gEDA-user: M68K bus and transmission line ringing

[msokolov@xxxxxxxxxxxxxxx (Michael Sokolov)]

Hello fellow gEDA/pcb users,

I know there are a few old-timers on this list (by that I mean hardware
engineers of old school), and my question is directed to those.  Would
anyone here happen to experience designing a system with the good old
Motorola MC68302?  (It's an old-school telecom processor, if I'm not
mistaken it's an early 1990s chip, but it most perfectly fits the late
1980s computing and communications environment, the era of MicroVAXen.
Has the most classic original 68000 core inside.)

I'm using MC68302 on my SDSL board, and I'm now in the process of
putting the finishing touches on this design before sending it off to a
layout contractor.  Here is my area of concern: my current schematics
(see http://ifctfvax.Harhan.ORG/OpenSDSL/OSDCU/) have the M68K bus
interconnecting the MC68302, RAM, flash, the SDSL transceiver's
microprocessor control port and the FPGA.  I have pull-up resistors on
the bidirectional control signals (AS, UDS, LDS, DTACK).  But I don't
have any series resistors yet, and I wonder if I should add some, and if
so, where?

I've been told that when a sufficiently fast-switching signal is driven
onto a sufficiently long net, one has to worry about this signal ringing
due to transmission line effects.  That of course raises the question of
just how fast it needs to be and just how long do the traces have to be
for the issue to become a concern.  I've been told that my MC68302
running at 16 to 25 MHz is fast enough, and that traces running half-way
across my 130x165 mm board would be long enough that I do need to worry
about this stuff.  I've been told that the solution is to insert series
resistors of ~30 Ohm into the nets close to the source.

Here are my questions which I hope might be answered by someone who has
some experience with MC68302 or any M68K-based design:

* Does the MC68302 in fact produce slew rates fast enough on the M68K
  bus signals it drives (address bus, data bus on writes, control
  signals) for the designer to be worried about transmission lines
  ringing?

* As I've mentioned before, my design will be laid out on a 130x165 mm
  PCB.  I can probably get the core microprocessor subsystem (MC68302,
  RAM and flash) fairly close together in one corner, but I also need
  the bus to go to the SDSL transceiver chip.  The latter is a mixed
  signal IC and its physical placement is rather inflexible as it's a
  critical component standing on the boundary between the digital and
  analog sections of the board.  The latter requirement would probably
  mean that my M68K bus traces *will* run half-way across my PCB.  Is
  running half-way across a 130x165 mm PCB long enough for me to be
  worried?

* If the answers to the previous two questions are affirmative (i.e.,
  that I do have a transmission line ringing issue which needs to be
  addressed), what's the proper way to address it?  Would series
  resistors take care of it?  If so, exactly where should I put them?
  On the address bus?  On the data bus?  On the control signals?

* I've been told that the series resistors I'm talking about go right
  after the source.  But where is the source on a bidirectional
  multipoint net?  Take the data bus for example: it's bidirectional and
  goes to multiple peripherals.  Series resistors between the MC68302
  and the main bus nets would take care of writes, but what about reads?

I hope someone can give me some insight, preferably backed by some
actual experience with M68K designs.

TIA a lot,
MS


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