When a signal exceeds its critical length, it becomes a distributed length transmission line, and its effects must be managed properly, or it will become significantly degraded. The critical length is the primary filter for determining if a net requires high-speed management. 1. If a signal can be routed to less than its critical length, it will most likely not have the high-speed concerns described in this book, which need to be mitigated. 2. Critical length is exceeded when the time to propagate a conductor's length is greater than 1/4 of the signal rise or fall time, whichever is faster. This is when the problems begin. 3. If the total interconnect of the net is greater than its critical length, it must be managed with the constraints and/or methods described in this book. 4. Total interconnect length includes pin-package, routing, and via-used lengths. 5. Create max length constraints to flag nets exceeding their critical length.
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