Wire and SPI¶

FRAMEWORK LIBRARY · I²C · BUS INSTANCE · FOUR IMPLEMENTATIONS

Wire and SPI are the two buses that nearly everything in a MeshCore node hangs off. They sit at the top of [arduino_base], without an author prefix and without a version number, because they do not come from the registry but from the platform's framework package.

[!NOTE] Source. This page was verified against the firmware itself: MeshCore v1.16.0, commit 03b6ef4, 28 July 2026 — files platformio.ini, src/helpers/ESP32Board.h, src/helpers/ui/ST7789LCDDisplay.cpp and src/helpers/ui/GxEPDDisplay.cpp.

What it does¶

Wire is the Arduino API for I²C: a two-wire bus that can carry dozens of devices, each with its own address. SPI is the API for the bus of the same name: faster, with four wires, and with a separate select line per device.

Neither is a library in the ordinary sense. They belong to the platform's Arduino core, so there is one of each per platform: Arduino-ESP32 on ESP32, the Adafruit nRF52 core on nRF52, arduino-pico on RP2040 and STM32duino on STM32. The API is the same, the behaviour underneath is not — in the number of available bus instances, in the free choice of pins and in the timing.

How MeshCore pulls it in¶

platformio.ini r.19-21

lib_deps =
  SPI
  Wire

Two lines in [arduino_base], without a prefix and without an @ version. They apply to all 507 build targets. For the STM32WL a third framework library joins them, SubGhz — see subghz.md.

How MeshCore uses it¶

SPI carries the radio. On every platform except STM32WL the transceiver sits on the SPI bus; the variant files define P_LORA_MISO, P_LORA_MOSI, P_LORA_SCLK and P_LORA_NSS for it. RadioLib does the rest — see radiolib.md.

Larger displays do not always share that bus. Where it clashes, a second bus instance is created:

src/helpers/ui/GxEPDDisplay.cpp r.13-15

#ifdef ESP32
  SPIClass SPI1 = SPIClass(FSPI);
#endif

And for a TFT display on some boards the bus is started with explicit pins:

src/helpers/ui/ST7789LCDDisplay.cpp r.31-34

    // Im not sure if this is just a t-deck problem or not, if your display is slow try this.
    #if defined(LILYGO_TDECK) || defined(HELTEC_LORA_V4_TFT)
      displaySPI.begin(PIN_TFT_SCL, -1, PIN_TFT_SDA, PIN_TFT_CS);
    #endif

Wire carries the rest. Sensors, clock chips, current meters, bus expanders and the smaller OLED displays all sit on I²C. The board decides on which pins:

src/helpers/ESP32Board.h r.44-50

  #if defined(PIN_BOARD_SDA) && defined(PIN_BOARD_SCL)
   #if PIN_BOARD_SDA >= 0 && PIN_BOARD_SCL >= 0
    Wire.begin(PIN_BOARD_SDA, PIN_BOARD_SCL);
   #endif
  #else
    Wire.begin();
  #endif

That the pins can be passed in here is an ESP32 property: on that platform the I²C controller can be mapped freely onto pins. On the other platforms they are fixed.

The text Wire occurs in 149 of the 590 source files, SPI in 83.

What it means for a node¶

Nearly every problem with a sensor that is not found, or a display that stays black, comes back to one of these two buses: wrong pins in the variant file, two devices on the same I²C address, or a display pushing the radio off the SPI bus.

The STM32WL is the exception as far as the radio is concerned: there is no external SPI connection to the transceiver there, because it sits on the same chip. It does use Wire, for whatever else hangs off the board.

Sources¶

Translated from Dutch by Anthropic Claude