I recently came across this verilogA module for analog multiplexer.. but even arfter reading up manuals I have not quite understood how the argument of @cross function works.... I understand up until the 'direction' argument.. .. I know the rest of them are simulator arguments(can recognise abstol) but not how they work ... can someone throw more light on it...???

module analog_mux(vin1, vin2, vsel, vout);
input vin1, vin2, vsel;
output vout;
electrical vin1, vin2, vsel, vout ;
parameter real vth  = 2.5;

  integer selector;
  real vout_val;

  analog begin
     selector = (V(vsel) > vth) ? 1: 0;

     @ (cross(V(vsel) - vth, 1, 1.0, vsel.potential.abstol))
        selector = 1;

     @ (cross(V(vsel) - vth, -1, 1.0, vsel.potential.abstol))
        selector = 0;

     if (selector == 1)begin
        vout_val = V(vin1);
     end
     else if (selector == 0)begin
        vout_val = V(vin2);
     end

     V(vout) <+ vout_val;
  end
endmodule

Marq Kole wrote on Jul 23^rd, 2007, 6:01am:


That's not quite how I'd describe it.  The simulator goes along picking timepoints, evaluating the models, and solving the circuit.  Now, suppose a crossing occurs, such that timepoint t5 is below the threshold and timepoint t6 is above it.  By interpolation, the simulator can compute the "actual" crossing time tc (which is only accurate to within tolerances and the quality of the interpolation ...); now it compares t6 to tc and checks to see if this difference is less than the time tolerance for the cross event.  If not, then it will choose a new timepoint that is closer to tc.

(Obviously, it is trying to choose large timesteps so that the simulation finishes faster; the cross event keeps it from stepping too far between timepoints.)