If using using multiple PCA9306 for I2C voltage level translation, do I only need one resistor on the master bus?

Question

I have three I2C bus devices: 1) gas sensor (3V), 2) temperature and humidity sensor (3V), and 3) ESP32 (3.3V). They all connect to an I2C bus master: ATMEGA32U4. Using the typical application circuit provided in the datasheet, I used 2.2K resistors for both bus master and bus devices. My question is, do I only need to connect a resistor to the bus master once, and not for every bus device added?

The 3 V sensors can be on the same bus (you only need one pullup resistor for SCL and one pullup resistor for SDA for each bus), and the 5 V bus can also be shared. It looks like you only need two PCA9306s -- one to level translate the 3 V bus and another to level translate the 3.3 V bus.

Thank you! It didn't occur to me that you can combine same level SCL/SDA buses.

Answer 1

A single set of pull-up resistors per bus segment is enough, unless special conditions apply or some standards say otherwise, such as in case of devices that have hot-pluggable I2C bus.

However the values of the resistors are important. If one device pulls a bus wire low, current flows through all pull-ups on the bus. For example the 2.2 kohm resistor on 3.3V bus alone provides 1.5mA of current, and the currents through other resitors easily provide over 5mA, and that may be too much for your sensors.

You need to read how much your chips can handle, as some sensor chips are not even rated to handle the 3mA of I2C standard.