Most battery operated 802.15.4/ZigBee nodes (think sensors or switches) will be in sleep mode 99.9% of the time, waking up periodically for a few milliseconds to check a sensor or poll the other radios. Total power consumption of the node will actually approach sleep mode power consumption.
This is important because engineers and vendors tend to emphasize active power consumption. In face, sleep mode power consumption is frequently buried in the back of the data sheet or may not be in the data sheet at all. Even if it takes a call to the vendor, it is worth finding out the sleep mode power drain of both the radio and controller because it will have a substantial impact on the battery life of the end nodes.
In a system that is off most of the time, active power drain can be less important than sleep mode power consumption. To illustrate this point take the hypothetical example of an end node that wakes up once a minute to perform a task that takes 12 milliseconds with equal amounts of time spent on transmission and reception. The rest of the time the node is asleep.
The total power consumed by just the controller and radio is 0.0062 mA, with sleep mode power representing over one-third of the total. This is why close attention should be paid to sleep mode power as well as active power.
Estimating actual system battery life is virtually impossible because one must factor in all the external components in the reference design as well as the sensors or indicators.
However, in a real-life temperature sensor node application, the microcontroller has active current of 8 mA and sleep current of 1.5 uA with watchdog timer on, while the radio has transmit and receive currents of 17 mA and 15 mA and sleep current of 0.7 uA.
The actual power consumed by this application for wakeup, sense, ADC conversion, transmit data, receive acknowledgement, and transition back to sleep mode is 0.0011 mAh, including external components and sensors for each transmission cycle.
At a rate of one transmission per minute, this node would consume 0.0706 mA per hour of operation. At this rate, two AA 2700mah lithium-ion batteries last about 5.2 years. Increasing the sleep mode current on both the microcontroller and the radio by 1uA each reduces the battery life to 4.8 years " about 10% less.
In addition to paying close attention to sleep mode power consumption of both the radio and the controller, engineers should also check the controller data sheet to verify true 1.8-volt operating range when designing battery-powered nodes.
Some microcontrollers that claim 1.8V operation in their marketing literature actually require more than 2.0V to operate properly. The 0.2-volt difference can cut practical battery life by up to 30%. This information may be buried in a footnote, so it is a good idea to contact the vendor directly to verify the supply voltage.
