One of the big buzzwords around the EDA world these days is "variation." Don't you just love buzzwords? Take a perfectly normal, slightly ambiguous word, capitalize it, add a another slightly ambiguous hyphenated suffix, and suddenly you've just solved a new problem for your customers. "Interface-driven'' "user-centric'', "platform-based" and "variation-aware."
Well, I'm not here to sling buzzwords. I'm here to help you find ways to make better use of our software so you can deal with the actual situations you have to face every day. And, of course, "variation" does exist. It exists in all sorts of forms in all parts of the design process. So today, let's talk about one of the most basic forms of variation you've been dealing with in design for years. Corners.
Specifically, I'd like to cover some of the features in Virtuoso IC6.1.5 which help you set up and run the massive number of corner combinations you have to define to verify your designs today.
First, it helps to realize that our definition of a "corner" can encompass any sort of variation you can define in ADE XL. It's not just limited to your classic PVT. You can create corners using any design variables, device parameters (to be the subject of a future article), or combination thereof. You can even create statistical corners based on sample points from Monte Carlo analysis (yet another future article).
The ability to create corners in many different ways opens the door to lots of efficient methods of circuit analysis.
But first, you've got to set them up.
If you're using IC6.1.5, you'll have noticed that we redesigned the Corners Setup UI. The basics of using the new form are covered in this video and this video. (And, of course, in the documentation.) The videos explain how to create corners, model groups and corner groups, as well as how to copy corners and enable or disable individual corners and corner groups for each simulation testbench.
Instead of repeating those topics here (you can always just watch the videos), I'll introduce some new features that have been added in recent ISR releases of IC6.1.5. Everything described here is available in ISR6 (released in Sept. 2011) or later.
Selective Corner Group Expansion
Rather than just expanding a group of corners completely, such that each column contains only one combination of values, you can now expand a corner group based on one or more selected parameters. There are 2 options for this. The first is similar to the original full corner group expansion, only you use just a selected set of parameters. Those parameters will be combinatorially expanded. The rest will remain as they were.
For a simple example, if I start with Temperature=0,100 and VDD=1.7,1.9 (a group of 4) and I select to expand based on VDD, I'll get one group with Temperature=0,100 and VDD=1.7 and a 2nd group with Temperature=0,100 and VDD=1.9.
The 2nd option is what we call a parametric set (ParamSet) expansion. For this case, each selected parameter must have the same number of values, then the corner group is expanded using the 1st value of each parameter, the 2nd value of each, and so on. Other values remained grouped as they were.
Using the same simple example, if I start with Temperature=0,100 and VDD=1.7,1.9 (a group of 4) and I select both Temperature and VDD for ParamSet expansion, I'll get one corner with Temperature=0 and VDD=1.7 and a 2nd group with Temperature=100 and VDD=1.9.
Bonus Tip: Once you've set up a lot of corners and corners groups, you can select their columns in the Corners Setup form and choose RMB->Create Corner Group. The tool will collapse the columns down, combining common variable values, into the smallest possible number of corner groups. This makes your corners setup much easier to manage.
Extra Bonus Tip: After you've gone through all this great work, you'll probably want to reuse these corner definitions for other designs. Simply use the Save icon at the top of the Corner Setup form to save them to a file. You can use the Load icon to load them in the next design, or use the cdsenv variable:
envSetVal( "adexl.gui" "defaultCorners" 'string "myDefaultCorners.sdb" )
in your .cdsinit file to have the same corners setup every time you create a new ADE XL view.
The ability to create, group and expand corners to suit different design needs makes it much easier to perform all your circuit verification and analysis.
While “shot gunning” and running a massive number of corners seems like the thing to do these days with all the computing resources, it may not since these can take a long time to run, be confusing, and take time to analyze and sort out. Not all designs are cut-n-dried to just have min-max corners and to have someone look for the “red” marked results and make sense out of it all. Monte Carlo statistics can more effective and can provide design centering and study for key specs. And, variation analyses is a step up from that. If running corners, this must be selectively done so to run the right combinations. Some corners are even mutually exclusive and may not occur, or have a low probability of occurring. So, why run them? How about understanding your design better?
On the subject of “variation” analysis which comes to mind is studying the variations through a sensitivity-like analysis such as offered by other EDA partners can be effective since runs faster and can give useful information early in the design process. For designs in advance nodes, variation analysis has been shown to be very effective.
On the subject of “aware” flows -- yes, this word is overused by all of us. I never liked this word when first saw it. I am trying to think of another description to use. I think "aware" is going the way of "novel". Novel is often not an allowed word in IEEE papers now.
Thanks for the comment. You make a lot of great points. Advanced features in Monte Carlo analysis, sensitivity analysis and worst case corners are some of the key needs of our customers right now. We have been putting a great deal of focus on these areas and, in recent ISR releases of IC 6.1.5, there are a ton of new features which you may want to investigate.
In particular, casually hiding under the "Sensitivity Analysis" run mode in ISR6 are a lot of powerful analysis and visualization options for examining the effect of variations in both device parameters and statistical variables on all aspects of circuit performance. Take it for a spin!
Look for new blog posts along these lines over the next few months...
Hi Bob, good comment, but 2 things: In many designs there are things a designer can easily overlook, and having the corner methodology is a fine technique, esp. if you include statistical corners! 2nd, I wonder what is missing in ADEGXL wrt sensistivity (I only dislike the GUI)? What can you do more with competion tools? Personally, I am doing many sweeps to see such sensitivities before doing corners - for design understanding.