Hi there,

Is it possible to write a pwl voltage source model in veriloga? I want to use a pwl voltage source with an enable signal, whenever the enable is high, the pwl source starts to work. The problem is, I don't know how to define the number of pairs for the model for different simulations.

Thanks,

... and here's one if you prefer string input for the pwl (e.g., pwl="0.0, 0.0, 0.5, 1.1, 1.0, -1.1,..."). It does use the non-standard string .len() operator from System-Verilog, and so may not be suitable for your application.

Code:

module strpwl(p, n);
   electrical p, n;
   inout p, n;

   parameter string pwl = "";
   parameter        dcval = 0.0;
  

   analog function real scanval;
      input s;
      inout pos;
      string s;
      integer pos;
      integer ipos;
      string  valstr;
            
      begin
         ipos = pos;
         while(pos < s.len() && s.substr(pos, pos) != ",")
           pos = pos+1;

         valstr = s.substr(ipos, pos-1);
         if(1 == $sscanf(valstr, "%g", scanval))
           pos = pos+1;
         else
           pos = -1;  // Error indicator
      end
   endfunction

   integer pos = 0;
   real    tnext, vnext;
   real    tprev = 0.0;
   real    vprev = dcval;
   real    slope;
  
   integer err;
  
   analog initial begin : blk
      real retVal;

      tprev = 0.0;
      vprev = dcval;
      
      tnext = scanval(pwl, pos);
      vnext = scanval(pwl, pos);
      if (tnext-tprev != 0.0)
          slope = (vnext - vprev)/(tnext - tprev);
      $strobe(tnext, vnext);
   end

   analog begin
      @(timer(tnext)) begin
         tprev = tnext;
         vprev = vnext;
        
         tnext = scanval(pwl, pos);
         vnext = scanval(pwl, pos);

         if(pos > 0)
           begin
              if(tnext - tprev <= 0) begin
                 err = 1;
                 $debug("Non monotonic time sequence.");
                 $finish;
              end
              if(!err)
                slope = (vnext - vprev)/(tnext - tprev);
           end
      end

      if(!err)
        V(p, n) <+ vprev + slope*($abstime - tprev);

   end

endmodule
 

This model works in spectre. However, if I run it in ams, I got the ncvlog error message complaining phrase:
   parameter real t[1:N] = {0,1};
It says "In Affirma-AMS, parameter array declaration is not supported"

How can I solve this problem?

Thanks,




Boris Troyanovsky wrote on Apr 9^th, 2010, 10:32am:

Another approach is to create an array variable and assign it a value in an initial step:

Code:

`include "disciplines.vams"

module test (a, b);

  parameter integer npairs = 4;

  inout a, b;
  electrical a, b;

  real times[0:npairs-1], values[0:npairs-1];
  integer i;

  analog begin

    @(initial_step) begin
      times[0] = 0;    values[0] = 0;
      times[1] = 1n;   values[1] = 0.25;
      times[2] = 2n;   values[2] = 0.5;
      times[3] = 5n;   values[3] = 0.75;
    end

    if (times[i+1] < $abstime) begin
      i = i + 1;
      $discontinuity(1);
    end
    V(a, b) <+ (values[i+1] - values[i]) * ($abstime - times[i])/(times[i+1] - times[i]) + values[i];

  end
  
endmodule // test 

But first try to use Patrick's suggestion: it is much simpler and more flexible.

Cheers,
Marq