Functions in Constraints
In some cases constraint can’t be expressed in a single line, in such cases function call can be used to constrain a random variable. calling the function inside the constraint is referred to as function in constraints.
- The function will be written outside the constraint block
- Constraint logic shall be written inside the function as function definition and function call shall be placed inside the constraint block
- Functions shall be called before constraints are solved, and their return values shall be treated as state variables.
constraint constraint_name { var = function_call(); };
Functions in constraints example
In the below example,
The function is called inside the constraint.
class packet;
rand bit [3:0] start_addr;
rand bit [3:0] end_addr;
constraint start_addr_c { start_addr == s_addr(end_addr); }
function bit [3:0] s_addr(bit [3:0] e_addr);
if(e_addr < 4)
s_addr = 0;
else
s_addr = e_addr - 4;
endfunction
endclass
module func_constr;
initial begin
packet pkt;
pkt = new();
repeat(3) begin
pkt.randomize();
$display("\tstart_addr = %0d end_addr =",pkt.start_addr,pkt.end_addr);
end
end
endmodule
Simulator Output
start_addr = 9 end_addr =13 start_addr = 2 end_addr = 6 start_addr = 0 end_addr = 1
