Re: gEDA-user: M68K bus and transmission line ringing

[Mark Cianfaglione <mcianfa@xxxxxxxxxxxxx>]

geda-user-bounces@xxxxxxxxxxxxxx wrote on 19/10/2007 07:25:19 AM:

> 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? 

I have fond memories of the 68K family...;-)

> 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?
> 

It's reasonably loaded. Too low a load was bad as the drivers of the 
processor were hefty so the overshoot and undershoot were nasty unless you 
dampened them with series resistors.

> 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.

We used to use 32 ohm series resistors to do that. You can get them in 
nice packages now that don't take up huge space requirements.

> 
> 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?

On heavily loaded systems (like yours) it's not really a huge issue 
although we would as a precaution put them in on the address and control 
lines anyway on certain designs. (I design telecom equipment so it had to 
run for 20 years as designed.) 

> 
> * 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?
> 

You would probably see ringing on the other chips if there is any. One 
saving grace in your situation is your bus speed. In reality the control 
signals have to be beautiful. The address can be loud as long as it 
settles before the minumum setup time but as it's a single source (unless 
you are doing DMA) it can be controlled by series terminations at the 
processor. The data is a little harder to control as it's a multi source 
bus. Essentially the strongest signal on the bus will be the worst quality 
to the other devices.

> * 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?

See above...

> 
> * 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?
> 

There's the issue. There is a possibility of using AC terminations. 
Essentially it's a series RC (32 ohms and a .1uf cap or so...) circuit 
that you would put on anywhere on the bus net to ground. This has the 
advantage of terminating the net but not causing extremely high drive 
currents as the termination is only effective on the transitions (the 
rising or falling edges). There are several suppliers of pre-packaged AC 
terminations for uses like yours.


I hope I've added to your body of knowledge...;-)

Mark


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user