On Friday 19 October 2007, Michael Sokolov wrote: >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. > That I can't give, but what I can suggest is to find a decent tome on transmission line theory just to give you some background. Also, any multi-point scheme is probably going to a cut & cry solution. Even the act of probing a trace to see what the signal looks like can change things enough that a working circuit will fail, or a failing circuit will work.
Circuit board trace widths & spacings to other traces all become data to be optimized in situations like that. The usage of a src termination r is often a grand compromise, and may have more to do with the FCC's radiation of a computing device pass/fail tests than in actual circuit performance. More than one consumer device has been rendered very undependable by those requirements. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Revenge is a form of nostalgia. _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user