5397-POLOLU, Pololu Isolated USB-to-I²C Adapter with Isolated Power
This USB-to-I²C adapter enables a PC or other USB host to act as the controller on an I²C bus, reading and writing data from I²C target devices. It supports clock stretching and I²C clock speeds over 1 MHz. The adapter features full galvanic isolation between the USB and I²C sides, preventing unwanted current flow and protecting the USB host from damaging voltages. This version delivers isolated power to the I²C side at 5V or 3.3V, selectable via an onboard switch (latched at startup).
Overview
This adapter functions as a bridge between I²C devices and a USB host, supporting bidirectional communication with clock stretching and high-speed I²C operation. It provides isolated and regulated 5 V or 3.3 V power (up to 200 mA) to the I²C bus.
Using a USB Type-C connector, the adapter appears as a virtual serial port (CDC ACM), recognized by Windows 10+, Linux, and macOS without needing drivers. It can be accessed through any programming environment capable of reading and writing binary data over a serial port. A Python library is provided for quick startup, and the STM32C071G8 firmware is open source for customization and transparency.
The I²C side features a JST SH-style 4-pin connector compatible with SparkFun’s Qwiic and Adafruit’s STEMMA QT systems. The four I²C pins are also broken out to 0.1″-pitch through-holes for use with standard male or female headers.
Connections
| Pin | Cable color | Function |
|---|---|---|
| 1 | Black | I²C ground (GND); not connected to USB GND |
| 2 | Red | I²C output voltage (VCC). When enabled via the “Enable VCC Out” command, provides 3.3V or 5V up to 200 mA. Voltage is set by slide switch position at startup. |
| 3 | Blue | I²C data line (SDA) |
| 4 | Yellow | I²C clock line (SCL) |
The output voltage (VCC) is selected using a slide switch latched at power-up. Changing the switch while powered has no effect until power is cycled or VCC is toggled via command. Blue (5V) and green (3.3V) indicator LEDs show actual output voltage; both are off when VCC is disabled.
Schematic
