STM32 Printf Float Variable Logging Via USART Configuration Guide

Aug 3, 2025 by ADMIN 66 views

Hey guys! Today, let's dive into a common challenge faced when working with STM32 microcontrollers – printing float variables using printf over USART. This is crucial for debugging and logging data, but it often requires some configuration to get it working smoothly. We'll explore the steps involved, common pitfalls, and how to overcome them, all while keeping it casual and easy to understand.

Understanding the Issue

When you try to directly print a float variable using printf in an STM32 project, you might encounter that the float value gets displayed incorrectly. This typically happens because the standard printf implementation in embedded systems is often a reduced version to save memory, and it might not include support for floating-point formatting by default. This is where we need to step in and configure our project to correctly handle floats.

The standard printf function is part of the C standard library, a powerful tool for formatting output in a human-readable way. However, in resource-constrained environments like microcontrollers, the full implementation of printf can be quite large, taking up precious flash memory. To mitigate this, embedded toolchains often provide a “slimmed-down” version of printf that excludes certain features, such as floating-point support, which are less frequently used in basic embedded applications. This is where the problem arises when we try to print float variables directly using the default configuration. The microcontroller simply doesn't have the necessary code to handle the floating-point format specifiers, resulting in incorrect or garbled output. Therefore, we need to explicitly tell the compiler and linker to include the floating-point support in our project.

When working with microcontrollers, it's essential to understand the limitations of the environment. Microcontrollers have limited flash memory, RAM, and processing power compared to desktop computers. This means that we need to be mindful of the code size and resource usage. Including full floating-point support adds to the code size, which is a trade-off we need to consider. In many embedded applications, floating-point calculations are necessary for tasks such as sensor data processing, control algorithms, and signal processing. The need to display these floating-point values for debugging or logging purposes is equally crucial. By understanding these constraints and requirements, we can make informed decisions about how to configure our projects. We must strike a balance between functionality and resource usage to ensure that our embedded systems operate efficiently and reliably. By explicitly enabling floating-point support and using it judiciously, we can leverage the power of floating-point arithmetic without overwhelming the microcontroller's resources.

Setting Up Your Project for Float Printing

1. Enabling Float Support in Your Toolchain

The first step is to enable floating-point support in your toolchain settings. How you do this depends on your IDE and compiler, but the general idea is to tell the linker to include the floating-point libraries. Let's look at some common IDEs.

For STM32CubeIDE:
- Go to Project Properties (right-click on your project in the Project Explorer and select Properties).
- Navigate to C/C++ Build > Settings > MCU Settings.
- Make sure that the Floating point unit is set to FPv4-SP or similar (depending on your STM32 core) and that Floating-point ABI is set to Hard FP. Setting it to Hard FP tells the compiler to use the hardware floating-point unit, which is more efficient.
- Next, go to C/C++ Build > Settings > Tool Settings > MCU Linker > Miscellaneous.
- In the Other flags section, add -u _printf_float to force the linker to include the floating-point version of printf.
For Keil MDK:
- Open Project Options (right-click on your project in the Project Explorer and select Options for Target).
- Go to the Target tab.
- Ensure that Use float ABI is set to FPv5 VFP. This setting enables the use of the hardware floating-point unit.
- Go to the C/C++ Compiler tab and add --fpu=VFPv5 to the Misc Controls section. This explicitly tells the compiler to use the VFPv5 floating-point unit.
- Go to the Linker tab and add --specs=nano.specs -u _printf_float to the Misc Controls section. The nano.specs setting uses a reduced C library, while -u _printf_float forces the inclusion of floating-point support for printf.
For GCC (command line or other IDEs):
- You'll need to pass the following flags to the linker:
  - -u _printf_float: This ensures that the floating-point version of printf is linked.
  - -mfpu=fpv4-sp-d16 -mfloat-abi=hard: These flags enable the hardware floating-point unit. The exact flags might vary depending on your specific STM32 core.

These settings ensure that your toolchain knows you want to use floating-point operations and includes the necessary libraries. Without these settings, the linker might exclude the floating-point parts of printf to save space, which is why you see incorrect output.

2. Implementing the _write() Function

In STM32 projects, you need to implement the _write() function. This function is a low-level function that printf uses to output characters. You mentioned you've already got this in your syscalls.c file, which is excellent! Let's quickly recap how this works and ensure it's set up correctly. The _write() function takes a file descriptor, a pointer to the character buffer, and the length of the buffer. It then sends these characters over the USART.

int _write(int file, char *ptr, int len) {
    for (int i = 0; i < len; i++) {
        // Assuming you have a function USART_transmit that sends a single character
        USART_transmit(ptr[i]);
    }
    return len;
}

This simple implementation iterates through the buffer and sends each character using a USART_transmit() function, which you'll need to define based on your specific USART setup. The key is that this function handles the actual transmission of data over the serial port. Without a properly implemented _write() function, printf won't be able to send any output, regardless of whether floating-point support is enabled. The file descriptor argument is typically ignored in embedded systems since we're usually only dealing with standard output. However, it's part of the function signature and needs to be included. By ensuring that the _write() function is correctly implemented and that it properly transmits characters over the USART, we can establish the foundation for using printf to output debugging information and other data from our STM32 microcontroller.

3. Setting Up the USART

Make sure your USART is properly initialized. This includes setting the baud rate, configuring the GPIO pins for TX and RX, and enabling the USART peripheral in your STM32CubeIDE configuration or your code. This step is crucial because printf relies on the USART to transmit the formatted output. If the USART is not properly configured, you won't see any output, regardless of how well the floating-point support is set up. The baud rate determines the speed of the serial communication, and it needs to be the same on both the microcontroller and the receiving device (e.g., your computer's serial terminal). The GPIO pins used for TX (transmit) and RX (receive) must be correctly configured as alternate functions for the USART peripheral. This tells the microcontroller to route the USART signals to these pins. Additionally, the USART peripheral itself needs to be enabled in the microcontroller's clock configuration. This ensures that the USART module is powered and ready to transmit data. All these steps are essential to establish a working serial communication link. If any of these configurations are missing or incorrect, the output from printf will not be transmitted correctly, making it difficult to debug your code. Therefore, it's crucial to double-check the USART initialization to ensure that it's set up properly before trying to print floating-point values.

4. Using printf with Float Specifiers

Now that you've set up your toolchain and USART, you can use printf with float specifiers like %f, %e, or %g. Here's an example:

float myFloat = 3.14159;
printf(