Smarter Design Strategies

Designing for a worst-case scenario may guarantee a device will work, but it’s also the least-efficient approach.

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By Jon McDonald
I had an interesting discussion with a customer recently. They were involved in the architectural specification of a fairly complex piece of silicon. They spent a significant amount of time designing the part to work under worst-case power characteristics and defining the power supply requirements for the device in this worst-case use mode.

The conversation started with the question: “If we already are designing everything to support worst case power characteristics, why should we care about modeling anything else?” For a system used in a steady state, this may is a reasonable point of view. The system we were discussing had numerous general-purpose processing elements and was targeted for a wide range of end-user applications.

In trying to come up with a good analogy I started thinking about my Suburban and why I would want to know anything other than the worst-case gas mileage. I have a fuel indicator that shows the instantaneous mileage. Driving uphill, or under some heavy load, I often see it drop down to 1 or 2 mph. If I had to plan all my trips and refueling stops around that worst-case mileage, I would be stopping at a gas station every 30 miles. Luckily I do a little better than the worst case in most of my driving. For planning it’s much better to think about the mileage resulting from the expected load. By watching the instantaneous mileage I get feedback that helps me improve the efficiency of my driving under different conditions and allows me to plan the next stop based on the load I’m carrying.

I believe 20 years ago if you were designing something that plugged into the wall you may have considered your power availability to be infinite. Knowing your system was designed to work under a worst-case power draw may have been perfectly acceptable. Today I think most engineers would agree that some level of power conservation should be attempted in virtually all systems.

By modeling the power consumed by my device accurately under various use cases the end user of my device can get some feedback as to how efficiently they are using the device in their application.

Getting back to the original question, if I only care that I can get my device to work, then worst-case analysis may be good enough. If I have a reasonably complex system with multiple ways in which it can be used to solve a given problem, then it becomes valuable to have a model that gives me more accurate feedback as to how efficiently various approaches to solving a problem will use the system.

—Jon McDonald is a technical marketing engineer for the design and creation business unit at Mentor Graphics.