When measuring the frequency of a clock, I got the error information, " the internal timestep is too small".

My code  is list in the following:

         @(cross(V(clk)-vth, +1))begin
               current_time = $realtime;
                       if (last_time > 0.0)
               period = current_time - last_time;
               freq = 1/period;
                       last_time = current_time;
         end
         
         V(pout) <+ period;
         V(fout) <+ freq;


I think the problem comes from the statement, V(fout) <+ freq; .
Why it has no problem when using "V(pout) <+ period;",  while it has the problem when using the statement "V(fout) <+ freq;"?

How to solve this?

Thanks !

Stefan wrote on Jun 18^th, 2008, 3:18am:


Thanks, Stefan.

Then how to normalize the frequency output?
I'm puzzled by this problem.  the "V(pout) <+ period" works ok, while the "V(fout) <+ freq" does not work. I think they have the same quantity level. Why one works, but the other does not work?

sheldon wrote on Jun 19^th, 2008, 6:34pm:


Sheldon,

My application is to construct a pll model.

I had used the method you mentioned ever before. But the problem is still there.

The period has a quantity level 1E-9,  it works ok. While the frequency has a quantity level 1E9, it does not work.

So there may be other reasons that cause this prolbem.

Ken Kundert wrote on Jun 19^th, 2008, 10:54pm:


My supply voltage is 1.2V.
V(fout) is 0.279044V.
V(pout) is 3.5837V ( after x1E9 in the module), but this result is ritht.

MY code:

`include "disciplines.vams"
`include "constants.vams"

module period_freq_meas (clk, pout, fout);

           input clk;
           output pout, fout;
           electrical clk, pout, fout;
           
           parameter real vhi = 1.2 ;
           parameter real vlo = 0 ;
           parameter real vth = (vhi+vlo)/2 ;
           
   real last_time, current_time, period, px, freq;

   analog begin
         @(initial_step)begin
                       last_time = 0.0;
                       period = 0.0;
         end
         
         @(cross(V(clk)-vth, +1))begin
               current_time = $realtime;
                       if (last_time > 0.0)
               period = current_time - last_time;
               px = period*1E9;
               freq = 1/px;
                       last_time = current_time;
         end
         
         V(pout) <+ period*1E9;
         V(fout) <+ freq;
               
                 @(final_step) begin
                       $strobe("freq = %g", 1/period) ;
                       $strobe("period = %g", period);
                       $strobe("fx = %g", 1/px);
                       $strobe("px = %g", px);
                 end
   end
endmodule

And the stimulus:

X0 clk pout fout period_freq_meas
Vin      clk      0      SFFM(0.6 0.6 2E8 5 2e7)

Please help to check it.

Thanks!~