On The Implementation of Finite State Machines

With FSMs, the designer can statefully control data processing logic and thus implement arbitrary functions up to a full CPU. FSMs can be implemented with design elements using localparams (for named constants and states), register modules (for outputs and holding state), comparison modules (eq, lt, gt, neq), multiplexer modules (to select the next state), and wires to name signals and tie everything together.

Once the designer has defined the state transition diagram, they can build-up the design of the FSM incrementally using design elements by:

  1. defining any constraints on outputs (e.g. only asserted in certain states)
  2. defining the basic operations on the datapath, as Boolean composition of the control inputs and outputs
  3. defining the transformations on the datapath (what do certain operations means when in given states)
  4. defining the state transitions (given a certain transformation, what is the next state)
  5. defining the control outputs (which transformations assert a given output, as simple Boolean logic on the transformations)

In the state transition diagram, each node and its edges are designed as a "checker" module which takes the current state and some inputs, and if they are equal to the expected values, raises a "match" output. This "match" output is then used to set the next state. There are three ways to compose these "checker" modules to calculate the nest state:

  1. A chain of multiplexers which either pass along the current state or previously selected state, or replace that with a new selected state. This design is easy to follow, but depends on the tool's logic optimizer to combine terms in the chain of logic. log2N bits are required for N states.
  2. A parallel tree of multiplexers outputting either a chosen next state or zero, plus one extra bit of logic which outputs either zero or the current state if no other multiplexer is outputting a chosen next state. All the multiplexer outputs are OR-reduced to the final next state. This design has a shorter critical path, but requires a bit more manual design effort, and converges a lot of wasted calculations (the outputs of checkers which could never be active in the current state) into a large OR gate, which suggests a more efficient alternative. log2N bits are required for N states.
  3. Separate parallel "checkers" whose output sets or clears a single flip-flop. Each flip-flop represents a state. The FSM is in a given state if one and only one flip-flop is set (one-hot encoding). This design is the most parallel, has the shortest logic path, and may support pipelined operation or even overlapping states given suitable state encodings. N bits are required for N states.

There's an apparent problem where a chain of multiplexers imposes a priority order on "match" outputs, causing some concurrent state transitions to be missed, or a tree of multiplexers funneling to a final OR gate before the state register could merge multiple set "match" outputs into nonsense state values, or multiple bits could become set in an otherwise one-hot encoding. However, these cases are impossible for DFAs (Deterministic Finite Automata), where identical combinations of current state and current signals cannot lead to multiple, different states. To create such a condition in the above constraints/operations/transformations/transitions/control (COTTC) scheme, we would have to explicitly write multiple checkers which test for the same transformation but select different next states. Although logically allowable, the conflict will be visible to the user.


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