Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!husc6!bloom-beacon!mgm.mit.edu!wolfgang From: wolfgang@mgm.mit.edu (Wolfgang Rupprecht) Newsgroups: comp.sys.m68k Subject: Re: DTACK* Message-ID: <1613@bloom-beacon.MIT.EDU> Date: Wed, 14-Oct-87 17:08:05 EDT Article-I.D.: bloom-be.1613 Posted: Wed Oct 14 17:08:05 1987 Date-Received: Fri, 16-Oct-87 03:59:18 EDT References: <4215@pyr.gatech.EDU> Sender: daemon@bloom-beacon.MIT.EDU Reply-To: wolfgang@mgm.mit.edu (Wolfgang Rupprecht) Organization: Independent Software Consultant Lines: 34 In article <4215@pyr.gatech.EDU> jkg@pyr.UUCP (Jim Greenlee) writes: >I have a basic fundamental (probably stupid) question about DTACK*. I was >told (by a Motorola person) that it was possible to wire DTACK* directly >to ground and have a 68000 system run at full speed, provided the memory >access time was fast enough. This advice is in direct conflict with the timing diagram for the 68000. In the 8 Mhz. part, you have only 120 ns to get /DTACK high after /AS goes high. (ref: moto data book for the 68000). I had the pleasure of debugging someone elses design (always fun), where they violated this spec. Timing spec whats that? ;-) What I observered via a logic analyzer is this (and I find this fascinating!): The /DTACK somehow gates the 60000's external address bus latches. If The /DTACK stays low, the address bus stayes 'stuck' at the previous address. The internal PC is quite alright, in fact the 68000 will continue running when this happens. It just fetches the data from last address again, all the time thinking it fetched the data for the current address. The 68000 will limp along like this for quite a while. When you let up on the \DTACK you will see that correct address on the address bus. The pc has infact been incremented by two, for every instruction-space fetch. Neat huh? Perhaps you could use this to single step, but why bother? It's just as easy to write a hack single step monitor using the trace bit in the status register. -wolfgang Wolfgang Rupprecht UUCP: mirror!mit-mgm!wolfgang ARPA: wolfgang@mgm.mit.edu (IP addr 18.82.0.114) Wolfgang Rupprecht UUCP: mirror!mit-mgm!wolfgang ARPA: wolfgang@mgm.mit.edu (IP addr 18.82.0.114)