Restricted Fmax due to minimum period

Thanks,

The restricted Fmax reported by Quartus is always based on worst-case silicon, voltage, and temperature corners. It’s a safe, guaranteed number to ensure reliable, repeatable operation across any production device, over temperature, process variations, and voltage shifts. Your specific part on the bench might be a very fast sample at room temperature, with clean power rails and excellent signal integrity — which is why you're seeing it exceed the spec.

But Altera (and any FPGA vendor) has to spec for the slowest, hottest, lowest-voltage corner case device — not the best-case one on your desk.

The minimum period restriction is often tied to internal clock tree delay balance, LE register switching performance, and clock pulse width requirements. Even if your simple clock divider logic is fine, the global clock network and clock control blocks are spec’d to a conservative value to avoid marginal operation in less ideal scenarios.

You’re right — your testing showed impressive margins. But production designs, especially in safety-critical or industrial environments, need to lean on the datasheet limits for reliable, field-safe operation.

You’ve really helped clarify and close the loop on a tricky subject — and it was a pleasure following your methodical, evidence-backed investigation!

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