I have a 50 bit variable, I want to generate a pattern as shown in example under. How do I write a constraint for it.
For Example
rand bit[10:0] var; (Taking an example of 10 bit variable actual pattern needed is for a 50 bit variable)
Pattern
1000000000
1100000000
1110000000
1111000000
1111100000
1111110000
1111111000
1111111100
1111111110
1111111111
Is there really a need to use constraints for something that can easily be done with a function call? Also, if you can write constraints, what should the randomize function do when all bits are ones?
Ben
In reply to nishitk:
rand bit[10:0] varb = 11'b10000000000;
constraint pattern { varb == const'( {1'b1, varb[10:1]} ); }
In reply to dave_59:
Why the need for the const?
In reply to ben@SystemVerilog.us:
A const cast means treat the expression as a constant value (I.e. non-random) value. The expression gets evaluated before randomization and is treated like a state variable.
In reply to dave_59:
It helps. Thanks Dave !
In reply to dave_59:
In reply to nishitk:
rand bit[10:0] varb = 11'b10000000000;
constraint pattern { varb == const'( {1'b1, varb[10:1]} ); }
i am getting this error plz help me OUT
// code here
class packet;
rand bit[10:0] varb = 11’b10000000000;
constraint pattern { varb == const’( {1’b1, varb[10:1]} ); }
endclass
module constr_blocks;
initial begin
packet pkt;
pkt = new();
repeat(10) begin
pkt.randomize();
$display(“\t value = %0d”,pkt.varb);
end
end
endmodule
output:ERROR VCP7501 “Undeclared type: const.” “testbench.sv” 5 59
In reply to Rohi_417:
It looks like your tool has not implemented a const cast. Maybe you have an older version. Please contact your vendor for support.
if I have some thing like this rand bit [4:0] variable[]; size of variable is 5, and i want to generate pattern like variable[0] = 10000; variable[1] = 11000, variable[2] = 11100, variable [3] = 11110, variable[4] = 11111; Why does the following constraint misbehave/not working?
class problem9;
rand bit [4:0] variable[];
bit [4:0] variable_1[5];
constraint on_variable {
foreach(variable[i]) {
if(i !=0) {
variable[i] == (variable[i-1]) | (1<<(4-i));
}
else {
variable[0] == 1 << 4 ;
}
}
}
constraint size{
variable.size()==5;
}
endclass
module check;
problem9 p9;
initial
begin
p9 = new();
p9.randomize();
foreach(p9.variable[i]) begin
$display("i = %0d, variable = %0b",i, p9.variable[i]);
end
foreach(p9.variable_1[i]) begin
if(i != 0) begin
p9.variable_1[i] = p9.variable_1[i-1] | 1 << (4-i);
end
else
p9.variable_1[0] = 1 << 4;
$display("p9.variable_1 here = %0b", p9.variable_1[i]);
end
end
endmodule
I am getting the output for variable_1 where constraint/randomization is not involved. but for variable I get an output as:
i = 0, variable = 10000
i = 1, variable = 10110
i = 2, variable = 10000
i = 3, variable = 10010
i = 4, variable = 11000
You have a problem with operator precedence in your constraint. Check the precedence of bitwise ‘or’ (|).
Here’s one way to do this.
package learning;
import uvm_pkg::*;
`include "uvm_macros.svh"
class packet extends uvm_object;
`uvm_object_utils(packet)
function new(string name = "packet");
super.new(name);
endfunction
static int count = 1;
rand bit [10:0] pattern;
bit [10:0] p = ~0;
constraint default_c {
pattern == (p << ($bits(pattern) - count));
}
function void post_randomize();
if ($bits(pattern) == count) count = 1; else count++;
endfunction
virtual function void printInfo();
`uvm_info(get_type_name, $sformatf("pattern = %b", pattern), UVM_LOW)
endfunction
endclass
class test extends uvm_test;
`uvm_component_utils(test)
rand packet pkt;
function new(string name="test", uvm_component parent);
super.new(name, parent);
endfunction
task run_phase(uvm_phase phase);
super.run_phase(phase);
repeat (50) begin
pkt = packet::type_id::create("pkt");
assert(pkt.randomize());
pkt.printInfo();
end
endtask
endclass
endpackage
module top();
`include "uvm_macros.svh"
import uvm_pkg::*;
import learning::*;
initial begin
run_test("test");
end
endmodule
ya got it, thanks Dave.
class abc;
rand bit[9:0] a,b;
static bit[9:0] count;
constraint a_c{
a == 10'b1111_1111_11<< count;
}
function void post_randomize();
count++;
foreach(a[i]) begin
b[i] = a[9-i];
end
b = ~b;
$display("count =%0p a =%b", count, b);
endfunction
endclass
module tb;
abc abc_inst;
initial begin
abc_inst = new();
repeat(10)
abc_inst.randomize();
end
endmodule