Multiple items matching in Verilog case statement

user_13774396 0
Viewed 2245

I have a state machine with several states that are very similar. I could write it for each state, like in the following example:

module CHECK_FSM (
GO,
DONE,
CLK, RESETN );

input       GO;
output reg  DONE;
input       CLK,RESETN;

reg[7:0]    state;

reg[7:0]    next_state; 

    //the actual registers
    always @(posedge CLK or negedge RESETN)
        if (!RESETN)    state <= 8'd0;
        else            state <= next_state;

    //next state logic
    always @(*)
        begin
        //defaults
        next_state = state;
        DONE = 1'b0; //low by default

        case(state)
            0: //S_INIT
                if(GO==1'b1)
                    next_state = 8'd1;
                else
                    next_state = 8'd0;
            1: //S_WAIT_1:
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
            2: //S_WAIT_2:
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
            3: //S_WAIT_3:
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
            //...
            127: //S_FINISH
                begin
                DONE = 1'b1;
                next_state = state; //stay put
                end

            default: next_state= 8'd0;
            endcase
        end

endmodule

I am wondering if there is a way to specify the case statement for multiple states. Something like:

        case(state)
            0: //S_INIT
                if(GO==1'b1)
                    next_state = 8'd1;
                else
                    next_state = 8'd0;
            [1-->126]:
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
            127: //S_FINISH
                begin
                DONE = 1'b1;
                next_state = state; //stay put
                end

            default: next_state= 8'd0;
            endcase

I want to avoid repeating the statements (that are all the same) to be more clear and to avoid errors if I need to modify it afterwards (my actual FSM is more complicated than this...). Any ideas ?

PS: The code is meant to be synthesized. In my case, the synthesis tool does not support SystemVerilog statements, so I am hoping for a Verilog solution.

verilog case
2 Answers

case statement syntax allows for specifying multiple case item values separated by commas:

1, 2, 3, 4,
5, 6, 7, 8: begin
    // your code
end

That will be quite cumbersome for 126 values.

In your case, since you are only using values 0 to 127, you don't need 8 bits for your state variables. You could change them to be 7-bit ([6:0]), and then your case would be full. Then, you could use the default for your states 1-126 without having the long comma-separated list.

case (state)
    0       : //...
    127     : //...
    default : //1-126 ...
endcase

For the benefit of tools which do support SystemVerilog syntax, IEEE Std 1800-2017, section 12.5.4 Set membership case statement describes the case inside construct:

case (state) inside
    0       : //...
    [1:126] : //...
    default : //...
endcase

Your synthesis tool may or may not support it, but it is worth a quick try.

user_1977580

The case inside statement in SystemVerilog would do exactly what you want. But since you have constrained yourself to Verilog,it might be more practical to use an if/else chain than a case statement for the example given

if (state==0) begin : S_INIT
                if(GO==1'b1)
                    next_state = 8'd1;
                else
                    next_state = 8'd0;
              end
 else if (state >0 && state < 127) begin
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
            end
else if (state == 127) begin : S_FINISH
                DONE = 1'b1;
                next_state = state; //stay put
                end
else begin : S_DEFUALT
            default: next_state= 8'd0;
end

or you can still use a case statement in this form

case(1)
  state==0: //S_INIT
                if(GO==1'b1)
                    next_state = 8'd1;
                else
                    next_state = 8'd0;
  (state>0&&state<127):
                if(GO==1'b1)
                    next_state = state+1; //continue
                else
                    next_state = 8'd0; //go back to S_INIT
  state==127: //S_FINISH
                begin
                DONE = 1'b1;
                next_state = state; //stay put
                end
  default: next_state= 8'd0;
endcase
user_27556070
Related