Any system that shares a clock is synchronous. Two common communication methods for hardware are SPI, which has a shared clock, and is hence synchronous (https://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/SP...) and UART, which has no shared clock, indeed it can be implemented with a single wire, and is hence asynchronous. Internally the receiver's clock needs to be fast enough to correctly sample data being transmitted to it. (Ed: or I guess you might be able to do something clever with an async circuit fed directly by the signaling line.)

Communicating with hardware outside the CPU doesn't have to be synchronous or asynchronous, it's a design decision depending on what you're trying to accomplish. Languages that don't support both methods are missing half of the design space.

I believe it's relevant to point out that most CPUs are synchronous in response to the claim that "Processors really are fundamentally asynchronous."

It does seem like I could have phrased that differently, since a number of people have taken it to mean something rather different than I intended.