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AVR® 8-Bit RISC - picoPower Sleep Mode
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Techniques for Low Power Consumption in Sleeping Modes
The best way to describe a microcontroller's power consumption is by a power budget as shown to the right. Power budget uses the duty cycle between active and sleep mode to show the average power consumption of an application. The average current consumption enables battery life time calculations as well as other power supply considerations.
A power budget will unveil whether active or sleep power consumption is the most critical for the given application.
In most low power applications, the power budget points out sleep power consumption as the most critical parameter. Thus AVR picoPower technology employs a number of techniques that result in the lowest sleep mode power consumption of any microcontroller on the market today.
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True 1.8V Supply Voltage
Power consumption is the product of voltage and current, and enabling 1,8V operation ensures low power consumption as well as allowing the battery supply to drop to 1.8V before being out of range. The minimum operating voltages for today's microcontrollers are usually 1.8V - 2.0V but many microcontrollers have limitations requiring higher voltages for some of their functions. An example is the 1.8V TI MSP430 that require 2.2V to write to its parameter memory. AVR MCUs support true 1.8V operation. All features, including analog modules, Flash, EEPROM and RAM run at 1.8V.
Minimized Leakage Current
The absolutely minimum sleep current achievable is mainly limited by a microcontroller's leakage current. Leakage current is affected by temperature, supply voltage and most important; process. Atmel has used proprietary processes that have been specifically developed for low power operation based on years of research and experience. The leakage current of picoPower AVRs is less than 100 nA.
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Taking the BOD to the Sleeping BOD
Although zero-power Brown-Out Detectors (BOD), such as that used in TI®'s MSP430, can save a lot of power, they are notoriously slow, provide odd detection levels and can require a millisecond to detect a below-threshold voltage. The slow response time could put the controller at risk. Atmel's AVR BOD detects brown-out conditions in 2 microseconds but draws about 20 uA, adding substantially to the Power Down sleep current of 100 nA.
As part of the picoPower technology, Atmel has maintained the high performance and relatively high current of the BOD and saved power by simply turning it off when it is not needed. This approach results in the lowest overall power consumption and the highest possible performance with accurate detection at 1.8V, 2.7V and 4.5V.
Since there is no need for the BOD while the MCU is in the deep sleep mode, the picoPower BOD can be turned off in Extended Standby, Standby, Power Save and Power Down modes.
The picoPower BOD-disable feature is enabled by the application using a two-step secure operation and is fully automatic. When entering sleep, the BOD is disabled after the MCU has entered the sleep mode, and enabled to verify that the power supply is sufficient before the MCU is allowed to wake from sleep. If the power supply is not sufficient, the MCU will enter a reset mode before any code execution takes place. While in sleep mode, the only critical parameters to handle are the RAM and register contents. On AVR microcontrollers these contents are valid until ~0.3V Vcc, while the AVR's Power-on Reset (POR) triggers at ~1.0V. If a power supply voltage drop should occur while in sleep mode with the BOD disabled, the SRAM and register contents will be valid until a POR occurs. A POR will enable the BOD again and set the POR flag to be read by the application firmware.
The BOD disable feature completely eliminates any power consumption penalty for the BOD during sleep mode and the MCU has full protection in active mode.
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Ultra Low Power 32 kHz Crystal Oscillator
Ensure the lowest power consumption in sleep mode is important but at some point the application needs to wake up and do its actions. Wake up from sleep is either performed on reset or interrupts, and the most common interrupt for many low power applications is a RTC interrupt. The Real Time Counter (RTC) enables the application to keep track of time and wake up on timed intervals. AVR's RTC mode is called Power-save mode (PS). The latest 32 kHz crystal oscillator design utilized in the AVR MCUs reduces the current consumption in Power Save mode to a level comparable to Power Down mode.
Competing microcontrollers offer Power Save current consumption as low as 700 nA, including the oscillator and BOD. With a supply voltage of 1.8 volts, the AVR picoPower technology achieves the industry's lowest Power Save current consumption of 650 nA with the 32 kHz oscillator running and a sleeping BOD.
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