I²C Logic Level Converter

Modern MCUs work at 3.3V or even lower voltage, while many legacy I²C devices are still operating at 5V.

Two problems may arise:
  • Signal from the MCU operating at 3.3V is not high enough to be understood as high by the 5V-powered device.
  • Signal from the I2C device is to high and can damage the 3.3V MCUs with non-5V-tolerant I/Os.

The solution is called a logic level converter.

Breakout Board

This ready to use logic level converter from Sparkfun is very convenient on a breadboard.

There are two sides: one for high voltage (eg. 5V) and another for low voltage (eg. 3.3V).

For I²C, use the TXO/TXI pins as they are bi-directional. The two GND pins are connected. For serial, RXI/RXO can also be used but they are only mono-directional. 
Picture from Sparkfun

MOSFET-Based Circuit

Now, when I need a smaller footprint, I use the standard MOSFET-based circuit.

Each channel consists on one 2N7000 and two 10kΩ resistors. Two channels are required, one for SDA and another for SCL.

  • Reference: 2N7000 and 10kΩ resistors 
  • Cost: the 2N7000 is listed at less than USD0.15 per unit
Schematics for the SDA channel, to be duplicated for the SCL channel.

Integrated Circuit

Another option I've investigated includes integrated circuits.

The most common is the PCA9306, available in a variety of packages. There are many other ICs, some of them with specific features.
Logic diagram from Texas Instruments

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