FPGA Clock Timing / Setup Slack Calculator

f(max) and setup slack from tco, logic, routing and setup time — the timing-closure equation every FPGA report prints.

Target clock frequency (MHz)t(co) — clock-to-output of source FF (ns)t(logic) — combinational delay (ns)t(routing) — interconnect delay (ns)t(su) — setup time of capture FF (ns)t(skew) — clock skew helping the path (ns)Positive if the capture clock arrives later than the launch clock.

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Setup slack

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Critical-path delay

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Max frequency

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Clock period

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Timing

T ≥ tco + tlogic + troute + tsu − tskew ; hold: tco + tmin(logic) ≥ th + tskew

References: Weste & Harris, CMOS VLSI Design (timing) · Xilinx/Intel FPGA timing-closure user guides (setup/hold analysis)

Negative slack on ONE path caps the whole clock domain — that's why timing reports list the “critical path”. Fixes in order of cheapness: let the tools retime, add a pipeline register (one cycle latency for ~2× frequency), restructure the logic. Skew that HELPS setup hurts HOLD — the second equation — which no amount of slowing the clock fixes.

Clock Timing Calculator computes f(max) and setup slack of a synchronous path from its delay components — free, instant and private in your browser. FPGA students decoding their first timing report and ASIC newcomers use it to skip the datasheet algebra: type your numbers, read the answer with the substituted formula shown step by step, and share an exact permalink of the calculation.

About FPGA Clock Timing / Setup Slack Calculator

Clock Timing Calculator computes f(max) and setup slack of a synchronous path from its delay components using the standard engineering relation: T(clk) ≥ tco + t(logic) + t(routing) + tsu − t(skew); slack = T(clk) − T(path). Worked live: 0.5 + 6.2 + 1.8 + 0.4 ns of path against a 100 MHz clock leaves +1.1 ns of slack — met. The result recalculates on every keystroke, the worked-example panel shows your numbers substituted into the formula, and the Copy permalink button encodes the inputs in the URL so a colleague opens exactly your calculation. Everything runs client-side — nothing you type leaves your device.

How to use FPGA Clock Timing / Setup Slack Calculator

1Enter your values — the tool starts with realistic defaults for this exact use case, so the worked example is meaningful immediately.
2Read the live result and the worked-example panel, which substitutes your numbers into the formula step by step.
3Adjust any input to compare scenarios, then use Copy result or Copy permalink to share the calculation.

Why use FPGA Clock Timing / Setup Slack Calculator?

✓Implements the real formula — T(clk) ≥ tco + t(logic) + t(routing) + tsu − t(skew) — with the substitution shown, not a black box
✓Built for FPGA students decoding their first timing report and ASIC newcomers
✓Copy result and permalink buttons — share the exact calculation in a README, forum answer or design review
✓100% free, no sign-up, runs entirely in your browser (works offline once loaded)

Frequently asked questions

How do you calculate clock timing?+

F(max) and setup slack of a synchronous path from its delay components follows T(clk) ≥ tco + t(logic) + t(routing) + tsu − t(skew); slack = T(clk) − T(path). For example, 0.5 + 6.2 + 1.8 + 0.4 ns of path against a 100 MHz clock leaves +1.1 ns of slack — met. The calculator applies the same relation and shows the substituted arithmetic so you can verify every step.

What does negative slack actually mean for my design?+

Data arrives at the capture flip-flop after its setup window at the requested clock — the synthesis tool is telling you the silicon cannot run that fast. One failing path caps the whole clock domain; fix it or slow the constraint.

What are the standard fixes for a failing timing path?+

In rising cost order: let the tools retime/replicate registers, pipeline the path (one extra latency cycle often doubles f(max)), restructure logic (one-hot FSMs, carry-chain awareness), then floorplan. Hold violations are scarier — no clock slowdown fixes those.

Is the Clock Timing Calculator free and private?+

Yes — completely free with no sign-up or usage limits, and it runs entirely in your browser: the values you enter are never uploaded or stored on a server.

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