Ask for help on the constraint skill in systemverilog

SystemVerilog
1

Here is a simple transaction item I coded as below.


class my_trans extends uvm_sequence_item;

    function new(string name = "my_trans");
        super.new(name);
    endfunction : new

    rand logic [7:0 ] id;
    rand logic [47:0] info;

    constraint my_cons {
        id >= 8'h1;
        id <= 8'h9;
    }

The first constraint I want to use is the id could only be selected from 1 to 9. (just like what I coded …)
but another constraint I have no idea how to program is: The id in the current transaction can not be the same as the one in previous transaction. For example, if the id is randomized to 3 in this transaction to send (start_item/finish_item), and then the next id can’t be 3 again.

How to implement it in constraint ?

Thank you

2

In reply to zz8318:

If it’s only the previous transaction, you can do:

constraint not_previous {
      id != const'(id);
}

This assumes you are randomizing the same object, then cloning it before sending it.

If you are constructing a new object each time, you will have to save the id in another state variable and use it in the constraint.

3

In reply to dave_59:

Thanks Dave. I’ll follow your suggestion to take a try

4

In reply to zz8318:

Hi Dave,

If the situation is a bit more complicated that we have to follow below rules in the transaction. How to implement it in constraint ?

  1. the current id could not be the same as last one.
  2. if the last id is 1, then the current id could not be 3.
  3. if the last id is 2, then the current id could not be 9.

Thank you

5

In reply to dave_59:

Hi Dave,

I tried your suggestion but failed at below syntax error.

Error-[SE] Syntax error
Following verilog source has syntax error :
“/proj/tb/my_seq.svh”,
20: token is ‘const’
id != const’(id);
^
SystemVerilog keyword ‘const’ is not expected to be used in this context.

6

one quick solution can be(though not very efficient):: just after randomization you can store each 8bit wide randomized value of id and check if current value is generated before or not. if yes than re-randomize the class.