It is difficult to generalize without knowing more of the specifics such as logic family, clock rate, edge rate, trace impedance, etc. but I'll try. ;)
scott....@jci.com wrote: > All, > > I have a question regarding trace routing for high-speed clock signals. > > I have one driver, and two receivers. The distance between the driver and > 1st receiver is roughly 2.5cm, the distance between 1st receiver and 2nd > receiver is 3cm, and the distance between driver and 2nd receiver is 3.5cm. > > I actually have 2 questions: > > 1) What is a good rule of thumb for routing and termination of this > high-speed clock trace? Do it carefully. Sorry to be so obtuse but without knowing what the "speed" is in "high-speed" I don't know what else to say. > > 2) Is daisy chain routing preferred (meaning driver to 1st receiver, > then 2nd receiver), or should 2 traces of equal length be routed, 1 to each > receiver (each of these two traces will have twice the impedance of the > single trace emanating from the driver so that impedance matching will be > maintained)? Daisy chaining is to be avoided since it increases clock skew but it is much simpler to drive and terminate. Termination reduces reflections but leads to higher static and dynamic current flow. EMI is related to current flow so it is possible to increase EMI by terminating. Double the traces and you more than double the termination problems. There are partial solutions such as partial termination and diode clamping techniques such as "Forced perfect" but none are simple. This is why SI Engineers make good money. For your geometry your 2t reflection on a daisy chain would occur around 300 ps. This is probably acceptable unless you have some sub nanosecond logic in your circuit. Ok to generalize, you don't need to daisy chain if the total path is not over say 5 cm in length. (This assumes you are not running sub nanosecond or TTL logic.) You can probably get by without terminating or by partially terminating in say 200 ohms. Fred Townsend DC to Light Consulting > > > Any information, in general, or in specific on the subject is greatly > appreciated. > > Thank you. > > Best Regards, > > Scott Mee > Johnson Controls Inc. > Automotive Systems Group > EMC Product Compliance > > 616.394.2565 > scott....@jci.com > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Ron Pickard: emc-p...@hypercom.com > Dave Heald: davehe...@attbi.com > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > http://ieeepstc.mindcruiser.com/ > Click on "browse" and then "emc-pstc mailing list" ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
