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7903A – AVR – 2006/02

Introduction to Power Consumption of MCUs

Why is Low Power Important?

In the same way that fuel consumption is important in everything from scooters to oil

tankers, power consumption is a key parameter in most electronics applications. The most

obvious applications for which power consumption is critical are battery-powered

applications, such as home thermostats and security systems, in which the battery must

last for years. Low power also leads to smaller power supplies, less expensive batteries,

and enables products to be powered by signal lines (such as fire alarm wires) lowering the

cost of the end-product. As a result, low power consumption has become a key parameter

of microcontroller designs.

Low Power Parameters

Although power consumption is the product of operating voltage (Vcc) multiplied by the

current consumption (Icc), current consumption is usually the only measure considered

when describing the power characteristics of a chip. This is a mistake because decreasing

the operating voltage directly reduces the current consumption and the overall power drain.

Current consumption increases directly with the system clock frequency so keeping the

system clock as low as possible is critical to keeping power consumption down. The clock

frequency is affected by a number of factors that include the microcontroller’s surroundings

and peripheral set as well as the architecture and the instruction set. RISC microcontrollers

typically execute in a single clock cycle but some architectures divide the clock down in the

same way as CISC architectures do before feeding it to the CPU. This situation leads to

confusion about what clock frequency is really required to execute the target application.

Designers should pay close attention to the Instruction set architecture when reading the

current consumption numbers in a microcontroller’s datasheet.

Most datasheets provide power consumption numbers for the microcontroller with no

peripherals running. The additional current drawn by the peripherals must be taken into

account since all MCUs have peripherals and their contribution to power drain can be

significant.

Temperature is another factor. Since higher temperatures lead to higher power

consumption, designers should always consider the power numbers for worst-case

temperatures.

Active versus Sleep

In many applications, the processor does not run continuously and peripherals may be idle

much of the time. The overall power consumption can be lowered by taking advantage of

various “sleep” modes available on virtually all processors. The most common sleep

modes are Power Down” (PWD), Power Save (PS) and Idle.

In Power Down mode everything is shut down, including the clock source. In Power Save

mode everything is turned off except a 32 kHz clock running from a crystal to keep track of