What is Asynchronous Logic?

So, what even is this blog? Well, I want to record my efforts to understand asynchronous (clockless) logic, and help others who may endeavor to do the same. So, what does asynchronous mean?

Well, in ‘traditional’ logic designs, the system uses a clock to synchronize data. In order for data to be saved and made available to the next unit, a clock edge is sent to the registers. This is needed because the signals take time to settle, and that time isn’t constant. This means that the data can’t be saved until enough time has passed to be sure everything is ready.

In Asynchronous Logic, there is no clock. Instead of waiting for enough time to have passed for the signals to be settled, the circuit is aware of when it is ready. There are several ways to accomplish this:

• Matched Delays: The circuit is designed with a completion signal that is delayed from the inputs. The signal is used to inform the next module that the data is ready. Each module can define it’s own delay, instead of a system-wide clock period.
• Completion Detection: Since the transistors will continue to consume power until they are done switching, it is possible to measure the current drawn, and signal completion when it drops below a certain threshold.
• Null Convention Logic: NCL uses a different data encoding scheme to tell when data is finished. Each line is DATA or Null, so a boolean signal is represented by 2 lines (DATA0 and DATA1).

The common element of all of these is that the components/modules have to let the the next module when they are done, and the previous module when they are ready for more data. This is called handshaking. Each stage has to wait for the next to be ready for new data, and for the previous to have new data ready.

Recap:

• Synchronous Logic requires a clock to tell it when it is ready to do the next thing.
• Asynchronous Logic is designed to know when it is done internally, removing the need for a clock.

So, asynchronous sounds better, right? It can run at the best speed of the hardware, it doesn’t spend as much power on the clock, etc. The downside that has stood in it’s way up to this point has been that it is harder to design. However, as designs get smaller and faster, the advantages of asynchronous logic become more important.