ENUM ENCODING

Example 2,9 Enumerated Types Using the enum encoding Attribute... 

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

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. 

From the ENUM ENCODING attribute, you can see that and X are mapped to D, which is a don t-care (can be either O or T). Therefore, for synthesis, is equivalent to X which is equivalent to D . This is the reason your synthesis results are the same, regardless of whether you use or X . 

It is also possible to define your own encoding scheme through the use of the ENUM TYPE ENCODING attribute. See Qiapter 7 for more details on attributes. Box 3.7 illustrates the syntax of this t e of declaration. 

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