---
tags:
  - resource
created: 2025-11-06 17:37
---

## Summary

Notes on the use of the [rppal]() crate.
## Notes

### GPIO Pull Up/Pull Down Bias

The `rppal::gpio::InputPin::set_bias` method is used to configure the Raspberry Pi's **built-in pull-up or pull-down resistors** on an input pin. 

You should use `set_bias` when interfacing with digital input sensors or switches that do not have their own external pull-up/pull-down resistors. These internal resistors ensure the input pin has a defined logic level (either high or low) when the external component is disconnected or floating, preventing erratic readings caused by electrical noise. 

| When to Use          | Typical Connection                                                                                                                                                        |                                                               |
| -------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------- |
| **`Bias::PullUp`**   | When the input is connected to ground (GND) to signal a "low" state (e.g., a button connecting the pin to GND). The resistor pulls the pin voltage up to 3.3V by default. | One side of a button to the GPIO pin, the other side to GND.  |
| **`Bias::PullDown`** | When the input is connected to 3.3V to signal a "high" state (e.g., a sensor outputting 3.3V). The resistor pulls the pin voltage down to GND by default.                 | One side of a button to the GPIO pin, the other side to 3.3V. |
| **`Bias::Off`**      | When the external circuit already provides a defined logic level (e.g., an active sensor with a strong output signal, or an external pull-up/pull-down resistor is used). | Active sensor output connected directly to the GPIO pin.      |

## References