On Raspberry Pi boards, when the CPU is in halt state (powered, but CPU not running), GPIO3 is kept in high state due to an external pull-up resistor ... to the primary I2C bus could be a problem.

It can work in production – here’s Adafruit taking the 3.3 V-pulled-up Raspberry Pi I2C bus, and connecting ... The “Low” state (writing 0) is a hard pull down to GND.

Isolating a Bidirectional Open-Drain Bus The I2C bus, which contains at least one bidirectional open-drain bus, does not easily lend itself to optical or other isolation. The ideal bus model of the ...