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How to read datasheets

A voltage of 1.4 V is applied to an input of an IC whose input thresholds are specified as VIH(min) = 2.0 V and VIL(max) = 0.8 V. How is it interpreted?

  1. AIt is below VIH(min) of 2.0 V, so it is always interpreted as L
  2. BIt is above VIL(max) of 0.8 V, so it is always interpreted as H
  3. CWhether it is interpreted as H or L is not guaranteed
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Answer: C. Whether it is interpreted as H or L is not guaranteed

VIH(min) is the lowest voltage guaranteed to be recognized as H, and VIL(max) is the highest voltage guaranteed to be recognized as L. The range between them, 0.8–2.0 V, is an undefined region where the input may be recognized either way depending on the unit and temperature. If an input stays in this region for long, the output may oscillate, or the pMOS and nMOS of the input stage may turn on simultaneously and shoot-through current may flow. For slowly changing signals, use a Schmitt-trigger input with hysteresis.

Logic levels and noise margins H output L output Read as H Undefined Read as L VDD 0 V VOH(min) VOL(max) VIH(min) VIL(max) NMH NML Output guarantees Input thresholds NMH = VOH(min) − VIH(min) NML = VIL(max) − VOL(max)
Logic levels and noise margins · View in diagrams
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More How to read datasheets questions

  1. The driving IC has VOH(min) = 2.9 V and VOL(max) = 0.4 V, and the receiving IC has VIH(min) = 2.0 V and VIL(max) = 0.8 V. What is the low-level noise margin?
  2. Which correctly describes the problem with connecting the output of an IC powered from 3.3 V directly to a CMOS input powered from 5 V (VIH(min) = 0.7 × VDD)?
  3. What does it mean when a datasheet describes an input pin as "5 V tolerant"?
  4. Which correctly describes the change when the value of the pull-up resistor on an open-drain output is reduced?
  5. What is the main problem when many CMOS inputs are connected to a single output of a CMOS logic IC?