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Saving Power with Temperature Compensation

Greg Ehmann, Nov. 01, 2017 – There have been many blogs and articles written on power management utilizing dynamic voltage and frequency scaling (DVFS), a method by which a discrete voltage and frequency pair is chosen from a predetermined list based on an input requirement. For an example, read Don Dingee's blog entitled "DVFS is Dead, Long Live Holistic DVFS." Choosing this input requirement is where it all starts.

The most common input requirement is the required performance. Where do we find out the required performance? Well, the OS surely knows what tasks are running so it can estimate throughput. This throughput directly relates to the frequency and power state of one or more CPU cores. DVFS lets the OS reduce the frequency to match the desired throughput, while reducing voltage to the minimum level that supports safe operation at that frequency. But, how do we choose that voltage?

Energy saving through temperature compensation is an additional method that can be applied by which the voltage to a circuit may be lowered based on the actual operating temperature of the chip. Most designs today do not take advantage of this parameter and simply assume the worst case temperature when choosing the operating voltage. While this is the simplest method, it results in higher than required voltages and thus, consumes more energy. Since most designs rarely operate at the maximum temperature, temperature is another input requirement we can exploit in DVFS for additional energy savings.

Positive Feedback Loop

The concept of temperature compensation is simple and leads us into a welcome positive feedback loop where if the temperature of the device drops at some point, we can lower the operating voltage, meaning we consume less power. That's a good start. But, less power consumed also means less heat is generated and less generated heat frequently produces an even lower operating temperature, closing a positive feedback loop. This further reduction in temperature can then lead to a further reduction in the voltage, until the die reaches equilibrium.

This scenario may occur, for example, if I switch from watching a movie on my phone to just listening to music. The heat from the movie phase does not dissipate immediately and therefore, when the music phase starts, the voltage will have to be higher than the minimum because of the temperature. But, as the reduction in heat generation during the music phase lets the temperature fall, the voltage can be reduced.

On the flip side, as the temperature begins to rise, the voltage may need to increase to compensate for the higher temperature. This may cause more than a single change in voltage when the performance is high, but eventually the device will stabilize somewhere at or below the maximum voltage. There is no possibility of a run away loop as long as the device was originally designed based upon meeting the highest throughput at the maximum allowed temperature.

This scenario can be triggered by moving my phone from inside a cool building to outside in the warm sun while doing the same activity or by switching activities to one that requires higher throughput.

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