One-hot encoding

The fastest FSM implemoitation uses the one-hot method of encoding. In DC, the user will have to declare the FSM oicoding style using the set fem encoding style command. 

Example 2.10 State Machine Using the state vector and enum encoding Attributes 

SO = 4 b0001, S1 = 4 b0010, S2 = 4 b0100, S3 = 4 b1000 reg [3 0] / synopsys enum states V CURRENT STATE, NEXT STATE  

Example 2.10 shows the same state machine described in Example 2.7 using both the state vector attribute and the enum encodlng attribute. enum encoding has been used such that the first flip-flop, when on, implies the state SO, the second flip-flop implies state SI, and so on. Note that the coding style uses the state vector attribute as in Example 2.8 and does not require the declaration of state vectors. In general, the one-hot encoding style involves e use of one flip-flop for each state, the current state being determined by the flip-flop which is on. 

Example 2.11 describes an FSM which shows fail-safe behavior. The others clause ensures that if the FSM were to go to the state defined by encoding ll , then it is reset to the state SO. 

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Since we have to handle sets of clock values, we encode the values by bit vectors in such a way, that every value has an associated bit. ) th this one hot encoding we are able to represent sets of clock values. E.g. the set 0,3,4,5 is encoded by a bit vector 6 = 0... OllKKll, where the least significant bit encodes the zero value. 

To describe a fully specified (deterministic) finite state machine without output in PTL, we first introduce a propositional variable for each input symbol and one for each state. Our interpretation for the variables is such that when a variable is assigned true, that symbol (state) is the machine s current symbol (state) the symbols (states) are mutually exclusive. This may be compared with a one-hot encoding scheme. In PTL, the mutual exclusion of variables vO, vl, v2,. .., vn can be expressed by [] ( vl v2. .. vn V vl v2. ..vn V V vl v2. ..vn) or in short-hand notation [] El (... 

Here is another example of a Mealy FSM, this one uses one-hot state encoding. 

Asynchronous Reset Unspecified Encoding Bit Length 4 Encoding style one hot... 

In the dc shell script file used to synthesize the FSM, notice that the script changes the encoding style to one-hot inspite of the encoding being different in the source code. Figure 5.3 shows the FSM inferred. The same dc shell script can be used for Verilog code along with the appropriate file names. 

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