Hi,
I dont think you can "inject" a 5v signal onto an arbitrary line because that could blow any chip that tries to pull that line down to zero volts (logic zero).
You can try to pull it up with a light current but if the current is allowed to go too high it blows one of the other chips.
It's quite easy to provide an MCU with a signal that it can handle without going over the input voltage level. Sometimes a current limiting resistor is enough other times you can provide a Schottky diode as a clamp to +3.3v also with a current limiting resistor, as long as the positive supply line can always sink the current.
But injecting a signal like you describe probably is not a good idea.
I dont think you can "inject" a 5v signal onto an arbitrary line because that could blow any chip that tries to pull that line down to zero volts (logic zero).
You can try to pull it up with a light current but if the current is allowed to go too high it blows one of the other chips.
It's quite easy to provide an MCU with a signal that it can handle without going over the input voltage level. Sometimes a current limiting resistor is enough other times you can provide a Schottky diode as a clamp to +3.3v also with a current limiting resistor, as long as the positive supply line can always sink the current.
But injecting a signal like you describe probably is not a good idea.