Soft constraints are
default constraints which hold true until contradicted by another similar
constraint.
Let us understand this concept with the
following examples.
1. We have a packet class with address variable, which is limited in between 10 and 100.
class packet;
rand bit[7:0]
address;
constraint address_c
{
address inside
{[10:100]};
}
endclass
program test();
packet p;
initial begin
p=new();
assert(p.randomize());
$display("address value is %0d" , p.address);
end
endprogram
Output: address value is 53
The output is 53 which is in between 10 and 100.
2. Let us apply a inline constraint stating
that address should be equal to 200.
class packet;
rand bit[7:0]
address;
constraint address_c
{
address inside
{[10:100]};
}
endclass
program test();
packet p;
initial begin
p=new();
assert(p.randomize() with { p.address==200;} );
$display("address value is %0d" , p.address);
end
endprogram
Output: Error
Error-[CNST-CIF] Constraints inconsistency failure
testbench.sv, 19
Constraints are inconsistent and cannot be solved.
Please check the inconsistent constraints being printed above and rewrite
them.
"testbench.sv", 19: test.unnamed$$_3.unnamed$$_1: started at 0ns failed at 0ns
Offending 'p.randomize() with {
(p.address == 8'hc8);
}
'
address value is 0
testbench.sv, 19
Constraints are inconsistent and cannot be solved.
Please check the inconsistent constraints being printed above and rewrite
them.
"testbench.sv", 19: test.unnamed$$_3.unnamed$$_1: started at 0ns failed at 0ns
Offending 'p.randomize() with {
(p.address == 8'hc8);
}
'
address value is 0
The output is 0, here the randomization failed due to
constraint conflict i.e. we have given an inline value for address which is out
of the range i.e. in between 10 to 100, in such scenarios we need to switch off
the corresponding constraints as shown below.
rand bit[7:0]
address;
constraint address_c
{
address inside
{[10:100]};
}
Which
endclass
program test();
packet p;
initial begin
p=new();
p.address_c.constraint_mode(0);
assert(p.randomize()
with { p.address==200;} );
$display("address value is %0d" , p.address);
end
endprogram
output: address
value is 200
3. Let us change the code slightly and use
soft keyword before address variable in constraint block as shown below.
class packet;
rand bit[7:0]
address;
constraint address_c
{
soft address
inside {[10:100]};
}
endclass
program test();
packet p;
initial begin
p=new();
//p.address_c.constraint_mode(0);
assert(p.randomize() with { p.address==101;} );
$display("address value is %0d" , p.address);
end
endprogram
output: address
value is 101
In the above code there is no need to use additional
constructs like constraint_mode(0) , here the inline constraint of {p.address
== 101 } will have the priority and the solver solves this instead of above
declarative constraint ( constraint address_c { soft address inside {[10:100]}; } )
These soft constraints can be used to check the error/illegal scenarios.
These soft constraints can be used to check the error/illegal scenarios.
So the output here is
101.
Code Link : http://www.edaplayground.com/x/59M8
Happy Learning.
Please provide your feedback if any.
Thanks,
Vikas Billa
