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.
2 Answers
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
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.
