I²C address list and conflict checker

How I²C addresses work

Every device on an I²C bus answers to a 7-bit address, from 0x08 to 0x77. Many chips let you pick one of two, four or eight addresses by tying pins to ground or to the supply; others have a single fixed address. Two devices on the same address answer at the same time, and both readings are garbage.

Some datasheets give 8-bit values, the address shifted left with the read/write bit added: the VL53L0X is listed as 0x52/0x53, the BMP180 as 0xEE/0xEF. Shift right by one to get the 7-bit address libraries and scanners use: 0x29 and 0x77.

Reserved addresses

From the NXP I²C-bus specification (UM10204). Use only 0x08 to 0x77 for your devices.

When two devices clash

First look for an address pin: moving one device is the cleanest fix. If both addresses are fixed, as with a DS3231 clock and a DS1307, or you need two identical sensors with no address pin, use a TCA9548A multiplexer, which gives you eight separate branches behind one address, or put the second device on another I²C bus. The ESP32, ESP32-S3 and RP2040 have two I²C controllers; the ESP32-C3 has one.

Finding what is on your bus

In ESPHome, the i2c component scans the bus at boot and lists the addresses it finds in the log. In Arduino, the classic I2C scanner sketch does the same over the serial monitor. If a device doesn’t show up, check power, SDA and SCL, the pull-ups and its address pin.

Pull-up resistors

I²C lines are open-drain: chips only pull them low, and resistors pull them back up. Too strong a pull-up and the chips can’t pull the line low enough; too weak and the line rises too slowly for the speed. The specification gives both limits.

Rmin = (VDD − 0.4 V) ÷ 3 mA        (VDD > 2 V; 20 mA at 1 MHz)
Rmax = tr ÷ (0.8473 × Cbus)
tr = 1000 ns at 100 kHz, 300 ns at 400 kHz, 120 ns at 1 MHz

All the devices in the list