How are clock tree DRC constraints different from clock tree timing constraints, and why does CTS need both?

Short Answer

Timing constraints (setup/hold via skew and latency) ask whether the clock arrives at the right time at every register. DRC constraints (max transition, max capacitance, max fanout) ask whether every individual clock net is electrically sound, independent of when it arrives. A tree can be timing-clean and DRC-broken, or DRC-clean and timing-broken -- they're genuinely separate axes CTS has to satisfy simultaneously.

Technical Explanation

• Timing constraints (setup/hold via skew and latency) ask whether the clock arrives at the right time at every register.
• DRC constraints (max transition, max capacitance, max fanout) ask whether every individual clock net is electrically sound, independent of when it arrives.
• A tree can be timing-clean and DRC-broken, or DRC-clean and timing-broken -- passing one check says nothing about the other.
• CTS has to satisfy both simultaneously, which is why report_timing and report_constraint are separate commands checking separate things, not two views of the same result.

Follow-up Question & Model Response

"Which of the two -- timing or DRC -- would you check first when debugging a clock tree problem, and why?"

Candidate Model Response: Timing first, generally -- because a DRC-clean but timing-broken tree at least tells you where the real skew/latency problem is, while a timing-clean-looking report can be hiding an electrical DRC issue that report_timing was never going to surface in the first place.