I would like to build a behavoiral model of an rectifier in Verilog AMS. It consist of two diodes in which the anodes and cathodesare connected with each another. I built a model, simulated with simvision and failed to finished simulation,The system shows there is an illegal value when the value of cross function is 0.7. I tried different ways to rebuild it but still can't find solution yet.  I appreciate any help from you. Thanks in advance.

here is my scripts.

`inlucde "desciplines.vams"

module diode (a,c);

inout a,c;
electrical a,c;

analog begin
   @(cross((V(a,c)+I(a,c)),0.7))
    ;
    if ((V(a,c)+I(a,c))>0.7)
            I(a,c) <+ 0.7;
            else begin
                    if ((V(a,c)+I(a,c))> -0.7)
                              V(a,c) <+ 0;
                    else
                               I(a,c) <+ -0.7;
                    end
     end
endmodule

a much better diode model would use an exponential..
for convergence issues its better to use a limexp function to express the relationship between the voltage and current..
But in YOUR case of something thats basically a bidirectional diode clamp..
I would just add TWO exponentials.. with the opposite polarity voltage in each expression

I(branch) <+ Is*limexp(V(branch)/Vt);
I(branch) <+ -Is*limexp(-V(branch)/Vt);
(Vt = kT/q if my recollection serves me correctly..
and if that doesn't help you know which of your text books to go back to.. I'd recommend starting with something simple.. like Millman's Microelectronics... (the one my college used for the Junior Circuits sequence many years ago..)
BTW -- I didn't look, (and If I'm not sure, I'll always crack open the right book before finishing the model!!)
BTW the Orange Verilog-A book has examples of a diode model.. and so does the Cadence Software, in ahdlLib..
good luck..
jbd

Eugene and JBD,
   Sonis is using a tip I presented in my book on Verilog-AMS (see section 6.2 on page 71). I think the problem he is having is result of the values of the contributions not matching up with the values used in the conditions. I would expect that
1. there would be @cross statements to detect thresholds at both 0.7 and -0.7
2. the conditionals would be rewritten as
if (V(a,c) + I(a,c) > 0.7)
 V(a,c) <+ 0.7;  <-- this is different
else begin
 if (V(a,c) + I(a,c) < -0.7)  <-- this is different
   V(a,c) <+ -0.7;  <-- this is different
 else
   I(a,c) <+ 0;   <-- this is different

-Ken