Circuits
Synchronous Vs Asynchronous Design
Much of today’s logic design is based on two major assumptions: all signals are binary, and time is discrete. Both of these assumptions are made in order to simplify logic design. By assuming binary values on signals, simple Boolean logic can be used to describe and manipulate logic constructs. By assuming time is discrete, hazards and feedback can largely be ignored. However, as with many simplifying assumptions, a system that can operate without these assumptions has the potential to generate better results.
Asynchronous circuits keep the assumption that signals are binary, but remove the assumption that time is discrete. This has several possible benefits:
No clock skew - Clock skew is the difference in arrival times of the clock signal at different parts of the circuit. Since asynchronous circuits by definition have no globally distributed clock, there is no need to worry about clock skew. In contrast, synchronous systems often slow down their circuits to accommodate the skew. As feature sizes decrease, clock skew becomes a much greater concern.
Lower power - Standard synchronous circuits have to toggle clock lines, and possibly pre-charge and discharge signals, in portions of a circuit unused in the current computation. For example, even though a floating point unit on a processor might not be used in a given instruction stream, the unit still must be operated by the clock. Although asynchronous circuits often require more transitions on the computation path than synchronous circuits, they generally have transitions only in areas involved in the current computation.
Note that there are some techniques in synchronous design that addresses this issue as well.
Average-case instead of worst-case performance - Synchronous circuits must wait until all possible computations have completed before latching the results, yielding worst-case performance. Many asynchronous systems sense when a computation has completed, allowing
Much of today’s logic design is based on two major assumptions: all signals are binary, and time is discrete. Both of these assumptions are made in order to simplify logic design. By assuming binary values on signals, simple Boolean logic can be used to describe and manipulate logic constructs. By assuming time is discrete, hazards and feedback can largely be ignored. However, as with many simplifying assumptions, a system that can operate without these assumptions has the potential to generate better results.
Asynchronous circuits keep the assumption that signals are binary, but remove the assumption that time is discrete. This has several possible benefits:
No clock skew - Clock skew is the difference in arrival times of the clock signal at different parts of the circuit. Since asynchronous circuits by definition have no globally distributed clock, there is no need to worry about clock skew. In contrast, synchronous systems often slow down their circuits to accommodate the skew. As feature sizes decrease, clock skew becomes a much greater concern.
Lower power - Standard synchronous circuits have to toggle clock lines, and possibly pre-charge and discharge signals, in portions of a circuit unused in the current computation. For example, even though a floating point unit on a processor might not be used in a given instruction stream, the unit still must be operated by the clock. Although asynchronous circuits often require more transitions on the computation path than synchronous circuits, they generally have transitions only in areas involved in the current computation.
Note that there are some techniques in synchronous design that addresses this issue as well.
Average-case instead of worst-case performance - Synchronous circuits must wait until all possible computations have completed before latching the results, yielding worst-case performance. Many asynchronous systems sense when a computation has completed, allowing