Simultaneous use of SPI and I2C with Mini ESP32C3 with 0.42" OLED display board
April 24, 2026
This repository contains a few PlatformIO/pioarduino projects for a common mini ESP32-C3-based development board that sports an onboard 0.42" OLED display and a USB-C connector.
The image above, borrowed from a vendor site, corresponds exactly to the board used to create the projects, including the absence of any markings that could identify the original manufacturer or vendor.
Table of Content
- 1. Hardwired I2C Pin Assignement
- 2. Proposed pins_arduino.h
- 3. pioarduino
- 4. Arduino
- 5. Configuration
- 6. Projects
- 7. References
- 8. Change Log
- 9. License
The common pin assignment of the generic ESP32C3 Super Mini boards is
static const uint8_t TX = 21;
static const uint8_t RX = 20;
static const uint8_t SDA = 8;
static const uint8_t SCL = 9;
static const uint8_t SS = 7;
static const uint8_t MOSI = 6;
static const uint8_t MISO = 5;
static const uint8_t SCK = 4;
static const uint8_t A0 = 0;
static const uint8_t A1 = 1;
static const uint8_t A2 = 2;
static const uint8_t A3 = 3;
static const uint8_t A4 = 4; Vendors of the mini board with the OLED display generally display a pinout diagram that corresponds to the above assignment, but then add an additional information which corresponds to the physical connection of the OLED display to the SoC.
OLED SDA = 5
OLED SCL = 6
The I/O pins 8 and 9 can indeed be attached to the I2C controller, but since there is only one I2C controller, the OLED display cannot be used with that pin assignment. Consequently, example projects for the board with an onboard display use a generic ESP32C3 Super Mini board definition and override the I2C pin assignment in the source code. That creates a problem because I/O pins 5 and 6 cannot be attached to the I2C and SPI controllers simultaneously. To use a SPI connection along with an I2C device, it would be necessary to override the variant pin definition with an explicit redefinition in the source code when initializing the SPI device as done with I2C.
The preferable approach is to define a proper pin assignment in a new variant pins_arduino.h file. Two versions are proposed here that differ only with regard to the pin to be attached to the SS signal of the SPI interface.
The objective of these definitions is to make it easy to use the SPI and I2C interfaces of the mini ESP32C3 board with onboard I2C display with a minimum impact on the other capabilities of the SoC.
static const uint8_t TX = 21;
static const uint8_t RX = 20;
static const uint8_t SDA = 5; // connected to onboard OLED
static const uint8_t SCL = 6; // connected to onboard OLED
static const uint8_t SS = 8;
static const uint8_t MOSI = 10;
static const uint8_t MISO = 9;
static const uint8_t SCK = 7;
static const uint8_t A0 = 0;
static const uint8_t A1 = 1;
static const uint8_t A2 = 2;
static const uint8_t A3 = 3;
static const uint8_t A4 = 4;- Pinout Diagram: link
- Advantage: All valid analogue input pins are free.
- Disadvantage: I/O pin 8 is a strapping pin and it controls the user LED.
static const uint8_t TX = 21;
static const uint8_t RX = 20;
static const uint8_t SDA = 5; // connected to onboard OLED
static const uint8_t SCL = 6; // connected to onboard OLED
static const uint8_t SS = 0;
static const uint8_t MOSI = 10;
static const uint8_t MISO = 9;
static const uint8_t SCK = 7;
static const uint8_t A0 = 0;
static const uint8_t A1 = 1;
static const uint8_t A2 = 2;
static const uint8_t A3 = 3;
static const uint8_t A4 = 4;- Pinout Diagram: link
- Advantage: The user LED remains available.
- Disadvantage: One of the five analogue input pin is no longer available.
Of course, any I/O pin from 1 to 4 (A1 to A4) could be connected to the SS signal of the SPI controller with the same result as obtained by choosing 0 (A0). Choosing A3 or A4 instead of A0, A1 or A2 may be better for signal routing purposes because all the SPI connections would then be on the same header.
In most cases either board definition should work. The exception would be trying to use GPIO 8 as an active high SS signal for some bizarre technical reason. This would probably not work since GPIO 8, as a strapping pin, is pulled to 3.3 volts. Until the IO pin is set low in the startup code, the SPI slave would be trying to read spurious data on MOSI with an invalid clock signal.
That said, the esp32c3_oled_mini definition is probably the better choice if there is no need for five analogue inputs. To belabour the point, with that definition, the user LED will not be turned on whenever a SPI transfer occurs. Indeed, IO pin 8 remains available to control the user LED.
The sketches in this repository are meant to be compiled in the pioarduino fork of PlatformIO. The former is arguably better than the latter when using recent ESP32 microcontrollers.
Unlike the Arduino IDE, the pioarduino IDE is not a stand-alone application. It is an extension that can be installed in Visual Studio Code (VSCode for short) editor. The extension is also available in the marketplace. VSCodium is a less well-known fork of VSCode that claims to have removed telemetry/tracking. Version 1.3.18 which was used while developing these sketches, does not contain the AI additions that have apparently been added to VSCode.
The pioarduino IDE extension is available in the extension marketplace of both code editors. Consequently, its installation is a simple three-step procedure shown in the IKEA-like instruction diagram below.
All of the sketches in the repository have been successfully compiled with the pioarduino-espressif32 platform version 55.03.37 released February 11, 2026. This contains the version 3.3.7 of the Arduino ESP32 core based on ESP-IDF v5.5.2.260206. All projects should compile without problems with the newest version of the platform, which is currently 55.03.33 based on v3.3.8 of the ESP32 Arduino core, but this has not been tested.
It is possible to test the proposed board definitions and variant pins_arduino.h files in the Arduino IDE. Unfortunately, the manual installation will be lost when the esp32 package is updated.
The April 23 update of this repository changed the value of the "variants_dir" key in the board definitions manifests (esp32c3_oled_mini.json and mini_esp32c3_oled.json) to the relative path of the boards directory. In the original version, the absolute path was provided which had a zero probability of working for most users. The relative path works in Linux and, hopefully, it works in other operating systems. If it doesn't then something like the following error will happen during compilation.
...
compilation terminated.
Compiling .pio/build/esp32c3_oled_mini/FrameworkArduino/Esp.cpp.o
*** [.pio/build/esp32c3_oled_mini/src/main.cpp.o] Error 1
In file included from /home/michel/.platformio/packages/framework-arduinoespressif32/cores/esp32/Esp.cpp:20:
/home/michel/.platformio/packages/framework-arduinoespressif32/cores/esp32/Arduino.h:43:10: fatal error: pins_arduino.h: No such file or directory
...
In that case, replacing the relative path "../boards" with the absolute path of the boards folder as the value of the "variants_dir" in the .json board definition files should work.
If the following error occurs,
Processing esp32c3_oled_mini (board: esp32c3_oled_mini; platform: https://github.com/pioarduino/platform-espressif32/releases/download/stable/platform-espressif32.zip; framework: arduino)
-------------------------------------------------------------------------------------------------------------------------------------------------------------
UnknownBoard: Unknown board ID 'esp32c3_oled_mini'
then the problem might be with the relative path "../boards" in the project configuration file platformio.ini. Again the solution could be to use the absolute path of the boards folder as the value of the boards_dir key. If there is a problem with the boards_dir entry in the configuration file, the lib_dir entry will probably be wrong also and, again, using the absolute path may be the solution.
All sketches in the super_mini_esp32c3_sketches repository can be uploaded to the ESP32-C3 Mini with a 0.42" OLED display because the latter is essentially an ESP32-C3 Super Mini with the extra constraint that the OLED display's SDA and SCL signal lines are connected to GPIO 5 and 6. Consequently, this repository only contains projects that are applicable to the ESP32-C3 OLED Mini where using a bespoke board definition is helpful.
The first project 01_pin_names is similar to the sketch of the same name in super_mini_esp32c3_sketches but it is modified to show the pin assignment defined in the proposed board definitions for the Mini with OLED display. Similarly, 27_i2c_oled, is a reprise of the same project in the
in super_mini_esp32c3_sketches modified to use either board definition proposed herein.
More interestingly, 28_spi_master_i2c_oled and 29_spi_slave_i2c_oled are new projects that show how either of the board definitions proposed here facilitates the use of the Mini with OLED when using the I2C and SPI (as master or slave) controllers simultaneously. The 24_spi_master and 25_spid_slave projects are here only to help in testing the SPI and I2C peripherals of the Mini with OLED when a second board of that type is not available.
Purpose: Display the I/O pin numbers and names and various macro definitions of ESP32-C3 mini board with onboard 0.42" OLED display or the generic ESP32-C3 super mini board
Notes:
-
Can be compiled for the generic ESP32-C3 Super Mini board using the
nologo_esp32c3_super_minienvironment or for the ESP32C3 Mini with OLED display using either theesp32c3_oled_miniormini_esp32c3_oledenvironment in theplatformio.iniconfiguration file. -
Before compiling the program choose the correct environment in the project's
platformio.iniconfiguration file.[platformio] default_envs = esp32c3_oled_mini ; SS = 0 ;default_envs = mini_esp32c3_oled ; SS = 8 ;default_envs = nologo_esp32c3_super_mini
This version will yield slightly different results when run on a generic ESP32-C3 Super Mini board than the version from the super_mini_esp32c3_sketches because it will signal the absence of the
ARDUINO_ESP32C3_OLED_MINIandARDUINO_MINI_ESP32C3_OLEDmacros which are defined in the ESP32-C3 Mini board with OLED display board definitions.
Purpose: Test of SPI communication as a SPI master device.
Notes:
-
This is the SPI master device which must be connected to a slave SPI device which could be another ESP32-C3 Mini running the
29_spi_slave_i2c_oledsketch or a generic ESP32-C3 running the25_spi_slavesketch. -
Use this sketch to test the ESP32C3 Mini with OLED display as a SPI slave device when a second Mini with OLED display is not available.
-
This is essentially the same example found in the
24_spi_masterexample in super_mini_esp32c3_sketches.
Purpose: Test of SPI communication as a SPI slave device.
Notes:
-
This is the SPI slaver device which must be connected to a slave SPI device which could be another ESP32-C3 Mini running the
28_spi_master_i2c_oledsketch or a generic ESP32-C3 Super Mini running the24_spi_mastersketch. -
Use this sketch to test the ESP32C3 Mini with OLED display as a SPI master device when a second Mini with OLED display is not available.
-
This is essentially the same example found in the
25_spi_slaveexample in super_mini_esp32c3_sketches.
*Purpose: Test the I2C peripheral of an ESP32-C3 Mini with a 0.42" OLED I2C display.
Notes:
-
Requires an ESP32C3 Mini with OLED display. Use either the
esp32c3_oled_miniormini_esp32c3_oledenvironment in theplatformio.iniconfiguration file. -
This is essentially the same example found in the 27_i2c_oled example in super_mini_esp32c3_sketches except that the I2C SDA and SCL pins are correctly identified in the board definition.
Purpose: Simultaneously test the SPI and I2C peripherals of an ESP32-C3 Mini with a 0.42" OLED I2C display.
Notes:
-
This is the SPI master device which must be connected to a slave SPI device which could be another ESP32-C3 Mini running the
29_spi_slave_i2c_oledsketch or a generic ESP32-C3 running the25_spi_slavesketch. -
Requires an ESP32C3 Mini with OLED display. Use either the
esp32c3_oled_miniormini_esp32c3_oledenvironment in theplatformio.iniconfiguration file.
Purpose: Simultaneously test the SPI and I2C peripherals of an ESP32-C3 Mini with a 0.42" OLED I2C display.
Notes:
-
This is the SPI slave device which must be connected to a master SPI device which could be another ESP32-C3 Mini running the
28_spi_master_oledsketch or a generic ESP32-C3 running the24_spi_mastersketch. -
Requires an ESP32C3 Mini with OLED display. Use either the
esp32c3_oled_miniormini_esp32c3_oledenvironment in theplatformio.iniconfiguration file.
-
ESP32-C3 0.42 OLED by Kevin (Kevin's Blog), 2025-02-12
- They appear to be “ABRobot ESP32-C3 0.42 OLED” devices, at least when first powered on they display the text “ABrobot"
- ArduinoIDE: board = ESP32C3 Dev Module
- olikraus/u8g2, <U8g2lib.h>
- SDA and SCL hardwired as 5 and 6 in display u8g2() constructor
-
ESP32-C3-ABrobot-OLED by ZhuHai ESP (zhuhai-esp), last update 2024-07-31
- PlatformIO: board = airm2m_core_esp32c
- olikraus/u8g2, <U8g2lib.h>
- SDA and SCL hardwired as 5 and 6 in display u8g2() constructor
-
ESP32-C3 OLED by kelemenb 2025-03-26 on Shared Inventions
- PlatformIO: board = esp32-c3-devkitm-1
- olikraus/u8g2, <U8g2lib.h>
- SDA_PIN=5, SCL_PIN=6 in Wire.begin()
-
SP32-C3 Tiny OLED Tutorial: Pinout, Setup & First Programs by Dziubym 2025-11-11
- Arduino IDE: board = ESP32C3 Dev Module (as per accompanying video)
- olikraus/u8g2, <U8g2lib.h>
- SDA_PIN=5, SCL_PIN=6 in Wire.begin()
-
ESP32-C3_OLED by Stefan (peff74) last update 2025-08-17
- Arduino IDE: board = ?
- olikraus/u8g2, <U8g2lib.h>
- OLED_SDA=5, OLED_SCL=6 in display u8g2() constructor
-
ESP32-C3 OLED Test by Bilal Akhtar (bilalakhtar) last update 2025-06-18
- Arduino IDE: board = ?
- olikraus/u8g2, <U8g2lib.h>
- SDA and SCL hardwired as 5 and 6 in display u8g2() constructor
- ESP32-C3-0.42 OLED last update 2022-07-21
- 01Space variant with I2C OLED and I2C Qwiic connector
- Arduino IDE: board = ESP32C3 Dev Module (
ESP32-C3-GraphisTest.inosketch), - SDA_PIN=5, SCL_PIN=6 in Wire.begin()
- MOSI=7, MISO=8, SCK=10 (no SS assignment) shown on pinout diagram which contradicts the chosen board.
| Date | Change |
|---|---|
| 2066-04-24 | Instructions for use in Arduino IDE |
| 2026-04-23 | Use relative path for "variants_dir" in the board definition manifests |
| 2026-04-21 | First commit in GitHub |
Copyright 2026, Michel Deslierres. No rights reserved
All code in this repository that is not under a specific license is in the public domain. In those jurisdictions where this may be a problem, the BSD Zero Clause License applies.
SPDX-License-Identifier: 0BSD