Esp-println Prints To USB With UART Feature Only: Bug?

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esp-println Prints to USB Even When Only UART Feature Is Enabled: Discussion

Hey everyone! Let's dive into a quirky issue some of us have been encountering with the esp-println crate. It seems like, under certain conditions, the library insists on printing output to USB, even when we've specifically configured it to use only the UART feature. Sounds a bit odd, right? Let's break it down and see what's going on.

Bug Description

So, what's the buzz? Basically, the issue is that even when you've set up esp-println with the intention of directing all print output to the UART (Universal Asynchronous Receiver/Transmitter) port, some sneaky output still finds its way to the USB connection. This is unexpected because, in theory, disabling the USB feature should prevent any data from being sent that way. For those who are trying to debug or use specific communication channels, this can be a bit of a headache.

To Reproduce

Okay, so how can you reproduce this? Here are the steps to replicate the issue:

  1. Create a new project for your ESP32-S3. Note that while this issue has been confirmed on the ESP32-S3, it's not entirely clear whether other SoCs (System on Chips) are affected. So, if you have other boards lying around, feel free to experiment!

  2. Add esp-println to your project's dependencies in your Cargo.toml file. Make sure to configure it like this:

    esp-println = { version = "0.16.1", default-features = false, features = [
  "critical-section",
  "esp32s3",
  "uart",
] }

    Important: Setting default-features = false ensures that you're only enabling the features you explicitly specify. In this case, we're enabling critical-section, esp32s3 (to target the ESP32-S3), and uart (to use the UART for printing).

  3. Write a program that includes some esp-println statements to print text. Nothing fancy needed here; just a few lines to output some data.

  4. Upload the program to your ESP32-S3 using cargo run or your preferred method.

  5. Connect to the board via USB.

  6. Open the monitor using espflash monitor or another serial monitor.

Now, here's the critical part: you'll notice that the text you're printing is being displayed in the monitor over the USB connection. But wait! We only enabled the uart feature, so this shouldn't be happening, right? That's the bug!

This issue is reproducible in the main branch, ensuring that it's not just an isolated incident in a specific version or branch.

Expected Behavior

So, what should happen instead? The expected behavior is that when only the uart feature is enabled, no output should be coming through the USB. All esp-println output should be directed exclusively to the UART pins, allowing you to monitor the output using a UART-to-USB adapter or another UART-based interface. This is crucial for debugging scenarios where you need to isolate communication channels and ensure that data is only being transmitted through the intended interface. Imagine setting up a sensor that communicates via UART, and you don't want any interference or confusion from USB output. This is where the expected behavior becomes essential.

Environment

To provide a bit more context, here's the environment in which this issue was observed:

  • Target device: ESP32-S3. You can use the espflash board-info command to get detailed information about your target device. This will help ensure that the issue is consistent across different hardware configurations.
  • Crate name and version: esp-hal version 1.0.0. This is the hardware abstraction layer crate that provides access to the peripherals of the ESP32-S3. Knowing the specific version helps in identifying potential compatibility issues or bugs that may have been introduced or fixed in certain releases.

Possible Causes and Solutions

Now, let's put on our detective hats and try to figure out what might be causing this issue and what potential solutions or workarounds might exist.

Potential Causes

  1. Feature Flag Interaction: There might be some unintended interaction between the uart feature and other features within the esp-println crate or its dependencies. It's possible that some underlying code is still being compiled in that uses USB, regardless of the feature flags.
  2. Default Configuration: The esp-println crate might have some default configurations that enable USB output unless explicitly disabled through some other means. This could be an oversight in the crate's design.
  3. Underlying HAL: The hardware abstraction layer (esp-hal) might be configured to route some output to USB by default, and esp-println is simply picking up that configuration.
  4. Monitor Interference: It's also possible that the monitor program (espflash monitor) is somehow intercepting and displaying UART output over USB. This is less likely, but still worth considering.

Potential Solutions and Workarounds

  1. Double-Check Feature Flags: Ensure that you have absolutely no other features enabled that might pull in USB functionality. Review your Cargo.toml file and any other relevant configuration files.
  2. Explicitly Disable USB: Look for any configuration options within esp-println that allow you to explicitly disable USB output. This might involve setting a specific environment variable or passing a configuration parameter to the esp-println initialization function.
  3. Investigate esp-hal Configuration: Check the esp-hal documentation and examples to see if there are any settings that control the routing of output. You might need to configure the HAL to disable USB output.
  4. Use a Different Monitor: Try using a different serial monitor program to see if the issue persists. This will help determine whether the problem is with espflash monitor or with the underlying code.
  5. Contribute to esp-println: If you're feeling adventurous, dive into the esp-println source code and see if you can identify the cause of the issue. If you find a solution, submit a pull request to the crate's repository. The open-source community will thank you!

Additional Tips and Tricks

  • Verbose Logging: Enable verbose logging in esp-println to get more detailed information about what's being printed and where it's being sent. This might help you pinpoint the source of the unexpected USB output.
  • Debugging with a JTAG Debugger: If you have access to a JTAG debugger, use it to step through the code and see exactly what's happening when esp-println is called. This can provide valuable insights into the issue.
  • Community Support: Reach out to the ESP32 and Rust communities for help. Post your issue on forums, chat groups, or Stack Overflow. There's a good chance that someone else has encountered the same problem and can offer a solution.

Conclusion

The mystery of esp-println printing to USB when only the UART feature is enabled is indeed a puzzling one. By meticulously following the reproduction steps, examining potential causes, and trying out various solutions, we can hopefully unravel this issue and contribute to making the esp-println crate even more robust and reliable. Remember, the key to solving these kinds of problems is persistence, attention to detail, and a willingness to dive deep into the code. Happy debugging, and may your UART always be your UART!

Next Steps

As a next step, it would be beneficial to:

  1. Test on other ESP32 variants: See if the issue persists on other ESP32 chips like the ESP32-C3 or ESP32-S2.
  2. Create a minimal reproducible example: Strip down the code to the bare minimum required to reproduce the issue. This will make it easier to debug and share with others.
  3. Open an issue on the esp-println GitHub repository: This will bring the issue to the attention of the crate's maintainers and allow them to investigate further.

By working together and sharing our findings, we can help improve the ESP32 ecosystem and make it easier for everyone to build amazing projects.