What is STA - Static Timing Analysis - And How Do You Ensure the Chip Will Work at the Right Frequency?

So far we’ve covered:

• Synthesis
• Place & Route
• Layout creation

But there’s one critical problem we haven’t solved yet:

How do you ensure all signals arrive on time?

If one gate received a signal too late - all the hardware might not work. If the timings aren’t correct - the chip will fail.

This is exactly the job of:

STA - Static Timing Analysis Or: static timing analysis.

This is the stage where you ensure the chip can operate at the planned clock frequency.

Why Is Timing So Critical?

Modern chips work at insane speeds:

• 1GHz = one thousand million operations per second
• 5GHz = five thousand million operations per second

In each clock cycle:

1. A signal must leave one place
2. Pass through gates
3. Pass through wires
4. Reach the destination
5. Be captured in a flip-flop

All this must happen in a very short time - sometimes less than a nanosecond.

If something takes more time - the chip won’t work.

What’s the Problem with Time?

Every component in the chip takes time:

• AND gate - takes time to compute
• Wire - takes time for signal to pass through
• Flip-flop - takes time to capture a new value

This time is called: Delay.

If the sum of all Delays is too large - the chip won’t meet the clock frequency.

What Are Setup Time and Hold Time?

For a flip-flop to capture the correct value, the signal must arrive:

Setup Time

The signal must arrive before the clock edge by a certain time.

If it arrives too late - the value won’t be captured.

Hold Time

The signal must remain stable after the clock edge for a certain time.

If it changes too quickly - the value might be wrong.

STA checks both these conditions on every path in the chip.

How Does STA Work?

STA takes:

• The Netlist
• The Layout
• The constraints on clock frequency
• The specification of each gate (how much time it takes)

And runs analysis:

1. Selects every path in the chip
2. Sums all Delays in the path
3. Checks if it meets Setup and Hold
4. Calculates the margin (Slack) - did we arrive on time or not

If Slack is positive → path is valid. If Slack is negative → problem! The signal won’t arrive on time.

What Happens If There’s a Timing Violation?

If STA discovers a path that doesn’t meet timing:

1. Need to change the design
2. Redo Placement or Routing
3. Add Buffers (amplifiers) along the way
4. Move blocks closer together
5. Change the constraints

This is an iterative process.

Sometimes multiple rounds are done of:

• Synthesis
• Place & Route
• STA
• Fixes

Until reaching a design that works at the required frequency.

Why Is It Called “Static”?

Static = without dynamic execution.

STA doesn’t run simulation. It doesn’t activate the chip.

It only:

• Measures the paths
• Calculates the times
• Checks if there’s enough time for each signal

It’s very fast - but also complete.

STA checks all paths, not just some of them.

An Analogy to Clear Things Up

Think about a train system:

Setup Time: The train must arrive at the station before releasing the passengers.

Hold Time: The doors must remain open long enough for passengers to get off.

STA: Checks that every train on all lines arrives on time - without checking actual travel.

Why Is STA So Important?

Because:

• Without it, you can’t know if the chip will work at the required frequency
• It discovers problems before manufacturing
• It ensures the design is ready for the next stage
• It prevents expensive failures

STA is the “stamp of approval” that timings are correct.

What Is the Result of STA?

At the end of the process, you get a report containing:

• List of all paths
• Critical paths (slowest ones)
• Timing violations (if any)
• Recommendations for improvement
• Confirmation that the chip will work at the required frequency

Only after STA passes successfully - can you advance to the next stage.

Where Do We Go From Here?

After STA passes:

1. Run additional simulations
2. Test the design on FPGA
3. Perform chip emulation
4. And advance toward Tapeout

In other words:

STA is the stage that ensures the chip is not only logically correct - but will also work at the required speed.

Summary

STA - Static Timing Analysis is:

• Timing analysis of all paths in the chip
• Checking that all signals arrive on time
• Preventing Setup and Hold violations
• Confirmation that the chip will work at the required clock frequency
• A critical process before moving to manufacturing

This is the stage that ensures the chip will be fast enough.

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In the next post, we’ll learn about Simulation, FPGA, and Emulation - tools that allow testing the chip before manufacturing, and seeing it “run” in the real world.