A pull-up resistor is a component used in digital circuits to ensure that a particular pin or line is pulled to a high voltage level (usually the supply voltage) when it is not actively being driven low by another component. Here’s a detailed explanation of why it’s called a “pull-up” resistor:
1. **Functionality**: The primary function of a pull-up resistor is to "pull" the voltage level of a digital input to a high state (logic high, often represented as 1) when no other component is driving the line low. This prevents the input from floating, which can cause unpredictable behavior or noise.
2. **Name Origin**:
- **Pull-Up**: The term “pull-up” comes from the action the resistor performs. It “pulls” the voltage up to a high level (e.g., Vcc or Vdd) when the line is otherwise disconnected or floating.
- **Resistor**: The component is a resistor because it provides a resistance between the input pin and the supply voltage, allowing the input to be set to a high state without needing additional power or active circuitry.
3. **Why "Pull-Up"?**:
- In contrast to a “pull-down” resistor, which pulls a line to a low state (ground) when it is not being driven high, a pull-up resistor connects the line to a high voltage level. This helps ensure that the line does not remain in an undefined or floating state, which could lead to erroneous readings or signals.
4. **Usage**:
- Pull-up resistors are commonly used in various applications, including digital inputs, open-collector or open-drain configurations, and to ensure defined logic levels in microcontroller and digital logic circuits.
In summary, the “pull-up” part of the name reflects the resistor's role in pulling the voltage level up to a high state when it would otherwise be floating.