Checksum: 00113
Path: utzoo!utgpu!anakin
From: anakin@gpu.utcs.toronto.edu (Anakin Research)
Date: Sat, 12-Nov-88 23:56:43 EST
Message-ID: <1988Nov12.235643.5124@gpu.utcs.toronto.edu>
Organization: University of Toronto Computing Services
Newsgroups: comp.sys.amiga
Subject: Re: Lucas board
References: <6053@killer.DALLAS.TX.US>
Reply-To: anakin@gpu.utcs.UUCP (Anakin Research)
Keywords: Lucas 68020 68881

 
Walter and other interested folk! 
 
I've a lot of messages and I'll reply them soon, but anyone who draws out
a schematic from a node list deserves an instant reply. You say:
 
>There are a couple of things that bother me about the Lucas board. The first
>is that the documentation says that you might have to select parts for U9.
>Any design that is requires this is a commercial no-no. I question the
>stability over temperature of a design like this.
 
>The second is that there seems to be an incompatability between the 68020
>and the 68000 when operated in byte mode. The 68k puts out UDS when the
>byte in bits 8-15 gets put out and LDS when the byte in bits 0-7 gets
>put out. The 68k manual says that that it puts the data out in both bytes
>when only one of the strobes is true. However, the 68020 always puts bytes
>out in bits 24-31. The problem with the Lucas board is that if you do a
>byte move to an odd address, the 68020 winds up writing to bits 8-15 where
>the 68k writes to bits 0-7. Am I overlooking something? I didn't see
>anywhere in the 68020 manual where is says that it duplicates data on the
>bus when in byte mode.
 
Yes indeed this is a commercial no no, but in the end I could not solve
the async problem any other way. I know its klugey, but it works. So far
I've noticed no temperature problems. I felt it was a small trade off in
order to have boards that work at 12 through 20 Meg. (25 with a PAL change)
as many cannot afford the more expensive parts. In my day work I could
never get away with it. God, I love the Public Domain!
 
There is no byte incompatability in byte mode to an odd address. 
The internal multiplexor duplicates the byte in all four places(D0-7
D8-15 D16-23 D24-D31) whenever SIZ0 = 1 and SIZ1 = 0. In byte transfers
A0 and A1 have no meaning. (See page 5-5 of the 020 USER MANUAL)
 
The crystal socket has 14 pins only four of which are used by the
oscillator. I'm afraid I used the others as vias when I was laying
out the board. Honest, it was tight and I never thought I'd get caught.
I'm so ashamed. 
 
                                Brad Fowles