I'm curious what you mean by 'synchronous' here, for example an I2C bus is clocked by the master, ignoring clock stretching and that clock need not be in any wa

I'm curious what you mean by 'synchronous' here, for example an I2C bus is clocked by the master, ignoring clock stretching and that clock need not be in any way synchronous with the processor clock on the slave. And to clarify, we're talking about PHY level protocol clocks here, not time clocks.