I understand the benefits of using differential pairs for signals running at 100MHz and above.
(1) Can you speak about the impact of using differential clocks in a parallel bus?
(2) Do the differential clocks maintain the noise suppression characteristics when daisy-chained in a multidrop environment? Has anyone tried this and had positive experiences vs. single ended multidrop clocks?
Thanks for your interest in High-Speed Digital Design.
In general, I have found differential distribution to be a very effective means of combating ground bounce in the transmitting package, ground bounce in the receiving package, as well as the ground shifts that occur on either side of the connectors in high-speed systems. These benefits accrue in multi-drop configurations as well as point-to-point configurations.
I have found differential distribution to be of little value in reducing the impact of crosstalk generated locally by other traces on the same PCB. This is because the crosstalk function from nearby traces falls off very steeply with distance. The impact of this is that differential pairs cannot be placed particularly close to any aggressive signal. For example, imagine a system that has one aggressive trace, and a nearby victim trace that is receiving unacceptable amounts of crosstalk. Now I propose to protect the victim trace by splitting it into a differential pair, and using a true differential receiver. Assuming that I don't want to affect the layout density, I plan to implement the centerline of the new pair right on top of the original victim trace. In other words, when we split the victim one member of the pair will have to move closer to the offending source, while the other moves away. Unless the two traces of the pair are extremely close together (less than a third of the original separation between centerlines), the extra crosstalk we pick up from the nearby side of the differential pair overwhelms any "balancing" effect we might have hoped to gain from the far side of the differential system. To mitigate this effect you have to separate the the victim pair from the aggressive signal. In the final analysis, it's usually the extra separation that is providing most of the crosstalk benefit, not the fact of balanced signal distribution. When you want to battle crosstalk picked up on a PCB (over a solid ground plane), increasing the trace separation will probably result in a more dense design than using differential distribution.
For clocks, I see differential signaling used a lot, both in point-to-point distribution and in multi-drop distribution. The multi-drop aspect does not diminish the ground-bounce-canceling properties of differential signaling. There are only a few clock nets in a system (compared to the number of data nets) and it isn't that difficult to provide this extra measure of protection.
For parallel bus signals, I rarely see differential distribution used because it doubles the number of wires required. That will cause the phenomenon known as "routing headaches" among your PCB layout staff.
Dr. Howard Johnson