Debugging Unconstrained Paths in Tempus

25 Mar 2026 • 2 minute read

Introduction: When Timing Paths Go Silent

Imagine running timing signoff late in the design cycle and encountering a frustrating message: “No constrained timing paths found with the given description.”

No violation. No slack. Just…nothing.

For design and signoff engineers, unconstrained timing paths are one of the most dangerous blind spots. These paths escape timing analysis not because they are clean, but because something is missing, disabled, or incorrectly modeled.

This blog walks through what unconstrained paths are, why they occur, and how Cadence Tempus Timing Signoff Analysis Solution helps debug them. It presents a series of videos created to demonstrate the debugging practices in Tempus.

Why Unconstrained Paths Matter in Timing Signoff

An unconstrained path is not analyzed for setup or hold checks. That means:

Violations can be completely missed
Signoff coverage is incomplete
Silicon risk increases significantly

The unconstrained paths usually arise due to:

Missing or incorrect constraints
Disabled or inactive timing arcs
Clock propagation issues
Path exceptions, such as false paths or clock groups
Library or modeling problems

Understanding why a path is unconstrained is just as important as fixing it.

Core Learning Themes and Debug Techniques

The foundation of every debugging exercise is identifying why Tempus did not constrain a path, which check arcs failed, whether arcs are inactive or disabled or if there is any clock reference issues at the endpoint.

Identifying Inactive or Disabled Timing Arcs

This subject describes how constant propagation or user-applied constraints disable critical timing arcs required for setup and hold analysis, and how to trace and resolve such unconstrained paths in Tempus.

Clock Propagation and Clock Sense Issues

This section covers scenarios where a timing path becomes unconstrained because the clock waveform fails to reach the endpoint reference pin, often due to clock sense or clock propagation constraints.

False Paths Exceptions and Clock Group Constraints

This topic explains how unconstrained timing paths can result from unintended false path constraints or how clock-based false paths or logically exclusive clock groups can cause unconstrained paths between clocks. It then demonstrates how to identify, analyze, and validate false paths using Tempus reporting commands.

The Debugging Flowchart: A Repeatable Methodology

A dedicated debugging flowchart ties everything together, guiding engineers through:

Checking if the path exists
Identifying unconstrained reasons
Validating clocks and constraints
Investigating library and arc issues

This reinforces a methodical, repeatable approach rather than ad-hoc debugging.

Conclusion: From Blind Spots to Signoff Confidence

Unconstrained timing paths are not just tool warnings—they are signoff risks.

Through hands-on labs, realistic scenarios, and structured debugging workflows, the unconstrained timing path debugging videos empower engineers to: