Signal integrity is critical for reliable operation of high-speed interfaces, and the MachXO5-NX family defines overshoot/undershoot limits on input pins to protect long-term device reliability. Those limits are expressed against the Unit Interval (UI). This article explains how UI is determined for static and aperiodic control signals — such as EN, PGD, and RST — that assert only once and have no fixed operating frequency, and it corrects a common misreading of the 20 µs condition.

For periodic signals, where UI is derived from the signal frequency, see the companion article Overshoot and Undershoot Specs Explained (UI Calculation + Examples), which walks through worked periodic examples.

What UI (Unit Interval) Means

The Unit Interval is defined as UI = 1/f. For a periodic signal it comes straight from the frequency — for example, a 500 kHz square wave gives UI = 1/500 kHz = 2 µs. The overshoot/undershoot table then expresses each allowable voltage level as a percentage of one UI, and that percentage is the duration the signal is permitted to remain at that level.

"100% UI" therefore means the full UI duration — a time budget, not a fixed pulse width. Importantly, the %UI rules only apply when UI is less than 20 µs (equivalently, f greater than 50 kHz, since 1/20 µs = 50 kHz).

The 20 µs Is Not the Overshoot/Undershoot Pulse Width

A frequent misreading is that the "UI < 20 µs" condition means the overshoot or undershoot pulse itself is 20 µs wide. It does not.

The 20 µs is a threshold for applicability of the table, not a pulse width or bandwidth. When UI < 20 µs (f > 50 kHz), the percentage-based timing rules apply. When UI ≥ 20 µs (f ≤ 50 kHz), the table no longer applies in the same way, and designers should instead confirm the peak stays within the absolute maximum ratings. Real overshoot transients are typically only a few nanoseconds wide regardless.

Which UI to Use: Periodic vs. Static/Aperiodic Signals

The correct UI depends on whether the signal repeats:

• Periodic signals derive UI from frequency (UI = 1/f). Faster signals have a shorter UI and therefore a tighter absolute time budget for the same %UI allowance; slower signals allow more time for the overshoot to decay, as long as UI stays under 20 µs. The companion article shows this with two 500 kHz examples — one that exceeds its allowed duration and fails, and one that stays within it and passes.

• Static and aperiodic signals — EN, PGD, RST and similar control pins — assert only once and have no fixed frequency, so there is no 1/f to compute. For these signals the specification fixes UI = 20 µs. In effect, the applicability boundary itself becomes the reference UI, giving the %UI allowance a consistent basis even when no clock is present. This is the key difference from the periodic case.

Applying the Spec to an Aperiodic Signal — Worked Example

To check compliance, compare both the peak level and its duration against the table using the fixed 20 µs UI:

• For a 1.8 V VCCIO signal, the overshoot threshold is VCCIO + 0.5 V = 2.3 V.
• At the VCCIO + 0.5 V level the table allows 100% of UI, so the time budget is 100% × 20 µs = 20 µs.
• If a peak slightly exceeds 2.3 V (for example, 2.322 V) but the overshoot lasts only about 5 ns, that duration is far inside the 20 µs allowance, so the signal passes.

The voltage and timing figures above are illustrative to demonstrate the method — they are not measured results.

Bottom Line

Both "UI < 20 µs" and "100% UI" refer to the allowance window, not the pulse width. For periodic signals, compute UI = 1/f (see the companion article). For static and aperiodic signals such as EN, PGD, and RST, use the fixed UI = 20 µs. In either case, convert the applicable %UI into a time budget (%UI × UI), then confirm your measured peak and its duration fit within it.