Implication constraints and if else constraints in SystemVerilog
Table of Contents
Implication constraints
The implication operator can be used to declaring conditional relations between two variables. implication operator is denoted by the symbol ->. The implication operator is placed between the expression and constraint.
expression -> constraint
If the expression on the LHS of implication operator (->) is true, then the only constraint on the RHS will be considered.
Implication constraint example
In the example below,
If addr_range == small, then addr will get a value less than 8.
i.e implication operator is used to mention condition between the two variables addr_range and addr.
class packet;
rand bit [3:0] addr;
string addr_range;
constraint address_range { (addr_range == "small") -> (addr < 8);}
endclass
module constr_implication;
initial begin
packet pkt;
pkt = new();
pkt.addr_range = "small";
$display("------------------------------------");
repeat(4) begin
pkt.randomize();
$display("\taddr_range = %s addr = %0d",pkt.addr_range,pkt.addr);
end
$display("------------------------------------");
end
endmodule
Simulator Output
------------------------------------ addr_range = small addr = 6 addr_range = small addr = 2 addr_range = small addr = 1 addr_range = small addr = 4 ------------------------------------
if else constraints
if else block allows conditional executions of constraints. If the expression is true, all the constraints in the first constraint/constraint-block must be satisfied, otherwise all the constraints in the optional else constraint/constraint-block must be satisfied.
if else constraints example
Above an example of implication operator usage is written with if else condition.
In the example below,
If addr_range == small, then addr will get a value less than 8.
class packet;
rand bit [3:0] addr;
string addr_range;
constraint address_range { if(addr_range == "small")
addr < 8;
else
addr > 8;
}
endclass
module constr_if_else;
initial begin
packet pkt;
pkt = new();
pkt.addr_range = "small";
$display("------------------------------------");
repeat(3) begin
pkt.randomize();
$display("\taddr_range = %s addr = %0d",pkt.addr_range,pkt.addr);
end
$display("------------------------------------");
pkt.addr_range = "high";
$display("------------------------------------");
repeat(3) begin
pkt.randomize();
$display("\taddr_range = %s addr = %0d",pkt.addr_range,pkt.addr);
end
$display("------------------------------------");
end
endmodule
Simulator Output
------------------------------------ addr_range = small addr = 1 addr_range = small addr = 4 addr_range = small addr = 6 ------------------------------------ ------------------------------------ addr_range = high addr = 12 addr_range = high addr = 15 addr_range = high addr = 9 ------------------------------------