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From: albaugh@dms.UUCP (Mike Albaugh)
Newsgroups: comp.sys.m68k
Subject: Re: What's the difference between the MC68661 and the MC68681?
Message-ID: <843@dms.UUCP>
Date: 13 Oct 89 18:30:35 GMT
References: <14121@jumbo.dec.com>
Organization: Atari Games Inc., Milpitas, CA
Lines: 35

From article <14121@jumbo.dec.com>, by ehs@jumbo.dec.com (Ed Satterthwaite):
> In article <9306@pyr.gatech.EDU>, byron@pyr.gatech.EDU (Byron A Jeff) writes:
> 
> ...
>> 
>> One caveat from the first time I used it: you must let it generate it's
>> DTACK in it's own good time. I put it in a 68008 test board that had
>> a grounded DTACK and the 68681 screwed up it's read FIFOs (4 byte
>> FIFO) when it didn't get it's DTACK.
>> 
> 
> Just so if you're running the 68681 on an asynchronous bus, which is what
> the original poster wanted.  But you don't have to wait for DTACK if you
> are running the '681 synchronously.  Maybe everyone knows that, but the

	Just a note here... 680x0's sometimes get _real_upset_ if you give
them both DTACK and VPA. The fine print in the manual says "don't do that",
but you sometimes end up doing it unintentionally, like adding just one
synchronous peripheral to a system that has DTACK grounded or having a device
at 0xFFF... that DTACKS, while your autovector stuff is looking at the FC
lines and asserting VPA. One of the real bears about this is that _most_
of the time it doesn't seem to matter (works anyway), and then under certain
(timing? temp?, phase of moon?) circumstances it just plain weeds the processor.

					Mike

BTW: I personally feel that autovectors are "the way to go" in an "operating
system" sort of environment, because interrupt entry/scheduler interactions
usually end up sending you through the same place for at least a whole
class of devices, but "de gustibus non est disputandum" :-)

| Mike Albaugh (albaugh@dms.UUCP || {...decwrl!pyramid!}weitek!dms!albaugh)
| Atari Games Corp (Arcade Games, no relation to the makers of the ST)
| 675 Sycamore Dr. Milpitas, CA 95035		voice: (408)434-1709
| The opinions expressed are my own (Boy, are they ever)