mmm@cup.portal.com (Mark Robert Thorson) (11/24/90)
Mark Thorson's pc design tips: 1. serialize critical signals, like clocks (i.e. a single line begins at an output and visits each input driven by that output, without any forks in the layout) 2. high-current signals, like RAS and CAS, should be run at a 90 angle to everything else to minimize inductive coupling 3. corners should be rounded or angled with no 90 bends, except for DC signals and power; corners cause reflections 4. no loops or circles in the layout; they act like antennas 5. a decoupling capacitor for every chip; Rogers under-the-chip capacitors are good, except with chips that run hot (and don't forget to insert them before stuffing the board with chips!) 6. locate the clock drivers in the same package, to minimize skew 7. don't run other signals through the package which buffers the clocks, RAS, CAS, etc., to minimize effects of cross-coupling through the power rails 8. series termination: 100 ohms located near driver (in my experience looking at the effects of termination on a scope, it seems mostly important whether termination is there or not, rather than the exact value which is used) 9. parallel termination: 220 ohms to Vcc, 330 ohms to GND at far end from driver 10. on slow-speed boards, block out as much copper as possible, to minimize wear on the etchant (this might not be a consideration, depending on the pricing policy of your pc shop) 11. at the same time, large areas of copper should be interrupted by space, to prevent formation of a solder drop when soldering 12. isolate strong sources of noise or circuits highly sensitive to noise with a moat on the power layers; i.e. the power for that section would be provided along one edge or in one corner, with lots of decoupling 13. when checking clearances around chips, allow for emulation probes, DIP clips, insertion/extraction tools, heatsinks, and cards inserted in edge connectors 14. always put a few blank DIP patterns in areas where more chips are likely to be needed (e.g. clock buffers, bus interface PLD's, etc.) 15. to minimize gold, remove the edge connector fingers from unused positions 16. when using chips that have on-chip oscillators with external crystals, keep the crystal close to the chip 17. orient chips with their long axis perpendicular to the flow of air 18. don't let the production people clear shorts in multilayer pc's by using a high-current pulse; this common technique causes unreliability by damaging conductors on either side of the short
markh@gamwich.hw.stratus.com (Mark Himelfarb) (11/27/90)
More points: Separate power and ground planes. This is absolutely essential in noise reduction. Do not allow traces to run parallel for long runs. Crosstalk has a nasty habit of occurring at the wrong times. Those series resistors may not necessarily be at 100 ohms; the actual value will depend upon the impedance of the pc board. I have seen series resistors used in DRAM arrays with values from 22 ohms to 47 ohms. Series resistors should also be used with newer technology devices (AS and FCT) which have etches longer than approximately 6 inches. There are lots of other issues, but the better pc layout software packages should flag these issues and others. Mark Himelfarb