On Oct 6, 2007, at 7:25 PM, Andy Peters wrote: > On Oct 6, 2007, at 6:37 AM, Randall Nortman wrote: > >> On Sat, Oct 06, 2007 at 07:21:22AM -0400, Bob Paddock wrote: >>> >>>> I route power inwards under the chip, with vias to the power >>>> planes and >>>> the decoupling cap on the other side. >>> >>> Sometimes doing that adds enough inductance to cause problems, >>> in high frequency boards. >> >> What does "high frequency" mean? I'm in the <50MHz range. > > It's not the frequency that's important, it's the rise time of the > signals.
Yep. That, and the response time of inputs. I know of a recent NASA mission where a processor chip was "derated": operated at a speed lower than normal spec in hopes of improving reliability. The board was made with relatively bulky components (again for reliability), and therefore somewhat spread out. Theoretically adequate for a chip as slow as the derated chip, but.. The chip itself was, of course, much faster than its derated spec. And when they tested the "cold case" (-20 C, I believe), that made it even faster. The system failed the test. It turned out that better bypassing (capacitors closer to the pins) was needed. Moral: useful reliability is a property of systems, not of components. Don't let component reliability considerations damage the system design. John Doty Noqsi Aerospace, Ltd. http://www.noqsi.com/ [EMAIL PROTECTED] _______________________________________________ geda-user mailing list [email protected] http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
