Hi.
I admit it, I'm new to VerilogA. I need a resistor in which the resistance changes depending on the voltage.

So a simple resistor is:

module res(p,n);
inout p,n;
electrical p,n;
parameter real r=0 from [0:inf);
analog
   V(p,n) <+ r*I(p,n);
endmodule

I have a set of voltage threshold values and resistances that are defined at the top of my code (`define etc), basically a set of linear approximations to an exponential increase in resistance with voltage. So I have:

if (V(p,n) < VD2)
Resistance = RA
else if etc, where RA and VD2 are both defined in the file above.

Can anyone tell me why I get a syntax error in the if statement, already looked up if statements in the VerilogA Reference Manual. Is it that the values VD2 etc are not actually physically relevant (i.e. not actual voltages etc)

The error I get is:
  line 26: "if <<--? (V(p,n) <= VD2)"
  line 26: Error: syntax error

How would you change the resistance of a resistor with voltage, temperature etc.
Thanks.
Ed Fisher

Ok so I worked through the maths for a voltage dependent resistor. That should work quite well but in this case all I need to do is conditionally change the resistance of the linear resistor model depending on where in the SPAD (single photon avalanche diode) reverse breakdown region it is operating.

So now back to a syntax problem, ironically I'm getting errors when some of the syntax is exactly the same as the Varactor Verilog-A model that is referenced above.

Here is my model;
`include "disciplines.vams"
module Rspad(anode, cathode);

inout anode, cathode;
electrical anode, cathode;      

parameter real R0 = 1k from (0:inf)      //zero bias Rspad resistance
real v, r;                        //define variables used for calculation

analog begin
v = V(anode, cathode);                  //get voltage over component
r = R0;                                        //not used for calculation
                                          //used for logical decision only

if (v <= `V2)                              //conditional assignment of r
 r = `RA;                                //voltage dependant

else if (v <= `V3)
 r = `RB;

else if (v <= `V4)
 r = `RC;

else if (v <= `V5)
 r = `RD;

else
 r = `RE;

V(anode, cathode) = r*I(anode, cathode);      //calculate voltage

end                              
end module                  

So I get a number of syntax errors, the first is:
line 67: "real <<--? v, r;"
line 67: Error: syntax error
line 67: Error: illegal declaration
line 67: Error: parameter "r" requires initializer
line 70: "v <<--? = V(anode, cathode);"
line 70: Error: undeclared symbol: v.

Any help would be much appreciated. Also is y=`x the correct way if x is defined using `define x 2;

Thanks very much.
Ed Fisher