I2C line broke for non-powered external sensor

[baonguyen21]

The RTC Ds3231 is all way powered, via 3V3 and via its own battery. With the code test for the RTC DS3231, the I2C communication to the RTC would fail if there is external I2C sensor that connected to SDA and SCL without being power (disconnected Vcc and GND).

[baonguyen21]

Phantom Power : https://forum.arduino.cc/index.php?topic=642971.0

[udellc]

Is there a way to disconnect only the VCC line to the sensor, but keeping it grounded? Would that fix the issue?

[udellc]

It looks like isolating the i2c bus will be necessary. This one is an updated version of the one recommended on the discussion board posted above:
http://www.ti.com/product/TCA4311A
I think we may need to make a new version of our Loom i2c shield, which includes an option to populate the TCA9548 i2c MUX but also with the TCA4311A buffer on the shield's i2c input.
The shield should also include an i2c pass through after the buffer, for being able to use isolated i2c devices without the MUX if desired.

[baonguyen21]

This issue is not resolved yet. The buffer require some more space that the current Hypnos V2 does not have. Requested feature will be postponed to later version.

[baonguyen21]

For now, the system is still unstable if the all of the I2C devices are not powered before communicating with the RTC. The I2C Multiplexer can be not powered, and system will still work.