Hi !
I have a RPi4 and a Pico and I want to make a I2C comunication between them. I wonder, if I power just one of them while they are connected through the GND, SDA and SCL, will do some hardware damage to one of them ? Because my RPi4 has just died (I read a short circuit on the 3.3V supply voltage) while I was working on the code for this I2C comunication. And I'm affraid that I've done something wrong... The only thing that it came into my mind is that I power them on at a time... Will this could lead to some hardware damage ? They have their own power supplies, the I2C Slave is not powered from the I2C Master.

 

No.
Sorry for the short answer

 

Connecting to an I2C port should not cause damage to an unpowered device. The I2C drivers are open collector, and inherently are off when not powered. The power source on the entire I2C connection should only be from one set of pull-up resistors that are inherently current limited. Unless something in the system is not the usual circuit setup, damage should not occur.

 

Having done it a thousand times already with various other projects I would suspect you had connected the power wires backwards on accident. Luckily I haven't done it to a PI yet, but I've been through three PICKits already.