DJ - On Thu, Sep 21, 2006 at 11:47:38AM -0400, DJ Delorie wrote: > I'm thinking I should design in some series resistors on the address > and data lines for my RAM expansion board (30MHz, about 6" of 8 mil > trace on DS FR4, 5v).
5 Volts? How quaint. You didn't give the thickness of the FR4 between the trace and the ground plane. Series resistors for a bidirectional trace go near the middle of their length. For unidirectional traces, place the resistor anywhere from the middle to the source end. > How does one go about calculating an ideal > resistance? The mcu spec doesn't have a min rise time spec, the ram > chips are 20ns (50MHz) and imply they want a 3ns rise time. Think of it this way: you want to charge a parasitic capacitance (guess 10 pF) with a rise time of 3 ns. That means you need a series resistance less than 3.0 ns / 10 pF = 300 Ohms. > I figure I can put in zero-ohm resistors for now, and experiment with > one of the lines, but I'd rather just come close enough on the first > try ;-) Any resistor in the range of 25 to 150 Ohms will damp the ringing. It takes a good FET probe and a fast 'scope to characterize the system well enough to optimize on the bench. Whenever I have this concern, I just put in 100 Ohms, observe that it works, and go on to the next chore. - Larry _______________________________________________ geda-user mailing list geda-user@moria.seul.org http://www.seul.org/cgi-bin/mailman/listinfo/geda-user