Regardless of the network top-logy, ZigBee/802.15.4 networks have three basic types of nodes: controller nodes, full function (FFD) nodes, and reduced function (RFD) nodes, each of which minimally has a radio, a microcontroller, 802.15.4 MAC and a network layer. Different types of nodes have different processing, memory and power consumption requirements.
Controller and full function nodes, such as those in gateway servers or electrical equipment, are usually hard wired to a power source, so power consumption, per se , is not an issue. Reduced function nodes, on the other hand, are connected to sensors and switches and are usually battery powered.
Battery-powered nodes should have a very long battery life. If possible the battery should outlast the industrial control equipment itself because many of these battery powered end-nodes will be in hard to reach places that will make replacing their batteries very difficult and expensive.
The ZigBee standard actually mandates a 2-year battery life for battery-powered nodes. However, where battery life is concerned, longer is always better. Imagine how annoying (not to mention expensive) it would be to replace all the sensors and servo controllers embedded in a flow control system.
The importance of Sleep Mode Power Consumption
Several factors affect power consumption, including the supply voltages of the radio and microcontroller, the active current drawn by the radio and microcontroller, the clock frequency at which the controller operates, the number of external components required in the system (particularly power amplifiers), and the code size, in as much as it affects the MCU clock frequency.
Radios and controllers are available with supply voltages that range from 1.8 to 3.6 volts. Lower is better, but be sure to verify the supply voltage specifications. Some vendors claim 1.8 volt operation, but actually require 2.2 volts to write to flash memory.
Atmel offers the only radios and controllers with “true” 1.8 volt operation. This ability to operate at 1.8 V actually allows the battery to last about 20% longer than it would with a 2 V supply voltage.
Radios draw different amounts of current during transmit and receive. ZigBee radios are available with transmit current consumption of 17 mA to 38 mA, and receive current consumption of 15 mA to 36 mA. Again, lower is obviously better.
However, in the case of radios, transmit power and receive sensitivity have an indirect effect on current consumption. If the radio's transmit power or receive sensitivity is too weak, power-hungry power amplifiers must be added to the design to beef up the signal. These external components can add up to 50 mA to node power consumption.
When designing battery-powered nodes, the rule of thumb should be to use a radio with the highest transmit power and the highest receive sensitivity to minimize or eliminate power-hungry power amplifiers. The controller should be supported with a MAC that executes in the minimum number of clock cycles. Both radio and controller should support multiple supply voltages with the low-end being no higher than 1.8-volts, with true 1.8-volt operation.
However, the most important factor affecting end-node power consumption and battery life is the sleep mode power consumption.
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.
There's a significant need for accurate counters and brownout detection. Brownout detectors (BODs) are frequently used to monitor the supply voltage and shut the system down before it falls below a certain threshold. If a controller operates below its minimum voltage it may execute code incorrectly, causing code runaway and the corruption of non-volatile memory — an event that can damage the application so it never works again.
While BODs protect the controller from code runaway, they also make a substantial contribution to sleep mode power consumption. An accurate BOD may consume as much as 20 uA. A common solution to this problem is to use a “zero-power@ brownout detector that consumes as little as a few nanoamps.
Although this approach keeps power consumption to a minimum, too little current may compromise the BOD's accuracy and speed so that the system fails before the brownout is noticed or a reset executed.
A BOD requires at least 20 uA to achieve sufficient accuracy and speed to protect the system. One way to achieve this accuracy without increasing sleep mode power drain is to turn the BOD off as soon as the controller goes to sleep and wake the BOD up before the controller is allowed to execute any code. This approach provides better brownout protection without compromising sleep mode power consumption.
ZigBee and 802.15.4 end nodes frequently need to keep track of the time, waking up periodically to poll a sensor or check in with a controller node. In these systems, an accurate real time clock must be running in both active and sleep modes while consuming a minimal amount of power.
Either a real-time crystal oscillator or a very low power oscillator (VLO) can be used to effect timed wake from deep sleep mode. If timing accuracy in not important, a VLO can be a good choice. However, if there is even a hint that the timing must be accurate, verify that the microcontroller has a very accurate 32 kHz oscillator.
ZigBee/802.15.4 end-nodes should have a battery life of at least several years. This can be achieved by paying close attention to the supply voltage and to sleep mode power consumption.
Using a radio with high receiver sensitivity and transmit power eliminates the need for power amplifiers and other external components, further extending battery life. By using a controller with the ability to put the brown-out detector in sleep mode, the system will be protected, without unnecessarily draining the battery.