Quick simulation for large designs using Ultrasim

If you haven't read the CAD tool information page, READ THAT FIRST.

Ultrasim is a fast event-driven circuit simulation engine.  It differs from Spectre in that the circuit is partitioned into a set of stages, usually defined by transistor connected together by their sources and drains (referred to as channel-connected components, CCCs, or channel-connected regions, CCRs).  Only when the inputs of the CCC change is the circuit simulation engine envoked on that CCC.  In addition, Ultrasim uses simplified device models to speed analysis. So you are trading off Spectre's accuracy with Ultrasim's speed.

Ultrasim work very well on digital designs.  For mixed-signal designs, you must use care in how the design is partitioned and the accuracy options you use to get good results on the analog pieces.  This will be true for SRAM sense amps, for example, or any PLL or DLL designs.

You will use Ultrasim for functional verification; that is, to run large number of patterns through your design to make sure that your design is logically correct.  You may also use Ultrasim for timing verification, although it is always a good idea, given the approximate nature of the device models, to simulate at least a few patterns in Spectre.  You will find, however, that Ultrasim is dramatically faster than Spectre on large designs.

• First, open a test schematic where you have all of your input voltages defined (ex: not your inverter schematic, but rather your inverter_test schematic) and launch 'ADE L'.
   
• Setup -> Simulator/Directory/Host.   Choose UltraSim as your Simulator.
   
• Session -> Load State.   Here, load a state with your three model libraries already defined. You will get the message 'you cannot load states for a different simulator'. Ignore it.
   
• Setup -> Model Libraries.   Check that you have all three model libraries defined. If you don't, copy-paste again from the Spectre tutorial page.
  Here's what's tricky. After you switch Simulators (Ultrasim <-> Spectre) your model libraries will be reset.
  So whenever you open ADE L,   DEFINE YOUR SIMULATOR FIRST,   then modify your remaining settings. It will not help to have all of your settings ready then switch simulators.
   
• Note that you can alternate between simulating your schematic and your extraction, just as you did with Spectre. (Adding 'calibre' before 'schematic' in Setup -> Environment) Check this before you simulate so that you are aware of what you are simulating.
  
  
  
   
• Now it's time to play with the various degrees of tradeoff between accuracy and speed. Go to Simulations -> Options -> Analog. 'Simulation Mode' defines the level of complexity - Digital Extended is the fastest, and Spice is the slowest but most accurate. Mixed Signal may be suitable for timing delays, for example, and Digital Fast may be suitable for figuring out the logical functionality of a huge design, but of course these terms are relative. Try running faster modes to get a sense of which one you need.
   
• You can leave 'Speed Option' and 'Analog Option' as Default.
   
• Moving onto the next tab 'Algorithm', click on DC Method. You can skip DC for very large designs.
   
• The tab 'Component' has an option called MOS Method. Again, the four options are fastest on the top of the menu and gets slower as you move down. This defines the accuracy of your MOS device model. You can leave it at Analog/MS Table.
   

• Analyses -> Choose.   You will see that only transient analysis and 'envlp' are available. Choose trans and define a suitable stop time.
   
• You can automatically have your outputs plotted by doing   'Outputs -> To be plotted -> Select On Schematic'. Click as many nets as needed, then press esc.
   
• Netlist and Run.

If you get an error saying 'T0, T1, ...  are not defined', you will most likely find that your three model libraries were not properly added.

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