SSD1306 OLED with LOLIN S2 Mini — I2C on GPIO33/35

A small OLED is the quickest way to give an S2 Mini project a face. The default I2C pins — GPIO33 for SDA, GPIO35 for SCL — are where D1 mini shields put I2C, so most OLED shields fit directly.

Power the display from 3V3. The sketch shows uptime and free PSRAM once a second.

Wiring

Every connection
LOLIN S2 Mini	SSD1306 OLED 0.96″ (128×64, I2C)	Wire	Notes
GND	GND	black
3V3	VCC	red
35	SCL	yellow	D1 mini “D1” position.
33	SDA	blue	D1 mini “D2” position.

3.3–5 V (on-board regulator) part, 3.3 V board. Most modules have a regulator and run from 3.3 V or 5 V. Their I2C pull-ups go to VCC, so on ESP32 and Pico power the OLED from 3.3 V to keep SDA/SCL at a safe level.

Board quirks

GPIO33 (SDA) and GPIO35 (SCL) are on the outer row where a D1 mini has D2 and D1, so D1 mini OLED shields plug straight on.
GPIO33–40 have no ADC on the S2, which makes them ideal for digital buses like I2C and keeps the ADC1 pins (GPIO1–10) free.
The display shows free PSRAM. The S2FN4R2 chip has 2 MB of PSRAM in the package; it reads 0 if PSRAM is disabled in Tools.

Code

ssd1306-s2mini.inoArduino C++Open in Playground Flash

// SSD1306 128x64 I2C OLED on a LOLIN S2 Mini.
// GND → GND, VCC → 3V3, SDA → GPIO33, SCL → GPIO35 (the S2 Mini's default Wire pins,
// in the same place as D2/D1 on a D1 mini). Address 0x3C.
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

Adafruit_SSD1306 display(128, 64, &Wire, -1);

void setup() {
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  Wire.begin(33, 35);  // SDA, SCL
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("SSD1306 not found at 0x3C - try 0x3D, and check the GND/VCC order on your module");
    while (true) delay(1000);
  }
  display.setTextColor(SSD1306_WHITE);
  Serial.println("OLED ready");
}

void loop() {
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0, 0);
  display.print("S2 Mini");
  display.setTextSize(1);
  display.setCursor(0, 28);
  display.printf("Uptime: %lu s", millis() / 1000);
  display.setCursor(0, 44);
  display.printf("PSRAM free: %u KB", ESP.getFreePsram() / 1024);
  display.display();
  delay(1000);
}

// SSD1306 128x64 I2C OLED on a LOLIN S2 Mini.
// GND → GND, VCC → 3V3, SDA → GPIO33, SCL → GPIO35 (the S2 Mini's default Wire pins,
// in the same place as D2/D1 on a D1 mini). Address 0x3C.
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

Adafruit_SSD1306 display(128, 64, &Wire, -1);

void setup() {
  Serial.begin(115200);
  uint32_t t0 = millis();
  while (!Serial && millis() - t0 < 3000) {}
  Wire.begin(33, 35);  // SDA, SCL
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("SSD1306 not found at 0x3C - try 0x3D, and check the GND/VCC order on your module");
    while (true) delay(1000);
  }
  display.setTextColor(SSD1306_WHITE);
  Serial.println("OLED ready");
}

void loop() {
  display.clearDisplay();
  display.setTextSize(2);
  display.setCursor(0, 0);
  display.print("S2 Mini");
  display.setTextSize(1);
  display.setCursor(0, 28);
  display.printf("Uptime: %lu s", millis() / 1000);
  display.setCursor(0, 44);
  display.printf("PSRAM free: %u KB", ESP.getFreePsram() / 1024);
  display.display();
  delay(1000);
}

Library: Adafruit SSD1306 (Adafruit (also install Adafruit GFX Library)) — compiled with Adafruit SSD1306 2.5.17.

Expected serial output

Serial Monitor · 115200Output

OLED ready

OLED ready

Common errors

“SSD1306 not found at 0x3C”

Try 0x3D; check that SDA is on 33 and SCL on 35. Full fix.

COM port disappears after upload

The S2 re-enumerates its USB after reset; reselect the port. Port troubleshooting.