Scripting STA – TCL Tricks to Catch Timing Bugs Early

From RTL to Reality – Issue #4

Title: Scripting STA – TCL Tricks to Catch Timing Bugs Early

“When your design scales, your eyes can’t catch every path — but your scripts can.”

💻 Why TCL Scripting in STA Matters

As Physical Design and STA engineers, you’re constantly working with massive timing reports. Hundreds of paths, thousands of violations. Manually checking? Not scalable. That’s where TCL scripting becomes your best friend.

🔍 What Can You Do with TCL in STA?

With a few lines of TCL, you can:

• Filter setup/hold violations by slack

• Extract worst paths by clock domain

• Automate summary reports for quick review

• Validate false paths or multicycle path definitions

• Highlight clock skew & latency mismatches

Let’s walk through some battle-tested TCL scripts you’ll use every week.

🛠️ Useful TCL Scripts for STA

1️⃣ Get All Violating Paths by Slack

📌 Use this to catch all setup/hold failures below 0ns.

2️⃣ Filter Setup Violations Only

🔍 Focus only on setup timing issues.

3️⃣ Filter Hold Violations Only

🧯 Useful when debugging short paths.

4️⃣ Summary of Slack Ranges

📊 Understand how deep your violations go — bucketed summary.

5️⃣ Identify Violations on a Specific Clock Domain

⏰ Isolate issues domain-wise. Helps debug cross-domain failures too.

6️⃣ Highlight Clock Latency Issues

📌 Use this before and after CTS to spot dangerous skew.

7️⃣ Automate Daily STA Sanity Check

📂 Creates organized report directories for daily regression.

🧠 Real Project Insight

In a recent project, STA showed no hold violations — but functional simulation failed. A quick TCL script revealed paths with positive hold slack < 0.05ns, dangerously close to margin.

Adding hold buffers blindly? Not the fix. The script helped us catch borderline cases before tapeout.

🧑🎓 Tips for New Engineers:

• Start small — don’t fear scripting. Copy, tweak, test.

• Build a library of reusable snippets

• Use comments and variables to make them readable

• Automate repetitive checks — focus on analysis, not clicking

📚 GATE Tie-In:

While GATE doesn’t test TCL directly, knowing how to debug STA helps you:

• Understand setup/hold path delays

• Appreciate clock-tree impacts

• Think algorithmically about data path integrity

This is how backend reality connects to front-end theory.

🎯 Challenge of the Week

• Run a timing report on your own block

• Use -min_slack and -from/-to to isolate domains

• Write a custom TCL that filters violations below -0.25ns

• Post your result summary on LinkedIn and tag #FromRTLtoReality

🗓️ Coming Next:

“Hold Time – The Silent Killer of Chip Stability” Learn why fixing fast paths is trickier than slow ones — and why blind buffering can cost more than you think.

📩 Enjoyed these TCL insights? Subscribe, share, and let’s automate smarter — From RTL to Reality.

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