How parallel case and full cases problems are avoided in System Verilog?

Answer 1

The full_case and parallel_case directives are dangerous because they tell the synthesis tool

something different about the design than what is told to the simulator.

To the Verilog simulator, full_case and parallel_case are buried inside of Verilog

comments and are completely ignored. To the synthesis tool, full_case and parallel_case

are command-directives that instruct the synthesis tools to potentially take certain actions or

perform certain optimizations that are unknown to the simulator.

A full case statement is a case statement in which all possible case-expression binary patterns

can be matched to a case item or to a case default.

e.g. Full case, sel=2'b11 will be covered by default statement.

The x-assignment will also be treated as a don'tcare for synthesis, which may allow the synthesis tool to further optimize the synthesized design. It's the potentially causing a mismatch to occur between simulation and synthesis. To insure that the pre-synthesis and post-synthesis simulations match, the case default could assign the y-output to either a

predetermined constant value, or to one of the other multiplexer input values

module mux3c

(output reg y,

input [1:0] sel,

input a, b, c);

always @*

case (sel)

2'b00: y = a;

2'b01: y = b;

2'b10: y = c;

default: y = 1'bx;

endcase

endmodule

// Use synopsys full_case statement to create the full case , but it treated differently in simulation and synthesis.

module mux3b (y, a, b, c, sel);

(output reg y,

input [1:0] sel,

input a, b, c);

always @*

case (sel) // synopsys full_case

2'b00: y = a;

2'b01: y = b;

2'b10: y = c;

endcase

endmodule

SystemVerilog use priority modified case statement to solve the full case problem.

The biggest difference between a full_case directive and a priority modified case statement

is that the priority keyword is part of the SystemVerilog syntax that will be interpreted the

same by simulators, synthesis tools and formal verification tools. In essence, the priority case

statement is a "safe" full_case case statement.

e.g.

priority case (...)

...

endcase

A parallel case statement is a case statement in which it is only possible to match any case

expression to one and only one case item.

e.g. A parallel case statement

module intctl1b

(output reg int2, int1, int0,

input [2:0] irq );

always @* begin

{int2, int1, int0} = 3'b0;

casez (irq) // synopsys parallel_case

3'b1??: int2 = 1'b1;

3'b?1?: int1 = 1'b1;

3'b??1: int0 = 1'b1;

endcase

end

endmodule

This is an example that demonstrates that adding the parallel_case directive makes the design

smaller and faster, but in the process it also adversely changes the functionality of the design.

SystemVerilog adds the new case statement modifier called "unique."

The unique keyword shall cause the simulator to report a run-time error if a case expression is

ever found to match more than one of the case items. In essence, the unique

case statement is a "safe" parallel_case case statement.

unique case (...)

...

default: ...

endcase

Guideline: Code all intentional priority encoders using if-else-if statements. It is easier for

the typical design engineer to recognize a priority encoder when it is coded as an if-else-if

statement.