What is a pull-down resistor used for in digital circuits?

What is a pull-down resistor used for in digital circuits?

• A. To pull voltage up to Vcc
• B. To ensure a defined logic low when an input is not actively driven
• C. To increase current
• D. To store charge for timing

Answer: (B)

A pull-down resistor provides a defined low logic level when an input line is not actively driven. Digital inputs are high-impedance and can float if nothing is connected, so a floating input may pick up noise and wander between logic levels. Connecting a resistor from the input to ground gives a weak path to ground, pulling the voltage toward 0 V and ensuring a stable logic low when nothing else is driving the line. When a driver actively drives the input high, the resistor simply draws a small current (approximately Vdd divided by the resistor value) and does not prevent the high level, as long as the source can source that current. The resistor value is chosen to keep power consumption reasonable while still overpowering leakage and noise. If you need the line to read high when not driven, you’d use a pull-up resistor to Vcc instead. Storing charge for timing would be the job of a capacitor, not a resistor.

A pull-down resistor provides a defined low logic level when an input line is not actively driven. Digital inputs are high-impedance and can float if nothing is connected, so a floating input may pick up noise and wander between logic levels. Connecting a resistor from the input to ground gives a weak path to ground, pulling the voltage toward 0 V and ensuring a stable logic low when nothing else is driving the line. When a driver actively drives the input high, the resistor simply draws a small current (approximately Vdd divided by the resistor value) and does not prevent the high level, as long as the source can source that current. The resistor value is chosen to keep power consumption reasonable while still overpowering leakage and noise. If you need the line to read high when not driven, you’d use a pull-up resistor to Vcc instead. Storing charge for timing would be the job of a capacitor, not a resistor.