All I3C Slaves come up in I2C mode, as they do not yet possess a Dynamic Address. If they have an I2C static address, then they may operate on a legacy I2C bus, using that static address. I3C Slaves may also support an I2C 50 ns Spike filter for Fm and Fm+ modes, and they may support other I2C features that are not supported by I3C (such as Device ID). All I3C Masters will emit the first START,7’h7E slowly enough to be seen through an I2C Spike filter, allowing the I3C Slaves to disable the spike filter once they see the first START,7’h7E.
If the I3C Slave does not have an I2C‑type static address, then it will simply wait for the START,7’h7E from an I3C Master. Such a Slave would be of no value on a legacy I2C bus, as I2C relies on each slave having a static address.
Note that I3C Slaves should not use I2C Clock stretching unless they know with certainty that they are on a legacy I2C Bus. I3C Masters are never required to support I2C Clock stretching, so the I3C Slave must not use that facility unless it has reason to know it is safe to do so (i.e., that it is on a legacy I2C bus). (Note also that an I3C Master may choose to support I2C Clock stretching when communicating with a legacy I2C Slave, though that is outside of the scope of the I3C Specification.)