When is a level shifter required in logic interfacing? In mixed-logic designs, under what condition is a level-shifter circuit necessary between two logic families or devices?

Introduction / Context:
Logic devices from different families or voltage domains often cannot directly communicate because their logic-level thresholds and absolute maximum ratings differ. A level shifter translates voltage levels so that a logic HIGH/LOW produced by one device is recognized safely and reliably by another without overstress.

Given Data / Assumptions:

• Different VCC rails (e.g., 3.3 V MCU to 5 V peripheral, or 1.8 V FPGA bank to 3.3 V sensor).
• Logic thresholds (VIH/VIL) and I/O protection structures vary.
• Bidirectional or unidirectional translation may be required.

Concept / Approach:

When supplies differ, a “HIGH” from the lower-voltage side may be too small for the higher-threshold device, and a “HIGH” from the higher-voltage side may exceed the lower-voltage device’s maximum rating, risking damage. Level shifters (translators) solve both problems by mapping voltage levels and, where needed, providing direction control and impedance buffering.

Step-by-Step Solution:

Verification / Alternative check:

Datasheet threshold tables and application notes confirm when direct connection is permitted (e.g., 5 V tolerant inputs) vs. when translation is mandatory.

Why Other Options Are Wrong:

“Always/never needed” is overly broad; identical supplies may obviate translation.

Same-supply systems typically do not need level shifting unless special I/O standards are used.

Fan-out alone does not mandate level translation.

Common Pitfalls:

Assuming “5 V tolerant” equals full 5 V logic compatibility; ignoring pull-ups that inadvertently lift lines beyond a device’s VCC; forgetting rise-time impacts with passive translators on fast buses.

Final Answer:

when the supply voltages are different