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Project Notes

#497 Publishing Internals

Publishing live ESP32 system internals to the web - an exploration of ESP32 features and web serving options, including JSON serialization.



The ESP32 is powerful enough to do some decent web serving. This project explores:

  • implementing a web server on the ESP32
  • generating JSON web responses
  • collecting ESP32 system internals, including:
    • temperature
    • hall-effect sensor
    • CPU and bus speeds
    • .. and other tidbits I can glean from references and sources…

The code is written to be built with the Arduino IDE + ESP32 board support.

Reading the Chip Temperature

The undocumented uint8_t temprature_sens_read() function is discplosed in esp32-hal-misc.c#L40 It returns the get chip temperature in Farenheit. The float temperatureRead() function returns celsius.

Reading the Hall Effect Sensor

The hallRead() function returns the magnetic field strength using the on-board hall-effect sensor.

Getting Operating Frequencies


  • getCpuFrequencyMhz() returns the CPU frequency in MHz. This gets the actual CPU speed, whereas the avr function clockCyclesPerMicrosecond() returns the frequency that the code has been compiled for (based on the F_CPU #define)
  • getXtalFrequencyMhz() returns the crystal frequency in MHz
  • getApbFrequency() returns the advanced peripheral bus (APB) frequency in Hz

While the crystal frequency should be static, the CPU and bus speeds can vary due to the power management algorithm being applied. See Power Management docs.

Rendering JSON

The /stats.json endpoint returns a JSON collection of current stats. Instead of a manual (and error-prone) process of building the JSON representation, the example uses the ArduinoJson library. It makes JSON generation trivial.

I’m using the StaticJsonDocument class that pre-allocates storage on the stack. Creating attributes and serializing the result is trivial, e.g.:

char buffer[200];
StaticJsonDocument<200> statsDocument;
statsDocument["uptime"] = millis();
serializeJson(statsDocument, buffer);

Developing the Web Page

I didn’t skimp on the web page: it uses a number of Javascript libraries, but all loaded from external CDNs. The ESP32 just serves the basic HTML page, and the stats JSON endpoint.

The page_design.html file was used during development. I load it in Firefox, and it has a hard-coded reference to the IP that the ESP32 is sitting on. In this way, I could fine-tune the web page before building it into the code that is written to the ESP32 itself.

The page uses some assets and libraries that are side-loaded i.e. not served directly from the ESP32:

Building and Running the Example

Copy settings.h.template to settings.h and fill in the WiFi access credentials before compiling. After uploading the program to the ESP32, connect to the serial console to find out what IP address it obtained..


Go to the IP address with a browser. the status page should load, and start updating statistics every second or so. To verify it is really working, bring a magnet close to the ESP32 to see changes the hall effect sensor.



There is no “circuit” to construct so far - simply plugging the ESP32 into the USB port!


Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.