What is an important basic rule for a designer of a board?

What is an important basic rule for a designer of a board?

• A. Leave unused logic pins floating
• B. Tie all pins to Vcc
• C. Terminate unused logic pins with a pull-down or pull-up resistor.
• D. Do not terminate pins

Answer: (C)

A fundamental rule in digital board design is that unused inputs must not be left floating. When a pin is not driven, its input can pick up noise and switch erratically, increasing EMI and causing unpredictable behavior or spurious toggling. Terminating unused pins with a pull-up or pull-down resistor defines a definite logic level (high or low) so the line stays stable. This biasing prevents random transitions while still letting the pin be repurposed later if needed, since the resistor is a weak tie rather than a hard short. If a pin becomes actively driven, the resistor simply forms a small, harmless load and won’t interfere with normal operation. Leaving pins floating or tying every pin directly to Vcc or GND wastes power and can create conflicts if a pin is driven in the opposite direction. Not terminating pins is just inviting noise and unstable states. So, the best practice is to use a pull-up or pull-down resistor to bias unused logic pins.

A fundamental rule in digital board design is that unused inputs must not be left floating. When a pin is not driven, its input can pick up noise and switch erratically, increasing EMI and causing unpredictable behavior or spurious toggling.

Terminating unused pins with a pull-up or pull-down resistor defines a definite logic level (high or low) so the line stays stable. This biasing prevents random transitions while still letting the pin be repurposed later if needed, since the resistor is a weak tie rather than a hard short. If a pin becomes actively driven, the resistor simply forms a small, harmless load and won’t interfere with normal operation.

Leaving pins floating or tying every pin directly to Vcc or GND wastes power and can create conflicts if a pin is driven in the opposite direction. Not terminating pins is just inviting noise and unstable states.

So, the best practice is to use a pull-up or pull-down resistor to bias unused logic pins.