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Logic levels and families

Digital logic · Intermediate · about 15 minutes · part of Zen’s Electronics Design Mastery

A logic input decides 1 or 0 by comparing its voltage with two thresholds. Mixing supply voltages can land a signal in the forbidden band between them. A 3.3 V output is too low for a 5 V 74HC input (it needs 3.5 V) but fine for a 74HCT, which has TTL thresholds. Going down from 5 V, a divider protects the 3.3 V input.

Logic levels and families: the circuit on the simulator's bench

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How it works

familyVIL maxVIH minVOH
74HC at 5 V1.5 V3.5 V≈ 4.9 V
74HCT at 5 V0.8 V2.0 V≈ 4.9 V
74LS (TTL)0.8 V2.0 V≈ 3.4 V
74HC at 3.3 V1.0 V2.3 V≈ 3.2 V

U1 runs on 3.3 V, so its high is about 3.2 V. U2 (now 74HC · CMOS) needs at least 3.5 V to see a 1.

Back down: R1 and R2 divide U2’s 5 V output to 5 × 3.3 / 5.5 = 3.0 V, safe for U1’s 3.3 V input, which needs 2.3 V. Without them, 5 V would push current through U1’s input protection diodes into its 3.3 V rail.

Try it on the bench

  1. With the input switch off, U1 outputs 3.2 V and U2 (74HC) flickers: 3.2 V is in its forbidden band.
  2. Select U2 and change its family to 74HCT: the same 3.2 V is now a solid 1. This is why 74HCT chips are used as level shifters.
  3. Delete R2: the 3.3 V chip now gets the full 5 V on its input, beyond its supply. Real chips survive that only through their clamp diodes, if at all.

Parts on the bench

In the course

Zen’s Electronics Design Mastery, module “Digital Electronics”.

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