what to randomize?
(1) primary input data <==one data
(2)encapsulated input data <== muti group data
(3)protocol exceptions,errors and violations
(4)delays
overview
1.randomization enables users to automatically generate random input sitimuls for functional verification
2.systemverilog enable user to specify random constrained (legal) values
3.random costraint should be specified using OOP
Randomization in SV
(1) keyword
rand bit [1:0] y;
randc bit [1:0] y;
(2)simple class with random variables
class BUS
rand bit [15:0] addr;
randc bit [31:0] data;
constraint range1{
addr > 1024;
data<16384;
}
endclass
(3)randomize() function <==启动一个随机约束
if success return 1
if failture return 0
BUS bus = new();
repeat(50) begin
if(bus.randomize() == 1)
$display(bus.addr.bus.data);
else
$display(“randomization failed”);
end
(4) constrain solver ===> seed
the same seed results in the same random value
(5)constraint blocks
constraint constraint_indentifier {
constraint_statmemts
}
(6)simples expressions
only one relational operator(< <= > >= …)
(7)set membership operator:inside
eg1: class BUS
rand bit[15:0] addr;
randc bit [31:0] data;
constraint range1{
addr inside {[0:1000],[1024:16384]};
data > 1000;
data < 100000;
}
endclass
eg2:
integer fives[0:3] = {5,10,15,20};
rand integer v;
constraint c3 {v inside fives};
constraint c4 !{v inside fives};
(8)weighted distribution:dist
property1: They are a relational test for test membership
property2: They specify a statistical distribution function for the result
:= <====the same value in the range every（可以对一个区间也可以对一个独立的数）
/= <=====to be equally divided by all values range_weight/n(对一个区间约束)
cannot be used with a randc
eg:
(9)bidirectional constraints
not procedural but declarative
all active at one time
(10) conditional constraints
implication operator –> 相当于 if..else
(11) unconstrainted
rand bit x ;
rand bit [1:0] y;
the same probability
(12)implication
class imp1;
rand bit x;
rand bit [1:0] y;
constraint c_xy{
(x==0) –> (y==0);
}
endclass
(13)implication and bidirectional constraints
class imp_Bid;
rand bit x;
rand bit [1:0] y;
constraint c_xy{
y > 0;
(x==0) –> (y==0);
}
endclass
(14)solve before
class solvebefore;
rand bit x;
rand bit [1:0] y;
constraint c_xy;
{
(x==0) –> y==0;
solve y before x;
}
(15) interative constraints
allows arrayed variable to be constrained in a parameterized manner using loop variables
class c;
rand bye A[4];
constraint C1
{
foreach(A[i])
A[i] inside{2,4,8,16};
}
constraint C2
{
foreach (A[j])
A[j] > 2*j;
}
endclass
(16) functions in constraints
class B;
rand int x,y;
constraint C{x <= F(y);}
constraint D{y inside{2,4,8};}
endclass
1.functions cannot contain output or ref arguments <===ref 相当于inout
2.functions should be automatic <===automatic可以立即返回值
3.functions that appear in constraint cannot modify the constraint <===只是简单调用而已
4.functons shall be called before constraints are solved, and their return values shall be treated as state variables
5.random variables used as function argumets shall establish an implicit variable ordering or priority
（17）in_line constraints ---xx.randomize()with{constraints_statememt}
equivalent to adding an extra constraint to any existing one in effect
(18) disabling random variables ---rand_mode() <==control a random variable is active or inactive
class packed;
rand integer src,dst;
endclass
int r;
packet packect_a=new;
packet_a.rand_mode(0);
packet_b.src.rand_mode(1);
(19)controlling constraints with constraint_mode() <===control a constraints is active or inactive
class packet;
rand interger src,dst;
constraint filter{src>2*dst;}
endclass
function integer toggle_rand(packet p);
if(p.filter.constraint_mode() ==1 )
p.filter.costraint_mode(0);
else
p.filter.constraint_mode(1);
toggle_rand = p.randomize();
endfunction
总结：constraints主要用于transaction中，