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From: cs1598@athena.ecs.csus.edu
Newsgroups: comp.sys.m68k
Subject: Re: HELP HELP HELP on some Technical Questions for the 680000!!!
Message-ID: <1991Apr4.105321.12087@csusac.csus.edu>
Date: 4 Apr 91 10:53:21 GMT
References: <1991Apr02.174845.5076@lynx.CS.ORST.EDU> <1991Apr2.220648.23079@colorado.edu> <1991Apr3.215104.21550@netcom.COM>
Sender: cs1598@athena.csus  (Michael Noriega)
Organization: California State University, Sacramento
Lines: 33

In article <1991Apr3.215104.21550@netcom.COM> mcmahan@netcom.COM (Dave Mc Mahan) writes:
>
> 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.

  VPA*, VMA*, and E* are all used in synchronous communication.  When
the 68000 looks for DTACK* and does not receive it, it checks for VPA*.
If VPA* is asserted then it now knows it is talking to a sychronous
device.  VMA* and E* are then used in communicating to the device.

  Also VPA* is used when an interrupt is auto-vectored.  In other words
if the vector number is not provided to the CPU (DTACK* will also not 
arive then) VPA* is asserted telling the CPU to look up the vector based
on the interrupt level.

      - Mike