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Issues in Modelling an INV output having 10mVp-p as its VSS

write2rammy

 Hallo All,

        I am designing a current starved INV which has an output high(VDD) and low (VSS). VDD = 1.8V, VSS = 200m+ 10mVp-p, 1GHZ. I wrote the following piece of code but it gives me an error saying "contribution statement  encountered inside an analog event"

 Assume Td_p,Td_n = 1p

     @(cross(V(Vin)-(V(V_swing)/2),+1)) begin
            V(V_ph) =  V(Vss);
    end        
    @(cross(V(Vin)-(V(V_swing)/2),-1)) begin
        V(V_ph)  =  V(Vdd);
    end        
        V(Vout) <+ transition(V(V_ph),0,Td_p,Td_n);
    end    

But the code works if I change V_ph as a real  variable. But I dont see the 1Ghz 10mVp-p sinusoid on 200m(dc) VSS. Instead it is a constant 200m low and 1.8 V high. Can someone help me with this.

  • Andrew Beckett

    You cannot have contribution statements inside an analog event, because it is then not executed all the time.

    Presumably what you want is something which alters whether it is sampling the Vdd or Vss nets, based on this input signal crossing. Something like:

    integer flag;

    @(cross(V(Vin)-(V(V_swing)/2),+1)) 
             flag=0;
    @(cross(V(Vin)-(V(V_swing)/2),-1))
            flag=1;
    if (flag==1)
        V_ph=V(Vdd);
    else
        V_ph=V(Vss);    
    V(out) <+ slew(V_ph,...);

    I'd suggest using slew rather than transition, because the V_ph variable will be continuous. In the previous code you were sampling it, but apart from when the switch is changing, the signal will be continuous here, and transition is not really designed for that (it will be a good way of slowing your simulation down).

    Regards,

    Andrew.

  • write2rammy

     Perfect.. Thank you for the explanation. It works with slew..

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