Common I2C/SPI Device Reference

I2C Devices

AHT20 — Temperature & Humidity

• Address: 0x38
• Init: Write [0xBE, 0x08, 0x00] then wait 10ms
• Measure: Write [0xAC, 0x33, 0x00], wait 80ms, read 6 bytes
• Parse: Status=byte[0], Humidity=(byte[1]<<12|byte[2]<<4|byte[3]>>4)/2^20100, Temp=(byte[3]&0x0F<<16|byte[4]<<8|byte[5])/2^20200-50
• Notes: No register addressing — write command bytes directly (omit register param)

BME280 / BMP280 — Temperature, Humidity, Pressure

• Address: 0x76 or 0x77 (SDO pin selects)
• Chip ID register: 0xD0 → BMP280=0x58, BME280=0x60
• Data registers: 0xF7-0xFE (pressure, temperature, humidity)
• Config: Write 0xF2 (humidity oversampling), 0xF4 (temp/press oversampling + mode), 0xF5 (standby, filter)
• Forced measurement: Write [0x25] to register 0xF4, wait 40ms, read 8 bytes from 0xF7
• Calibration: Read 26 bytes from 0x88 and 7 bytes from 0xE1 for compensation formulas
• Also available via SPI (mode 0 or 3)

SSD1306 — 128x64 OLED Display

• Address: 0x3C (or 0x3D if SA0 high)
• Command prefix: 0x00 (write to register 0x00)
• Data prefix: 0x40 (write to register 0x40)
• Init sequence: [0xAE, 0xD5, 0x80, 0xA8, 0x3F, 0xD3, 0x00, 0x40, 0x8D, 0x14, 0x20, 0x00, 0xA1, 0xC8, 0xDA, 0x12, 0x81, 0xCF, 0xD9, 0xF1, 0xDB, 0x40, 0xA4, 0xA6, 0xAF]
• Display on: 0xAF, Display off: 0xAE
• Also available via SPI (faster, recommended for animations)

MPU6050 — 6-axis Accelerometer + Gyroscope

• Address: 0x68 (or 0x69 if AD0 high)
• WHO_AM_I: Register 0x75 → should return 0x68
• Wake up: Write [0x00] to register 0x6B (clear sleep bit)
• Read accel: 6 bytes from register 0x3B (XH,XL,YH,YL,ZH,ZL) — signed 16-bit, default ±2g
• Read gyro: 6 bytes from register 0x43 — signed 16-bit, default ±250°/s
• Read temp: 2 bytes from register 0x41 — Temp°C = value/340 + 36.53

DS3231 — Real-Time Clock

• Address: 0x68
• Read time: 7 bytes from register 0x00 (seconds, minutes, hours, day, date, month, year) — BCD encoded
• Set time: Write 7 BCD bytes to register 0x00
• Temperature: 2 bytes from register 0x11 (signed, 0.25°C resolution)
• Status: Register 0x0F — bit 2 = busy, bit 0 = alarm 1 flag

INA219 — Current & Power Monitor

• Address: 0x40-0x4F (A0,A1 pin selectable)
• Config: Register 0x00 — set voltage range, gain, ADC resolution
• Shunt voltage: Register 0x01 (signed 16-bit, LSB=10µV)
• Bus voltage: Register 0x02 (bits 15:3, LSB=4mV)
• Power: Register 0x03 (after calibration)
• Current: Register 0x04 (after calibration)
• Calibration: Register 0x05 — set based on shunt resistor value

PCA9685 — 16-Channel PWM / Servo Controller

• Address: 0x40-0x7F (A0-A5 selectable, default 0x40)
• Mode 1: Register 0x00 — bit 4=sleep, bit 5=auto-increment
• Set PWM freq: Sleep → write prescale to 0xFE → wake. Prescale = round(25MHz / (4096 × freq)) - 1
• Channel N on/off: Registers 0x06+4N to 0x09+4N (ON_L, ON_H, OFF_L, OFF_H)
• Servo 0°-180°: ON=0, OFF=150-600 (at 50Hz). Typical: 0°=150, 90°=375, 180°=600

AT24C256 — 256Kbit EEPROM

• Address: 0x50-0x57 (A0-A2 selectable)
• Read: Write 2-byte address (high, low), then read N bytes
• Write: Write 2-byte address + up to 64 bytes (page write), wait 5ms for write cycle
• Page size: 64 bytes. Writes that cross page boundary wrap around.

SPI Devices

MCP3008 — 8-Channel 10-bit ADC

• Interface: SPI mode 0, max 3.6 MHz @ 5V
• Read channel N: Send [0x01, (0x80 | N<<4), 0x00], result in last 10 bits of bytes 1-2
• Formula: value = ((byte[1] & 0x03) << 8) | byte[2]
• Voltage: value × Vref / 1024

W25Q128 — 128Mbit SPI Flash

• Interface: SPI mode 0 or 3, up to 104 MHz
• Read ID: Send [0x9F, 0, 0, 0] → manufacturer + device ID
• Read data: Send [0x03, addr_high, addr_mid, addr_low] + N zero bytes
• Status: Send [0x05, 0] → bit 0 = BUSY