Expansion Board Base — XIAO Accessory Guide

Compatible XIAO Boards

Pin Usage Diagram

XIAO Pin    | Accessory Function       | Protocol
------------|--------------------------|----------
D0 / A0     | Grove A0/D0 connector    | Analog/Digital
D1          | User Button              | GPIO (INPUT_PULLUP)
D2          | SD Card Chip Select (CS) | SPI
D3 / A3     | Passive Buzzer           | GPIO (can be cut via trace)
D4 / SDA    | I2C Data                 | I2C (OLED, RTC, Grove I2C)
D5 / SCL    | I2C Clock                | I2C (OLED, RTC, Grove I2C)
D8          | SPI Clock (SCK)          | SPI (MicroSD)
D9          | SPI Data In (MISO)       | SPI (MicroSD)
D10         | SPI Data Out (MOSI)      | SPI (MicroSD)
TX          | Grove UART TX            | UART
RX          | Grove UART RX            | UART

Pin Conflict Warning

Pins OCCUPIED by Expansion Board

• D0/A0 — Grove A0/D0 connector
• D1 — User button (INPUT_PULLUP)
• D2 — SD card CS
• D3/A3 — Passive buzzer (can be disconnected by cutting trace)
• D4/SDA — I2C bus (OLED, RTC, Grove I2C)
• D5/SCL — I2C bus (OLED, RTC, Grove I2C)
• D8 — SPI SCK (MicroSD)
• D9 — SPI MISO (MicroSD)
• D10 — SPI MOSI (MicroSD)
• TX/RX — Grove UART connector

Pins remaining FREE

• D6, D7

Conflicts with other accessories

• COB LED Driver — conflicts on D0, D1, D2, D3, D8, D9 ❌
• ePaper Driver Board — conflicts on D0, D1, D2, D3, D8, D10 ❌
• RS485 Board — conflicts on D2 (SD CS vs enable), D4/D5 (I2C vs UART) ❌
• CAN Bus Board — conflicts on D8, D9, D10 (SPI) ❌
• Grove Vision AI V2 — can share I2C bus ✅ (uses SDA/SCL)

Note: The Expansion Board uses nearly all XIAO pins. It is designed as a standalone prototyping platform, not for stacking with other GPIO-heavy accessories.

Power Requirements

Arduino Setup & Usage

Required Libraries

# OLED Display:
# U8g2 library — https://github.com/olikraus/U8g2_Arduino
# Install via Arduino Library Manager: search "U8g2"

# RTC Clock:
# PCF8563 library — https://github.com/Bill2462/PCF8563-Arduino-Library
# Install via Arduino Library Manager or download ZIP

# MicroSD Card (SAMD21, RP2040, ESP32):
# SD library (built-in) — no installation needed

# MicroSD Card (nRF52840):
# SdFat library — https://github.com/greiman/SdFat
# Install via Arduino Library Manager: search "SdFat"

Complete Working Example — OLED Display

#include <Arduino.h>
#include <U8x8lib.h>
#include <Wire.h>

U8X8_SSD1306_128X64_NONAME_HW_I2C u8x8(U8X8_PIN_NONE, SCL, SDA);

void setup() {
    u8x8.begin();
    u8x8.setFlipMode(1);
}

void loop() {
    u8x8.setFont(u8x8_font_chroma48medium8_r);
    u8x8.setCursor(0, 0);
    u8x8.print("Hello World!");
}

Complete Working Example — RTC Clock with OLED

#include <Arduino.h>
#include <U8x8lib.h>
#include <PCF8563.h>
#include <Wire.h>

PCF8563 pcf;
U8X8_SSD1306_128X64_NONAME_HW_I2C u8x8(U8X8_PIN_NONE, SCL, SDA);

void setup() {
    u8x8.begin();
    u8x8.setFlipMode(1);

    Wire.begin();
    pcf.init();
    pcf.stopClock();

    // Set initial time
    pcf.setYear(20);
    pcf.setMonth(10);
    pcf.setDay(23);
    pcf.setHour(17);
    pcf.setMinut(33);
    pcf.setSecond(0);

    pcf.startClock();
}

void loop() {
    Time nowTime = pcf.getTime();

    u8x8.setFont(u8x8_font_chroma48medium8_r);
    u8x8.setCursor(0, 0);

    u8x8.print(nowTime.day);
    u8x8.print("/");
    u8x8.print(nowTime.month);
    u8x8.print("/");
    u8x8.print(nowTime.year);

    u8x8.setCursor(0, 1);
    u8x8.print(nowTime.hour);
    u8x8.print(":");
    u8x8.print(nowTime.minute);
    u8x8.print(":");
    u8x8.print(nowTime.second);

    delay(1000);
}

Complete Working Example — MicroSD Card

#include <SPI.h>
#include <SD.h>

#define SD_CS_PIN D2

void setup() {
    Serial.begin(115200);
    while (!Serial) delay(100);

    pinMode(SD_CS_PIN, OUTPUT);

    if (!SD.begin(SD_CS_PIN)) {
        Serial.println("SD card initialization failed!");
        return;
    }
    Serial.println("SD card initialized.");

    // Write a file
    File myFile = SD.open("test.txt", FILE_WRITE);
    if (myFile) {
        myFile.println("Hello from XIAO Expansion Board!");
        myFile.close();
        Serial.println("File written.");
    }

    // Read the file
    myFile = SD.open("test.txt");
    if (myFile) {
        Serial.println("File contents:");
        while (myFile.available()) {
            Serial.write(myFile.read());
        }
        myFile.close();
    }
}

void loop() {}

Complete Working Example — User Button

const int buttonPin = D1;

void setup() {
    pinMode(LED_BUILTIN, OUTPUT);
    pinMode(buttonPin, INPUT_PULLUP);
}

void loop() {
    if (digitalRead(buttonPin) == HIGH) {
        digitalWrite(LED_BUILTIN, HIGH);
    } else {
        digitalWrite(LED_BUILTIN, LOW);
    }
}

Complete Working Example — Buzzer

int speakerPin = D3;  // A3

void setup() {
    pinMode(speakerPin, OUTPUT);
}

void loop() {
    // Simple tone
    tone(speakerPin, 1000, 200);  // 1kHz for 200ms
    delay(500);
    tone(speakerPin, 1500, 200);  // 1.5kHz for 200ms
    delay(500);
}

TinyGo Setup & Usage

TinyGo OLED Display Example

package main

import (
    "machine"
    "image/color"

    "tinygo.org/x/drivers/ssd1306"
)

func main() {
    machine.I2C0.Configure(machine.I2CConfig{
        SDA:       machine.SDA_PIN,
        SCL:       machine.SCL_PIN,
        Frequency: 400000,
    })

    display := ssd1306.NewI2C(machine.I2C0)
    display.Configure(ssd1306.Config{
        Address: 0x3C,
        Width:   128,
        Height:  64,
    })

    display.ClearDisplay()

    // Draw a pixel pattern
    for x := int16(0); x < 128; x += 4 {
        for y := int16(0); y < 64; y += 4 {
            display.SetPixel(x, y, color.RGBA{R: 255})
        }
    }
    display.Display()

    println("OLED initialized (TinyGo)")
    select {} // Block forever
}

TinyGo User Button Example

package main

import (
    "machine"
    "time"
)

func main() {
    button := machine.D1
    button.Configure(machine.PinConfig{Mode: machine.PinInputPullup})

    led := machine.LED
    led.Configure(machine.PinConfig{Mode: machine.PinOutput})

    for {
        if button.Get() {
            led.High()
        } else {
            led.Low()
        }
        time.Sleep(10 * time.Millisecond)
    }
}

TinyGo Buzzer Example

package main

import (
    "machine"
    "time"
)

func main() {
    buzzer := machine.D3
    buzzer.Configure(machine.PinConfig{Mode: machine.PinOutput})

    // Simple beep pattern using manual toggling
    for {
        // 1kHz tone for 200ms (toggle every 500µs)
        for i := 0; i < 200; i++ {
            buzzer.High()
            time.Sleep(500 * time.Microsecond)
            buzzer.Low()
            time.Sleep(500 * time.Microsecond)
        }
        time.Sleep(500 * time.Millisecond)
    }
}

Note: For TinyGo SD card access, check tinygo.org/x/drivers for available SPI SD card drivers. The PCF8563 RTC may require a custom I2C driver in TinyGo.

Communication Protocol Details

I2C Bus (OLED + RTC + Grove)

SPI Bus (MicroSD)

Grove Connectors

Common Gotchas / Notes

1. ⚠️ Plug XIAO into the middle of the two female header connectors — Incorrect placement will damage both boards.
2. ⚠️ Plug XIAO first, then connect USB-C cable.
3. Buzzer on D3/A3 — Connected by default. Can be disconnected by cutting a trace on the board if D3 is needed for other purposes.
4. SD card CS is D2 — This is a common conflict point with other accessories.
5. OLED and RTC share I2C — Both are on the same I2C bus (SDA/SCL). No address conflict (OLED=0x3C, RTC=0x51).
6. nRF52840 SD card — Use the SdFat library instead of the built-in SD library for nRF52840.
7. Battery charging LED — Flashing = not charging or no battery; Solid = charging.
8. CircuitPython support — The board supports CircuitPython with MicroSD for library storage.
9. 5V servo connector — Available for direct servo connection without external power.
10. SWD debug pins — Broken out as male headers for debugging.
11. Not compatible with nRF54L15 or MG24 — These boards have different SWD pin layouts.

Reference Links

• Seeed Wiki: https://wiki.seeedstudio.com/Seeeduino-XIAO-Expansion-Board/
• Product Page: https://www.seeedstudio.com/Seeeduino-XIAO-Expansion-board-p-4746.html
• U8g2 Library: https://github.com/olikraus/U8g2_Arduino
• PCF8563 Library: https://github.com/Bill2462/PCF8563-Arduino-Library
• SdFat Library: https://github.com/greiman/SdFat