Controlling Crosstalk on Clock Lines

Clocks being delicate signals, it is natural that you should want to provide extra crosstalk protection for them. Gaining extra crosstalk protection involves two aspects: the physical means of providing more crosstalk protection and the logistical means of obtaining the correct physical result.

The physical means of providing extra protection are simple: Leave extra gaps around the traces or put the clock traces on separate layers encapsulated between solid reference ground planes. The reference planes may be either power or ground planes, provided there is a very low impedance connection and thus little noise between them.

The logistical means of providing extra protection are more complex. One must first complete the error-prone task of identifying each clock trace by either hand-drawn marks on a schematic or a list of schematic net names . Then the special routing requirements must be communicated to your layout person. The layout person then either accommodates your requests or ignores them . Remember that you and the layout people rarely share the same boss. No offense is meant to layout professionals, but realistically , a layout person generally has more than enough to do without accommodating your long list of intricate special requests. If you expect your special requests to be accommodated, you should be present when your layout professional begins working on your design. Providing snacks, pizza, or other goodies will sometimes help.

Written instructions for routing clock traces on a separate, protected layer are simple, compact, and easily understood . Many engineers therefore use this approach. Wasting a routing layer is, in their estimation, worth the cost if it accomplishes their goal without screw-ups .

A more elegant approach breaks your different trace types into separate classes. All modern auto-routers permit you to specify different minimum trace spacing requirements as a function of net class. Nets classified as clock nets can therefore be programmed to stay farther from other traces, creating less crosstalk. If your auto-router doesn't support routing by net class, get one that does.

POINT TO REMEMBER

• The physical means of providing extra crosstalk protection are simple; the logistical means are complex.