I wish to measure the delay between an enable signal and the clock signal's edge which occurred just before the rise of the enable signal.
I want this measurement to be accurate, therefore I use the last_crossing() function

I thought of this code, but it does not give the desired results due to order of execution. Apparently the simulator first calculates the expressions inside the cross block , and only then it calculates the last_crossing() block.
Therefore, when calculating dly ,en_time equals -1.


Code:

 @(cross(V(clk)-vth,1));
    clk_time=last_crossing(V(clk)-vth,1);

  @(cross(V(en)-vth,1)) begin      
      dly=en_time-clk_time;
    end

    en_time=last_crossing(V(en)-vth,1); 

Is there a different way to have accurate time calculation using last_crossing() and yet get the behavioral I need?


Regards,

Ari

Ken,

Thank you for your quick reply.

From the example you gave me I notice that the order of execution depends on the order of the lines.
Putting last_crossing() before the cross() make the expressions inside the cross() block to get the updated value calculated in the last_crossing() statement.

This is the code that works properly:

Code:

 @(cross(V(clk)-vth,1));
    clk_time=last_crossing(V(clk)-vth,1);

   en_time=last_crossing(V(en)-vth,1);
  @(cross(V(en)-vth,1)) begin      
      dly=en_time-clk_time;
    end
  
 

Yet, I wonder where in the LRM it says about this dependency.


Thanks again,

Ari