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problem with nodeset and ic Ringoscillator design

frasheed

Hello, 

I am simulating ring oscillator to match current and frequency values from simulation run by some other ppl. Those guys used SPF spice file and I have extracted DSPF file. After running simulation I am getting slight deviation in values, in order to tackle it , I need to see the response of active devices so I comment out all parasitic capacitors and set very low resistance to parasitic resistors that it will behave as a short. The simualtion run time was 50ns and before modifying R's and C's it was taking 24 min on average, but Now in 2 days it just passed through 3ns . When I did i set the nodeset values to certain nodes, but these nodes are not added in netlist and virtuoso gives me this error :

Ensure that the specified schematic names are valid.

/I0/I3/I0/net8 /I0/I3/I0/net31

These nodes are on the first level going down to the hierarchy (descend ). If I select any node from top level then its added to the netlist. After this I creted spectre .ic file and add these nodes there with their values, and then added this file as initial condition, even then the simulation is very very slow.

Any solution or suggestion that can help me to analyse the results only from the extracted transistors in my DSPF file.

Thanks

  • Dear frasheed,

    > I need to see the response of active devices so I comment out all parasitic capacitors and set very low resistance

    > to parasitic resistors that it will behave as a short. The simualtion run time was 50ns and before modifying R's and C's

    > it was taking 24 min on average, but Now in 2 days it just passed through 3ns .

    I am not sure, but I am thinking that when you set the resistors to very low values, you have created some very small time constants that are resulting in very small time steps. Have you examined the size of the time steps in your simulation?

    I have been faced with a very similar oscillator correlation issue. This is how I approached it:

    For the parasitic resistors, I add a scale factor as a parameter. Hence, if a parasitic resistor had a value of 10.0 ohms, I set it to the value of rscale*10.0. The value of rscale can be set to a number between 0.0 and 1.0. I then ran a series of simulations where I reduced the value of rscale from 1.0 to a low number. In this fashion, I could observe how the oscillator frequency and parameters varied as the parasitic resistances were reduced. This method also prevents the netlist from having too many small resistors.

    There is also a spectre parameter you can set to force spectre to ignore time constants below a specific value using the parasistics reduction option.

    I hope this helps!

    Shawn

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