Casting enum to logic

Question

Consider the following module declaration:

module DFF(d, q, CLK, RESET);
 parameter W = 2;
 input  [W-1:0] d;                      
 input  CLK;                                
 input  RESET;      
 output logic [W-1:0]   q; 

//....
endmodule

What is the proper way of instantiating it where d and q are of enum type? Here is my enum type:

typedef enum logic [1:0] {ENUM_IDLE = 0,
            ENUM_S1 ,
            ENUM_S2
            } T_STATE;

I would like to instantiate the DFF for a T_STATE variable type:

T_STATE d, q;
DFF dff_inst (.d(d), .q(q), .CLK(CLK), .RESET(RESET));

This generates compile/enumeration error. I have also unsuccessfully tried:

DFF dff_inst (.d(logic'(d)), .q(logic'(q)), .CLK(CLK), .RESET(RESET));

and

DFF dff_inst (.d(logic[1:0]'(d)), .q(logic[1:0]'(q)), .CLK(CLK), .RESET(RESET));

I would like to keep the DFF definition as is, but cast the enum type to logic.

Edit:

This one, suggested in IEEE Std 1800-2012, 6.24.1, also generates an elaboration error:

typedef logic [$bits(T_STATE) - 1 : 0] T_STATE_LOGIC; 
DFF dff_inst (.d(T_STATE_LOGIC'(d)), .q(T_STATE_LOGIC'(q)), .CLK(CLK), .RESET(RESET));

Shouldn't you have input logic [W-1:0] d; in your definition of DFF? — Mark Lakata
@MarkLakata, input logic [W-1:0] d is legal in SystemVerilog. logic has a smaller memory footprint and less over head in simulation compared to wire, as logic does not have strength information or conflict driver resolution (logic has single driver checked at elaboration when assigned in an always_comb/ff/latch block). You'll only see the benefit on very large projects. — Greg

Greg Greg · Accepted Answer · 2014-05-27T20:08:57

d doesn't need to be casted.

I could only reproduce the error with ModelSim, all the other simulators I have access to didn't generate any errors or warnings and simulated correctly.

For ModelSim, I found that this worked:

DFF dff_inst (.q(q[1:0]), .*);

and this worked:

DFF dff_inst (.q({q}), .*);

Working example on here

Casting enum to logic

1 Answers