 14 mhz
Message-ID: <15192@cbmvax.commodore.com>
Date: 16 Oct 90 23:17:39 GMT
References: <21053@orstcs.CS.ORST.EDU>
Reply-To: daveh@cbmvax.commodore.com (Dave Haynie)
Organization: Commodore, West Chester, PA
Lines: 43

In article <21053@orstcs.CS.ORST.EDU> rudolpe@upas.CS.ORST.EDU (RUDOLPh ERIC) writes:
>Here goes again:
>I installed a 16mhz 68000p12F (F!) in the rev 5 amiga 500. For the 14mhz signal,
>I tried CDAC* XOR 7mhz and (NOT) CDAC* XOR 7mhz with a 74Fast86.

If that's the only logic, you're doomed to failure.  Even ignoring the problems
with the E/VPA*/VMA* logic for now, any accelerated 680x0 must still respond
properly to the standard 68000 bus signals at 7MHz.  So, for example, that
means that you don't drive AS* on the bus until S2, in 7MHz time.  Same for
the DS* lines on read, and on write you wait until S4, again in 7MHz timeframe.
You don't look at DTACK* until the S4->S5 transisiton, 7MHz timeframe, and you
don't count on any valid data until the S6->S7 transition. 

That's the simple basics.  From there, you have to worry about the E/VPA*/VMA*
logic; you can't feed an 8520 the E from a 68000 clocked at 14MHz and expect
it to work right, if at all.  If you're trying this with other hardware, you
really need BR* and BG* to be clocked in and out of the 68000 on the right
clocks, or DMA devices may lock up.  

In short, there's a reason accelerator boards cost more than 74F86s and 
74F74s, and there's also a reason even some commercial accelerator boards
have trouble with expansion hardware.

>Now when I turn on the computer, it ALWAYS comes up dark grey first, then 
>bright green replaces it. I get 10 short flashes, and one long flash, and it
>tries to restart.

Your 68000 is having trouble communicating either with the 8520, Chip RAM,
or very likely both.

>I am under the impression that DTACK will hold off weird bus cycles.

DTACK* gates 68000 access to Chip RAM, true.  But you have to play by its
rules, as described above.

>In my opinion, this is a very very poor Hack.

In that, we are in complete agreement.

-- 
Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests"
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	Standing on the shoulders of giants leaves me cold	-REM