Full case synthesis directive

Figure 2-35 With full case synthesis directive no latches.

As the synthesized netlist shows, no latches are inferred for NextToggle when the full case synthesis directive is used. 

It is necessary to specify the full case synthesis directive, otherwise latches are inferred for Address. Alternatively, an initial assignment to Address before the case statement can also be made to avoid latches no synthesis directive is then necessary. This is shown in the following always statement. 

In this case, no latches are inferred for Z and NextState since the full case synthesis directive states that no other case item values can occur. However, the preferred style for not inferring latches is to use the default branch. 

The problem with this approach is that since it is impractical to list all possible values an integer can take, to avoid latches either the default case branch must be specified or the full case synthesis directive must be used. Another problem with this approach is not good readability. 

Here is an example of a full case synthesis directive. 

The two synthesis directives we have seen so far, full case and parallel case, can potentially cause functional mismatches to occur between the design model and the synthesized netlist. The problem is that these directives are recognized only by a synthesis tool and not by a simulation tool. In either of the cases, if the designer is not careful in specifying the directive, mismatches can occur. 

The full case directive tells the synthesis tool that all possible values that can possibly occur in CurrentState have been listed and the value of Next-State is a don t-care for all other cases, and therefore, the synthesis tool should not generate latches for NextState. However this may not be true in simulation. It could happen that CurrentState for some reason, gets a value of 2 b00. In such a case, the case statement simulates as if NextState value is saved, but in the synthesized netlist, the value of NextState may not be saved. 

Recommendation Use caution when using the synthesis directives full case and parallel case. Use only if really necessary. 

The major focus in this chapter will be on synthesis, with emphasis placed on more recent applications, particularly those where regiochemistry and stereochemistry are precisely controlled. The reader is referred to the earlier reviews for full mechanistic information and details of historic interest. Electrophilic addition of X—Y to an alkene, where X is the electrophile, gives products with functionality Y (3 to the heteroatom X. Further transformations of X and/or Y provide the basis for diverse synthetic applications. These transformations include replacement of Y by hydrogen, elimination to form a ir-bond (either including the carbon bonded to X or (3 to that carbon so that X is now in an allylic position), and nucleophilic or radical substitution. Representative examples of these synthetic methods will be given below. This chapter will include examples of heterocycles formed in one-pot reactions where the the initial alkene-electrophile adduct contains an electrophilic group that can react further. Examples of heterocycles formed in several steps from alkene-electrophile adducts will also be considered. Cases in which activation by an external electrophile directly results in addition of an internal heteroatom nucleophile are treated in Chapter 1.9 of this volume. 

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