Threshold Classifies Input - Electronic Circuits IV & XII: Digital Logic and Timing Closure Step by Step

A threshold-classifies-input scan checks each input voltage against exact thresholds, classifying inputs at or below the low threshold as LOW and at or above the high threshold as HIGH row by row. The arithmetic is exact only for the stated input voltages, thresholds, logic states, and rows in the ideal threshold-logic model. Real gates add noise-margin limits, supply tolerance, input leakage, finite rise time, loading, probe/calibration limits, finite precision, and measurement uncertainty.

A logic input is accepted only after exact voltage thresholds classify it. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

Equality at the low threshold is LOW

An input of 1 volt equals the low threshold, so the helper classifies it as LOW.

1\ \text{V} \le 1\ \text{V}\Rightarrow \text{LOW}

Equality at the high threshold is HIGH

An input of 4 volts equals the high threshold, so the helper classifies it as HIGH.

4\ \text{V} \ge 4\ \text{V}\Rightarrow \text{HIGH}