1.1.2. Asynchronous Design Hazards
Asynchronous design techniques, such as ripple counters or pulse generators, can work as “short cuts” to save device resources. However, asynchronous techniques have inherent problems. For example, relying on propagation delays can result in incomplete timing constraints and possible glitches and spikes, because propagation delay varies with temperature and voltage fluctuations.
Asynchronous design structures that depend on the relative propagation delays can present race conditions. Race conditions arise when the order of signal changes affect the output of the logic. The same logic design can have varying timing delays with each compilation, depending on placement and routing. The number of possible variations make it impossible to determine the timing delay associated with a particular block of logic. As devices become faster due to process improvements, delays in asynchronous designs may decrease, resulting in designs that do not function as expected. Relying on a particular delay also makes asynchronous designs difficult to migrate to other architectures, devices, or speed grades.
The timing of asynchronous design structures is often difficult or impossible to model with timing assignments and constraints. If you do not have complete or accurate timing constraints, the timing-driven algorithms that synthesis and place-and-route tools use may not be able to perform the best optimizations, and the reported results may be incomplete.
Additionally, asynchronous design structures can generate glitches, which are pulses that are very short compared to clock periods. Combinational logic is the main cause of glitches. When the inputs to the combinational logic change, the outputs exhibit several glitches before settling to their new values. Glitches can propagate through combinational logic, leading to incorrect values on the outputs in asynchronous designs. In synchronous designs, glitches on register's data inputs have no negative consequences, because data processing waits until the next clock edge.
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