AVR Freaks

ESD protection is already there, no series resistors on the external connector though.

Actually, the mux would not be directly connected to outside world.

Previously, there was an ASIC that had a built-in I2C master to access outside world through it. Now the replacement ASIC has only I2C pass-through to outside world, therefore the mux. Neither should require series resistors as per the manufacturer. I just want to use the mux as a precaution and make sure that if the external bus gets jammed, I can just reset the ASIC so that the rest of the system is unaffected.

In fact it needs some consideration if the ASIC pass-through feature also adds some series resistance like the mux would.

0R resistors would not hurt though.

The ASIC is commanded to join the external bus to internal when connection is detected and things let to stabilise for a while. But yes, there will always be a chance of contact bounce or whatever even after that.

Everything connected to the mux would use 3V3 bus. ASIC would level translate to 5V external bus. The 4052 mux would just benefit from 5V supply.

Not my ASIC so that's why :)

Well it seems that the ASIC is more than just a simple pass gate or mux. It seems to understand about the I2C protocol enough to switch data buffering direction between int->ext and ext->int when needed, and most likely clock is always buffered into int->ext direction, so clock stretching is not possible.

When shorting the external SDA, SCL or both to GND, I cannot see any problems whatsoever on the internal bus, of course the data read from external bus is not valid, but I can still control the ASIC through I2C to disconnect external I2C bus from internal.

Problem solved, no mux needed, no ASIC resetting needed because external bus does not jam internal bus.

To be honest, a mux would have solved a lot of I2C address problems, as there are a lot of components here, and without very careful design, the I2C component addresses would overlap.