The most common reason a designer will quote this design rule is because they want an excuse to avoid calculating impedance for a bus that has an impedance specification.

Why Designers Use the Critical Length Rule Therefore, I’m comfortable declaring that this concept should never be used as a design rule, and I’ll outline why below. If you are using this rule, you are just guessing. If a design rule has 9 different possible values that someone might follow, then it’s probably a useless design rule. Unfortunately, one must realize the following:Īll of the above values are contradictory, and the rule should only be used in a specific situation. I’ve seen designers quote that the critical length is 1/2, 1/3, 1/4, 1/5, 1/6, 1/8, 1/10, 1/12, and 1/20 the distance traveled during the rise time. The main problem here is the massive lack of context. This rule is quoted so often by new designers that you will see many proclaim themselves as experts just for reciting it. The rule should not be used in most cases. This high-speed PCB design rule is only correct in very specific situations.

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