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 _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
