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readforce does not work in DC simulation

Qilong

Hi all, 

    Recently I am trying to play with the readforce in DC simulation. My test circuit is a series RC network (R=1 Ohm, C=1pf both from analogLib), with the two terminals connected to gnd. No sources are used. Then I specified the voltage value on the floating plate of the capacitor in a file (with only one line in it: net_flaoting 1). Then I use readforce to read this file in DC simulation, with any other parameters like force, readns empty.  

    From the descriptions in the spectre simulator user guide and the reference, I am expecting a voltage of 0.5V on the floating plate of the cap after the DC run (I used rforce=1 Ohm). However only 0V is shown. On the other hand, if I specified initial state in the cap and use force=dev, the desired 0.5V will be calculated. Comparing this two cases, could I ask why the simulator does not use the statement I wrote in the file? 

    PS: As a crosscheck, I used the same file in a transient simulation with the same network by setting readic=filename.  Then I can see the initial state is calculated quite well. The floating node starts to settle down from the initial voltage of 0.5V (again rforece=1 Ohm), which seems the file is recognisable by spectre.   

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  • Qilong,

    The icpriority parameter is controlling the order of the readns/readic parameter and the "nodeset"/"ic" statements in the netlist (see "spectre -h nodeset" and "spectre -h ic". It is not about the ic parameter on capacitors or inductors. As far as I know, the initial conditions read from a file with readic will always override the initial conditions specified on the instance directly - no contradiction here at all. I don't understand what you mean by "Furthermore, is there any priority table that can used to check the order of use between ic, skipdc, readic and readns?" - maybe I've explained it though? Essentially a nodeset is a help at the beginning of DC convergence - the forces are applied, DC convergence is reached, and then the forces removed and it then attempts to continue with DC convergence. Initial conditions are held for the entire duration of the DC convergence (so they are a stronger force). The skipdc parameter controls whether a DC solution is found, or whether various advanced methods are used instead (such as waveless, rampup, sigrampup and so on).

    Regards,

    Andrew.

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  • Qilong,

    The icpriority parameter is controlling the order of the readns/readic parameter and the "nodeset"/"ic" statements in the netlist (see "spectre -h nodeset" and "spectre -h ic". It is not about the ic parameter on capacitors or inductors. As far as I know, the initial conditions read from a file with readic will always override the initial conditions specified on the instance directly - no contradiction here at all. I don't understand what you mean by "Furthermore, is there any priority table that can used to check the order of use between ic, skipdc, readic and readns?" - maybe I've explained it though? Essentially a nodeset is a help at the beginning of DC convergence - the forces are applied, DC convergence is reached, and then the forces removed and it then attempts to continue with DC convergence. Initial conditions are held for the entire duration of the DC convergence (so they are a stronger force). The skipdc parameter controls whether a DC solution is found, or whether various advanced methods are used instead (such as waveless, rampup, sigrampup and so on).

    Regards,

    Andrew.

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