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Sequential Logic Equivalence and Simplification

Different implementations of a circuit may vary in their state encoding or even in the amount of state they keep in terms of bits. One might be simpler or better than the other for a given purpose. At times we need to check the equivalence of deigns.

For a synchronous clock domain, if two designs are known to have the same state encodings, then the problem degenerates to that of combinational equivalence checking of their respective next-state functions. For each D-type flip-flop we need to check the combinational equivalence of its sourcing circuit in the two designs. However, even if the state encodings are the same, there can be un-used portions of the state space which must be treated as don't-cares for this check.

If the state encoding is known to be changed, then what can be compared? Perhaps we can compare the trajectory of states between two designs, building up our own mapping between the encodings in each design. Generally, two designs will have the same set of output terminals and so the basis of the mapping is equivalence classes formed around each possible setting of the output terminals.

This leads to the concept of full equivalence in terms of external observable behaviour. Do the two designs behave the sames as each other in black box testing: that is, without any knowledge of the internal behaviour. If so, they are said to bi-simulate each other.

Again, not all of the reachable state space may be used. The circuit might always be wired up externally so that one input is a delayed version of one output. Therefore the question arises, do a pair of designs follow the same state trajectory when interfacing with a specified reactive automaton ?

Commonly, the number of clock cycles of delay through a sub-system (its latency) is not important and perhaps can be disregarded in equivalence checking. This leads to the concept of temporally floating ports (not lectured in 2008/9), where a pair of designs are equivalent if the timing behaviour inside a port (subset of the external collections) appears equivalent, even though we would see differences if we looked at the relative timing of operations with respect to other ports. For example, the precise order in which an Ethernet hub chip sends a broadcast packet to each of its output ports does not matter, as long as it is actually delivered to each port, and from any given port this ordering cannot be perceived without peeking at the other ports. This sort of floating is similar to the temporal decoupling ides in the loosely-timed models, in that systems are designed, at the high level, to be tolerant to timing and performance variations in their major components. .

Two other variations in the problem definition arise for systems where the exact number of clock cycles is not considered important. This clearly also applies to asynchronous systems without a clock.

27: (C) 2008-11, DJ Greaves, University of Cambridge, Computer Laboratory.