/* model for an n-channel multiplexer
   each channel is simple modelled by a resistor between
   input and output

   in[0]----R------|
   in[1]----R------|---- out
   ....
   the value of the channel resistance is controlled by
   the bit pattern at the input, the selected channel
   is ron, all other are roff
   if a new channel is selected, the old on get switched
   of in ttransit_off, the new one is switched on in
   ttransit_on
*/

module multiplexer(in,ctrl,out);
 input [0:pow(2,size)-1] in;
 input [0:size-1] ctrl;                                 // LSB : MSB
 output out;
 electrical in,ctrl,out;

 parameter integer size=2 from [1:inf);
 parameter real vth = 1.4,                      // threshold for 0/1 on ctrl bi
ts
                ron = 1 from (0:inf),           // channel on resistance
                roff = 1M from (ron:inf),       // channel off resistance
                ttransit_off = 5n from [0:inf),
                                                // transit time from ron -> rof
f
                ttransit_on  = ttransit_off from [ttransit_off:inf);
                                                // transit time form roff -> ro
n

 integer rch [0:pow(2,size)-1];
 integer pow2 [0:size];
 integer channel;
 real    iout;

 analog
 begin
   @(initial_step)
   begin
     channel = 0;                                       // initial value for ch
annel
     for (i=0;i<size;i=i+1)
     begin
       pow2[i] = pow(2,i);                              // temporary variables
       rch[i]  = roff;                                  // set all channel to r
off
     end
     pow2[size] = pow(2,size);
     rch[0] = ron;                                      // initial channel 0 is
 on
   end

   generate i (0,size-1)
     @(cross(V(ctrl[i]) - vth))
     begin
       rch[channel] = roff;                             // switch of the old ch
annel
       channel = 0;
       for (i=0;i<size;i=i+1)                           // calculate new channe
l
         if (V(ctrl[i]) > vth) channel = channel + pow2[i];
       rch[channel] = ron;                              // switch on the new ch
annel
     end

   iout = 0;
   for (i=0;i<pow2[size];i=i+1)
     iout = iout + V(in[i],out) / transition(rch[i],0,ttransit_off,ttransit_on)
;

   I(out) <+ iout;
 end
endmodule