An Heltec ESP32 module with OLED screen was chosen for the ease of having a (tiny) OLED screen besides the standard capabilities of Wifi and Bluetooth connection and a USB interface for easy programming. An Heltec 32 Wifi Lora V3 was used. In this project I don’t expect to use the Lora radio.
To interface the 3.3V based microcontroller to the harsh +11V to +16V car environment, a dedicated LINBUS interface using the TJA1021 chip was selected. This takes care of all the level shifting between the voltages and splits the one-wire LIN bus into a two-wire standard serial interface TX/RX. The LIN-BUS<->UART board can be found on amazon or from Chinese webshops.
Currently a standard DC jack 5.5/21.mm is connected to the the LIN-interface voor the power supply and the blue LIN bus wire connects to an available LIN bus spade connector behind the campervan-side batteries of the Nugget. The ESP32 is powered through a USB cable. Eventually when making a more permanent design it can powered form the car battery side with an additional DC/DC convertor.
As Software Development Environment I use Visual Studio Code and the PlatformIO plugins. Within PlatformIO I make use of the Arduino framework for ESP32 and installed the ESP32 and Heltec libraries. The latter for OLED support. The code will eventually end up at github.
An introduction to coding and programming the ESP32 is beyond the scope of this post. Many good tutorials can be found, for example those:
PlatformIO tutorials or Random Nerds Tutorials
To interface with the LIN bus I forked two repositories of Mestrode. The first is a library to drive the dedicated functions of the TJA1021, and the second is the generic implementation of the LIN bus protocol using a serial interface of the ESP32. Both I had to fork as I needed to extend them. The transceiver library was using hard-coded TX/RX pins and for the Heltec board those pins were occupied by other functions. And for the interface library there was no spy-mode reading function implemented. I need to listen-only to the bus, while it is not in sleep mode. A new frame can come at any moment.
https://github.com/SevenW/Lin-Transceiver-Library
https://github.com/SevenW/Lin-Interface-Library
The actual code is not pushed to github yet.