How to Fix Error: 'USART_FLAG_TXE' was not declared in this scope

Identify the Scope Issue

• First, locate where the constant `USART_FLAG_TXE` should be defined. Typically, this would be found in your microcontroller's hardware abstraction layer (HAL) or a similar library header file.
• Ensure that the appropriate header file is included in your source file. For many STM32 microcontrollers, this would be something like:

  #include "stm32f1xx\_hal.h"
      

  Adapt the include directive based on your specific microcontroller.
• Check if this definition is within a conditional compilation block, such as `#ifdef` or `#ifndef`. It may be excluded based on certain preprocessor conditions not being met.

Ensure Proper Definitions and Inclusions

• Search for `USART_FLAG_TXE` in your HAL or peripheral library files. Confirm that it is indeed defined there. For instance, you might find it in a file like `stm32f1xx_hal_usart.h`.
• If the definition exists, check your project settings to confirm all necessary directories are included in your compiler's include path. This is essential so that the compiler can locate the necessary header files.
• You might need to configure your project to include specific hardware support files. Ensure these configurations are done properly in your IDE.

Verify Library Version Compatibility

• Ensure that the HAL drivers or equivalent libraries are compatible with your current project setup. If you've recently updated your compiler or project files, there may be version mismatches.
• Consult the library documentation or community forums to see if there have been changes or updates that might affect the definition of flags like `USART_FLAG_TXE`.

Explore Alternative Definitions

• If `USART_FLAG_TXE` appears deprecated or removed, search for alternative macros or settings that accomplish the same function. Libraries periodically refactor their codebase, replacing certain macros or functions with newer alternatives.
• Refer to the latest library documentation to check if alternatives are suggested for deprecated macros.

Custom Definitions

• As a last resort, should you be unable to resolve the issue via library updates or inclusion adjustments, consider defining the constant manually based on your understanding of the hardware register's bit patterns. Ensure this is consistent with other similar constants within the same domain.
• Example:

  #define USART_FLAG_TXE ((uint16\_t)0x0080)
      

  However, use this approach with caution and validate it thoroughly against the datasheet for your device.