SRAM simulation in LT Spice

Discussion in 'Programmer's Corner' started by anju_john, Mar 17, 2016.

  1. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Hi ,
    I am simulating the read and write operations of a 6T SRAM cell using LTSpice. But, i am not getting a proper output. The simulation results shows that the output (stored bits) changes with bitline even if the word line is low. Why it is so?

    The transistor (nmos ) output depends on the gate voltage right? Then why it changes with bitline?

    these are the results i got...

    Capture.JPG nmos.JPG sram.JPG sram_result.JPG

    Please help me.
    Thank you
     
  2. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I think you need to use a "real" FET and not the generic one. You can do this by right clicking the FET and picking one from the standard library.
    If this doesn't work post your .esc files.
     
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  3. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Thank you for the reply. I tried with real FET. But still the same problem. i have attached the file
     
  4. Bordodynov

    Active Member

    May 20, 2015
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    See
    Cell2.png
     
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  5. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Thank you so much....

    Could you please explain me the parameter .param Rleak = 1T
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Apparently it overrides the default value of the parasitic resistance that is parallel to the capacitor, so that the capacitor has lower leakage than it would with default value.
     
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  7. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Thank you..
    Could you please tell me how to find the static noise margin of a sram cell in ltspice. i tried with the attached .sp file. but it is showing the errors BITCAP is not defined and U is not a source or temp. How can i resolve these errors.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Not sure about that, but you might try typing 0.000000000001 instead of 1e-12.
     
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  9. anju_john

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    Mar 17, 2016
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    Thank you
    I tried that... still not working.
     
  10. Bordodynov

    Active Member

    May 20, 2015
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    Learn the syntax LTspice. Your file claims the Hspice syntax.
     
  11. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Sure, i will try to learn it. Actually i tried to modify my schematic file for the snm analysis. Is it possible to do in this way, since i am not much familiar with the software. Also, how to give those equations to the sources in a schematic.
     
  12. Bordodynov

    Active Member

    May 20, 2015
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    I fixed all your syntax errors. I do not understand your goals and what you do, and because nothing else did.
     
  13. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    Thank you so much. Actually my goal is to analyse the static noise margin of a 6T sram cell. For that i need to plot the butterfly curves and to measure the highest possible square inside the loops. When i referred some research papers, i saw that we need to mirror the inverted characteristics to get butterfly curves. i tried with dc sweep including two noise sources. but it was like nested sweep and i got some wrong results. so i modified the schematic based on some spice codes. here also i am getting only one proper curve. Since i am not much familiar and beginner in ltspice, i dont know the other possibilities. This is the actual problem. how can i mirror the inverter characteristics in order to measure snm.

    I need to get a graph as attached here
     
    • snm.png
      snm.png
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  14. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    the two curves you showed don´t look like any kind of mirror image. Shoouldnt you just ramp the x axix back and forth and graph y axis?
     
  15. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    I was also a little confused with that. i have attached the reference. could you please check it ?
     
  16. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Now I see that you should mirror it along the x=y axis.
     
  17. anju_john

    Thread Starter New Member

    Mar 17, 2016
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    How can we mirror x=y in LTspice
     
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