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.
Chris Baumann is Director of Atmel's BiCMOS Products business unit. Before joining Atmel in 1989, he held various positions at Texas Instruments and Honeywell. His B.S. degree in Electrical Engineering and M.S.E.E. degree are both from the University of Notre Dame.
