Microprocessor Design/Power Dissipation

In addition to power and performance, another useful metric for examining processors is in terms of the amount of power used. Power is a valuable commodity, especially in mobile or embedded environments, and in server farms. Processors that utilize less power are more highly prized in these areas than processors with more capability and better performance.

The primary problem in server farms like the ones used by Google is power.

Reducing the amount of energy used, without reducing the performance of the computer system, is one of the Grand Challenges in computer science.

Gene's Law
Less well known then Moore's law is Gene's Law, named after Gene Frantz. According to Gene's law, the power dissipation in embedded DSP processors will decrease by half every 18 months.

Two reasons to reduce power
The power used by a microprocessor causes 2 problems. Some techniques reduce only peak power; some other techniques reduce only average power.

The peak power problem

 * All the power used by a microprocessor is eventually converted to heat energy. If too much heat energy is allowed to build up inside the microprocessor, the temperature will rise high enough to destroy the microprocessor.

This problem is solved by the cooling system, which replaces that problem with another problem:
 * The higher the peak power used by a microprocessor, the more expensive the up-front cost of the cooling system necessary to keep that processor from destroying itself.

The average power problem

 * The higher the average power used by a microprocessor, the higher the cost to the person who uses that microprocessor. That person must not only pay for the electric power going into the microprocessor, but also pay for cooling to pump waste heat energy all the way from the microprocessor to the outside environment.

In some situations, there are other reasons to reduce power:
 * Laptop designers want a small, lightweight laptop. The higher the average power used by a microprocessor, the heavier the battery must be for a given runtime.

Heat
In microprocessors, power is mostly dissipated as heat energy. This conversion to heat energy is a function of the size of the wires and transistors, and the operating frequency of the processor.

As transistors get smaller, the depletion region gets smaller, and current leaks through the transistor even when it is off. This leakage produces additional heat, and wastes additional power.

Heat can also cause materials to expand, which can alter the electrical characteristics of the tiny transistors and wires.

Many small microcontrollers don't need to worry about heat because they generate so little, but larger modern general purpose processors typically need to be accompanied by heat sinks and fans to help cool the processor. If a processor is running too hot, typically it can be slowed down to a lower clock rate to help prevent heat build up.

As power is a function of the square of the voltage, approximately, if you can reduce the power supply voltage by half, you can reduce the power dissipation by possibly three quarters. Because of this, microprocessor chips are quite often designed to run at what were once considered impossibly low voltages. The initial microprocessor chips, the Intel 8080 and the Motorola MC6800, were designed to run at 5.0 volts. More modern microprocessors, like the AMD Athlon K7 chips, are designed to run at 1.65 volts or even lower.

It should be noted that, in order to prevent uncontrollable heat buildup, many modern general-purpose microprocessors dynamically turn off parts of the chip. A computer that is being used for purely integer calculations does not need its floating point unit, and so power to the entire FPU, except possibly the register stack, is turned off. Major sections of the microprocessor, then, can be turned on and off several times per millisecond. While this does cut down average power draw and heat dissipation, it does put extraordinary demands on the power supply for the chip, which can see power requirements that jump 50% in microseconds.

Resources

 * Frantz, G., "Digital signal processor trends", IEEE Micro, Vol.20, Iss.6, Nov/Dec 2000, Pages:52-59