20/09/2025 4:53 am
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I’ve read that unused inputs of logic gate ICs should always be tied either HIGH or LOW, but why this is necessary.What problems can occur if these inputs are left floating? Does it depend on the logic family (TTL vs CMOS), and what’s the recommended practice for handling unused inputs in a circuit?
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23/09/2025 4:30 am
Unused inputs of logic gate ICs should never be left floating because they can cause unpredictable circuit behavior. In TTL logic, unconnected inputs tend to float high due to internal biasing, but they remain highly noise-sensitive, which may lead to erratic switching and increased power consumption as the input transistors may conduct partially.
In CMOS logic, the problem is more severe since inputs have extremely high impedance; a floating input can settle at any undefined voltage, easily influenced by noise or static charges. This can cause random oscillations, unnecessary switching, higher power consumption, and even possible damage due to internal shoot-through currents.
To avoid these issues, the recommended practice is to always tie unused inputs to a defined logic level, either VCC (logic HIGH) or GND (logic LOW). This can be done with a direct connection or through a resistor (typically 1kΩ–10kΩ if current limiting is desired).
