Section 21.2

LRM-135

Change (changes in red and blue):

module mb;

logic [3:0] i,o;

ma #(.p1(3), .p2(int)) u1(i,o); //redefines p2 to a type of int

endmodule

$ can be assigned to parameters of integer types. A parameter to which $ is assigned shall only be used wherever $ can be specified as a literal constant.

For example, $ represents unbounded range specification, where the upper index can be any integer.

parameter r2 = $;

property inq1(r1,r2);

@(posedge clk) a ##[r1:r2] b ##1 c |=> d;

endproperty

assert inq1(3);

ma #(.p1(3), .p2(int)) u1(i,o); //redefines p2 to a type of int

endmodule

To support whether a constant is $, a system function is provided to test whether a constant is a $. The syntax of the system function is

$isunbounded(const_expression);

$isunbounded returns true if const_expression is unbounded. Typically, $isunbounded would be used as a condition in the generate statement.

The example below illustrates the benefit of using $ in writing properties concisely where the range is parameterized. The checker in the example ensures that a bus driven by signal en remains 0, i.e, quiet for the specified minimum (min_quiet) and maximum (max_quiet) quiet time.

Note that function $isunbounded is used for checking the validity actual arguments.

interface quiet_time_checker( clk, reset_n, en);

input reset_n;

input clk;

input [1:0] en;

parameter min_quiet = 0;

parameter max_quiet = 0;

generate

if ( max_quiet == 0) begin

property quiet_time;

@(posedge clk) reset_n |-> ($countones(en) == 1);

endproperty

a1: assert property (quiet_time);

end

else begin

property quiet_time;

@(posedge clk)

(reset_n && ($past(en) != 0) && en == 0)

|->(en == 0)[*min_quiet:max_quiet]

##1 ($countones(en) == 1);

endproperty

a1: assert property (quiet_time);

end

if ((min_quiet == 0) && ($isunbounded(max_quiet))

$display(warning_msg);

endgenerate

endinterface

quiet_time_checker #(0, 0) quiet_never (clk,1,enables);

quiet_time_checker #(2, 4) quiet_in_window (clk,1,enables);

quiet_time_checker #(0, $) quiet_any (clk,1,enables);

Another example below illustrates that by testing for $, a property can be configured according to the requirements. When parameter max_cks is unbounded, it is not required to test for expr to become false.

interface width_checker(clk, reset_n, expr);

input clk;

input reset_n;

input expr;

parameter min_cks = 1;

parameter max_cks = 1;

generate begin

if ($isunbounded(max_cks)) begin

property width;

@(posedge clk)

(reset_n && $rose(expr)) |-> (expr [* min_cks]);

endproperty

a2: assert property (width);

end

else begin

property assert_width_p;

@(posedge clk)

(reset_n && $rose(expr)) |-> (expr[* min_cks:max_cks])

##1 (!expr);

endproperty

a2: assert property (width);

end

endgenerate

endinterface

width_checker #(3, $) max_width_unspecified (clk,1,enables);

width_checker #(2, 4) width_specified (clk,1,enables);