SV transaction sequence dependency constraint?

Hi Davy,

Since you're looking just one step into the past, you could simply maintain a variable with the previous transaction type, use it in a constraint, and update its value after generating each new transaction. For example:

   cmd_t prev_cmd = WRITE;
   Packet p = new;
   initial
      repeat (20) begin
          // apply constraint
          assert( p.randomize()  with {if (prev_cmd ==WRITE)  cmd==IDLE } ) else $fatal;
          // do something with p here
          ...
          // update prev value for the next iteration
          prev_cmd = p.cmd;
      end

Now, if you want to generate more interesting sequences of transactions with a certain structure to them, you should create a new class - a sequence, which generates a stream of transactions based on some "control knobs" you set in the class instance. You can then have multiple (inherited) subtypes of that sequence class, each generating a different kind of sequence, with different knobs.

Regards,

   Zeev.

Originally posted in cdnusers.org by zeevk

Davy,

In cases like this, I've been using the "pre_randomize" function as follows...

//--- SV code start---

//define transactions
typedef enum { WRITE=1,READ=2,IDLE=4} cmd_t;
class Packet;
...
rand cmd_t cmd;
cmt_t last_cmd = IDLE;
...

constraint c1 {last_cmd == WRITE -> cmd == IDLE;}

function void pre_randomize();
last_cmd = cmd;
endfunction

endclass

Packet p;

initial
repeat(20) begin
p = new();
p.randomize();
end //--- SV code end

If someone knows a better way to do this, I'd like to hear about it.

David Walker

Originally posted in cdnusers.org by dbwalker0min

Hi Zeev and David,

Thanks a lot for the good ideas! I have tried David's program and all OK.

And there is an small errata for above program.
1. Assume only one instance of the class at one time
change "cmt_t last_cmd = IDLE;" to "static cmt_t last_cmd = IDLE;"
2. Change "function void pre_randomize(); " to "function void
post_randomize(); "

Best regards,
Davy

Originally posted in cdnusers.org by davyzhu

Hi Davy,

One of the most powerful aspects of system verilog generation is the randsequence construct. It is a very powerful feature and is very useful in generation of sequences. It is also very scalable. I will post an example later today on how to use it.

Nitin

Originally posted in cdnusers.org by nitin_sharma

Hi Davy,

Making the last_cmd field statis is dangerous - it will not work if you have several such generator threads running simultaneously, as all the packet class instances share the static last_cmd value.
Thanks,

    Zeev.

Originally posted in cdnusers.org by zeevk