This note that not all Verilog designs are synthesizable. Usually, only a very specific subset of constructs can be used in a design that is to be realized in hardware.

One important restriction that pops up is that every reg variable can only be assigned to in at most one always statement. In other words, regs have affinity to always blocks.

The following types of always blocks can generally be used.

 

In the former case, the * indicates that the block should be executed whenever any signal used in the block changes or, equivalently, that the block should be executed continuously. Therefore, regs that have affinity to combinatorial always blocks are implemented as signals computed from other signals using combinatorial logic, i.e. gates.

Registers that have affinity to always blocks of the latter type, on the other hand, are outputs of D flip-flops that are clocked on the rising edge of clk (falling edge if negedge is used). Inputs to the flip-flops are, again, computed with combinatorial logic from other signals.

Consider the following, somewhat contrived example.

 

Here, out_n is associated with the first always block, out with the second. out_n will be implemented with a single NOT gate that will drive out_n and be driven from out (note that it is a pure combinatorial logic). On the other hand, out will be driven by a flip-flop clocked from clk. The input to the flip-flop will again be computed by a NOT gate from out (which is driven by the aforementioned flip-flop). Optimizing synthesizers will combine the two NOT gates and use one NOT gate and one flip-flop.

Depending on the hardware you have available, other types of constructs can be used. For example, if the flip-flops have asynchronous resets, the following construct is also synthesizable.

 

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