Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!usc!apple!netcom!mcmahan
From: mcmahan@netcom.COM (Dave Mc Mahan)
Newsgroups: comp.sys.m68k
Subject: Re: HELP HELP HELP on some Technical Questions for the 680000!!!
Message-ID: <1991Apr3.215104.21550@netcom.COM>
Date: 3 Apr 91 21:51:04 GMT
References: <1991Apr02.174845.5076@lynx.CS.ORST.EDU> <1991Apr2.220648.23079@colorado.edu>
Organization: Dave McMahan @ NetCom Services
Lines: 39


 In a previous article, rainer@boulder.Colorado.EDU (Rainer Malzbender) writes:
>In article <1991Apr02.174845.5076@lynx.CS.ORST.EDU> rudolpe@prism.CS.ORST.EDU (Eric Rudolph) writes:
>>
>>I need to know about how VPA* VMA* E* and AS* are related. I have one
>>timing diagram for them and a little chart that tells the relative
>>timings, but I need to know WHAT they do and how they relate in English.
>>Things like "what would happen if the device didn't assert VPA" and 
>>"Why does the 68k assert VMA* and how does it relate to VPA*" stuff like
>>that.... I would be very grateful. I am designinga the famed speedup circuit
>>for the Amiga....
>
>VPA, VMA, and E are used to interface to older 6800-style peripheral chips.
>E is a clock that is the CPU clock divided by 10. VPA and VMA mean "valid
>peripheral address" and "valid memory address" (I think - all my hardware
>manuals are at home). They basically inform the 68000 that it is talking
>to a 6800 peripheral. I could dig up the databook later, but I'm almost
>positive you don't have to worry about these signals at all if you're
>designing for the Amiga. On my 68000 designs I never use VPA and VMA;
>sometimes I use E if I need a slower clock.

Be careful here!!  I know for a fact that the CIA chips on the amiga use the
E-clock to provide proper clocking for things like the disk drive.  You MUST
preserve the frequency and the relative phase of the clock with respect to
other signals, or your floppy disks won't work and your time-of-day clock
will run fast/slow.  Other strange things happen if you mess with the E-clock
on an amiga.  I'd post an article in comp.sys.amiga.tech and find out the
answers if you don't want to get burned.  Try and get a response from Dave
Haynie if you can.  He designed it.

>Rainer Malzbender                                 Save a dinosaur - buy DEC.
>Dept. of Physics (303)492-6829
>U. of Colorado, Boulder         rainer@boulder.colorado.edu 128.138.240.246

     -dave

-- 
Dave McMahan                            mcmahan@netcom.com
					{apple,amdahl,claris}!netcom!mcmahan