Constraint a random array

UVM
, ,
1

Hi ,
Can some one help in writing a constrain for [7:0]byte_array[32] . The constraint is each byte in next consecutive location should not change by more then 4 bits. I am not able to write this cleanly without using for loop. Any suggestions.

2

In reply to manav33:

The clean way of writing this IS using a
foreach loop. Why is that a problem for you?

3

In reply to dave_59:

In reply to manav33:
The clean way of writing this IS using a
foreach loop. Why is that a problem for you?

I tried using this . It looks correct ? I am doing ex or to count how many bits have changed and making sure its less then or equal to 4 .


rand [7:0]byte_array[32]
 bit [7:0] value_exor ;

int count;
int i = byte_array.size();
constraint c_byte_array { 
foreach(i) begin
       value_exor=byte_array[i]^byte_array[i+1];
				foreach(value_error[j]) begin
			                  if(value_error[j]==1)
                                         count++;
                                 end	
   count<=4;
end	
}
4

In reply to manav33:

That’s not how foreach works in a constraint.

constraint c_byte_array { 
foreach(byte_array[ii]) if (ii!=0) // guard against out of  bounds indexing
                    ($countones(byte_array[ii]) - $countones(byte_array[ii-1])) inside {[-4:4]}				
}
5

In reply to dave_59:

In reply to manav33:
That’s not how foreach works in a constraint.

constraint c_byte_array { 
foreach(byte_array[ii]) if (ii!=0) // guard against out of  bounds indexing
($countones(byte_array[ii]) - $countones(byte_array[ii-1])) inside {[-4:4]}				
}

Hi Dave

Thanks for your reply. If i understand correctly your code is making sure that between two consecutive memory locations the difference in number of “1”'s is not more then 4 correct ?

However i want the constraint such that between two locations no more then 4bits have toggled ( thats why i used Xor first to check how many bits have changed ), Can you please update constraint based on required scenario ? Thanks

6

In reply to manav33:

It was a quick guess for me. Show me an example where my constraint does not meet your requirements.