In what kind of situations that no pull would be configured as a input gpio

In the context of GPIO configuration, the choice between using a pull-up, pull-down, or no pull resistor depends on the specific requirements of the application and the external circuitry connected to the GPIO pin.

No Pull Configuration

When GPIO_InitStruct.Pull = GPIO_NOPULL; is used, it means that the GPIO pin is left floating, and no internal pull-up or pull-down resistor is enabled. This configuration is typically used in the following scenarios:

1. External Pull-Up or Pull-Down Resistors: If there are external pull-up or pull-down resistors connected to the GPIO pin, the internal resistors are not needed. Enabling both internal and external resistors could lead to conflicts or incorrect voltage levels.

2. Open-Drain/Open-Collector Outputs: In some designs, the GPIO might be used as an open-drain or open-collector output. In such cases, the external circuitry or another GPIO might provide the pull-up resistor, so the internal pull-up resistor is not used.

3. High-Impedance Input: For certain applications, such as analog inputs or high-impedance digital inputs, leaving the pin floating (no pull) might be necessary to avoid loading the signal source.

4. Specific Timing or Signal Integrity Requirements: In some high-speed digital interfaces, the use of internal pull-up or pull-down resistors might introduce unwanted delays or affect signal integrity. In such cases, the external circuitry is designed to handle these requirements.

Pull-Up and Pull-Down Configurations

1. Pull-Up: GPIO_InitStruct.Pull = GPIO_PULLUP;

   This configuration is useful when the GPIO pin is used as an input, and you want to ensure that the pin reads a high state when the switch is open.

2. Pull-Down: GPIO_InitStruct.Pull = GPIO_PULLDOWN;

   This configuration is useful when the GPIO pin is used as an input, and you want to ensure that the pin reads a low state when the switch is open.

In summary, the decision to use GPIO_NOPULL in the provided code snippet is based on the assumption that the external circuitry or application requirements do not necessitate the use of internal pull-up or pull-down resistors.