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7903A – AVR – 2006/02

The single-most important analog module in terms of power consumption during a sleep

mode is the brown-out detector (BOD). A BOD protects the microcontroller when the

supply voltage falls below its operating threshold by resetting the device. This keeps the

microcontroller in a defined state when the Vcc is below its operating threshold. The BOD

is not important to the microcontroller while it’s in sleep mode but it is extremely important

when it wakes up. Therefore, as a rule, most microcontrollers keep the BOD active during

sleep mode and it contributes substantially to sleep mode power consumption.

There are two ways of getting around BOD power consumption in sleep: making a “zero-

power” BOD or turning the BOD off altogether.

Since the BOD must be functional when the controller wakes up, making a zero-power

BOD may seem like the most attractive option. However, lowering the power to the analog

module can make it very slow and make it respond too slowly to an out-of-range voltage

supply. Since the microcontroller is not running any code or writing or erasing the Flash or

EEPROM in sleep mode, the BOD is not really necessary. However, it does need to be

operational the moment the controller wakes up. The solution to this problem is to have the

microcontroller shut down the BOD when it enters sleep mode and start it again just

before leaving sleep mode. This approach ensures the BOD is functioning when it is

needed without any current penalty while in sleep mode.

The 32 kHz Crystal Oscillator

In many applications, the time spent in active mode is insignificant compared to the time

spent in power save mode with everything turned off except a real-time clock and the

brown-out detector. In these applications, the power consumed in Power Save mode (sum

of PWD, BOD and the 32 kHz oscillator) is the most significant contributor to overall power

consumption. Therefore, lowering the current consumption of the 32 kHz oscillator can

significantly reduce overall system power consumption. Designers should be very thorough

in evaluating the current consumed with the crystal oscillator running.

Very Low-power Oscillators versus 32 kHz Oscillators

There are, in general, two ways of timed wake from deep sleep: either by a RTC or by very

low power oscillators (VLO). The difference between them is mainly the accuracy. The

RTC allows proper timing due to the very accurate 32 kHz oscillator while a VLO is very

inaccurate and not suited for time-critical functions.

The AVR picoPower Technology

The increasing use of battery and signal line powered applications mandate low power

solutions. Atmel has spent the last decade refining its technologies and architectures to

achieve the industry’s lowest power consumption. The culmination of this effort is its new

picoPower technology which will be implemented in new ultra low-power 8-bit RISC AVR

microcontrollers to be introduced during 2006.