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modgen: best way to change device m-factor?

analogfan

Hello,

The following question does not have a clear answer for me after reading some of the Cadence documentation on modgen. I have been using modgen only recently and find it very useful, but wondered about the following case. I believe you have to be using IC 6.1.7 to even see modgen:

Q: I have created a modgen in Schematic-XL and placed the devices how I want to in the Modgen layout editor through Module Generator. After saving and closing all of this, I later realize I want to change an NFET from m=2 to m=4. What is the best way to make this change?

Main problem I see is not having a simple way in Schematic-XL to update in the constraint manager the specific device I changed, so that the two additional devices (now m=4) show up within the modgen group. For example, if constraint manager showed Modgen(12) because I had 12 devices grouped, it should become Modgen(14) after I increase the m=2 to m=4. The "update constraints from layout/schematic" options do not seem to do anything.

What I've had to do is delete the original m=2 devices from the modgen constraint group, then add the m=4 devices back into the modgen group. But then sometimes when I open Modgen Layout Editor it resets the positions of all the devices I had placed before the change. I've tried making the schematic change only after I've had the Modgen Layout Editor open as well, but it doesn't work as smoothly either. It also isn't clear to me in which cases I use the "update all schematic constraints from layout" or "update all layout constraints from schematic" commands.

Does anyone have any suggestions on how to best modify an instance's m-factor after having already performed the layout placements in a modgen? And so placement doesn't reset?

Thanks,

af

  • Dimitra Papazoglou

    Hi af,

     Modgens are available in IC6.1.7 & earlier IC versions - It’s been a long long time since Modgens have been introduced.
    After changing your nfet from m=2 to m=4 you can double-click on the Modgen(12) in Constraint Manager. The Modgen Editor pops up and this time the modgen should be updated with the correct number of devices (14) trying to keep the pattern you set when devices were 12. Once you exit the Modgen Editor (having saved the updates- the ‘Exit’ button saves the updates automatically) the corresponding Modgen constraint should get updated in Constraint Manager assistant as well - Modgen(16) should be displayed now. 
    Do you see the same behavior? 
    As you said the placement of the devices is not always consistent in Modgen Editor. If you still see this behavior then please contact customer support.
    The “Updates all schematic constraints from layout” option should be used when the user modifies the constraint in the layout side and would like to update the corresponding schematic constraint according to the layout constraint changes. "Updates all schematic constraints from layout” works with the same logic.
    Regards,
    Dimitra
  • analogfan
    in reply to Dimitra Papazoglou

    Dimitra,

    Thanks for the instructions, but I see that changing the m factor resets the modgen placement of my devices. For my modgen(12), I initially had 2 rows x 6 columns. I changed the one device from m=4 to m=6, and it resets all 14 of the devices into one row. I also tried changing it m=4 to m=2, and that did same thing. I changed it back to m=4, and they were in the 2x6 grid again. Between each try I had exited the modgen without saving.

    I can contact customer support as you suggest. The behavior I would expect is that if I change m=4 to m=2, it would maintain my 2x6 grid but with 2 empty spots. Changing m=4 to m=6 should maintain 2x6 grid but put the 2 extras on side or something similar.

    Thanks,

    af

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