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XLP nanoWatt
Microcontrollers
&
Low Power Management
Industry Trend
 Many types of portable electronics
 Metering applications
 Medical devices
 Power consumption becomes one of the
most important concerns for designers
Power Consumption
 Dynamic
 Power used by the switching of the digital logic
 Voltage and temperature impact power usage in a
small way
 Mainly influenced by clock speed
 Static
 Power consumption when clock is disabled
 Transistor leakage currents
 Power used by voltage supervisors and other circuits
needed to resume normal operation from static mode
 Higher impact from voltage and temperature
Power Saving Modes
 Deep sleep mode
 The lowest of the static power modes
 Except a few RAM locations, the wake-up circuitry
and in some cases a low power oscillator used for
RTCC, everything is powered down
 Wake-up resets the device, and the firmware has to
check special registers to resume normal operation
state
 Used when long sleep times and very long battery life
are required
 Accurate timekeeping is possible
 No peripherals may run during deep sleep
 Typical power consumption is less than 50nA
Power Saving Modes
 Sleep mode
 Standard low power mode that predates nanoWatt
technology
 Core and most peripheral clocks are shut down
 General purpose RAM, registers and Program
Counter are preserved
 Wake-up times are very short, with little firmware
overhead
 Used when shorter sleep times and very short wakeup times are required.
 ADC (with own RC oscillator) and comparators may be used
during sleep
 Typical power consumption is between 50-100nA
Power Saving Modes
 Idle mode
 Dynamic reduction mode intended to allow for greater peripheral
functionality than the static modes
 Core clock is removed while still provided to the peripherals
 On some devices it is possible to apply the system clock only to
selected peripherals
 Idle mode consumes significantly more power than any of the
static modes
 Useful in cases in which high speed ADC, time-critical
communications or DMA transfers are needed
 It may significantly reduce power usage when the device is
waiting for data transfers, timer overflows and output compare
events
 Typical current consumption around 25% of normal run mode
Power Saving Modes
 Doze mode
 Dynamic reduction mode allowing full
peripheral and some core functionality
 System clock is applied to peripherals
 A user defined fraction of this clock is still
applied to the core
 Similar to IDLE mode, but core continues to
run at reduced speed
 Power consumption up to 75% of normal run
mode depending on application
Clock Switching
 IDLE and DOZE modes allow reduction of core power


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consumption while peripherals are still clocked at full
speed
Clock switching allows reducing the speed of clocks for
the entire device
The system clock source may be selected depending
upon the situation
Slower crystals or internal RC clocks may be used in
code sections that are not time critical
Computation intensive code or time critical sections may
conveniently switch back to a high speed clock source
Application Examples
 Methane gas / smoke sensor
 Device enters deep sleep and wakes up every
second to sample sensor data
 If data over threshold device switches to
standard sleep mode and samples sensor
data 10 times faster to confirm readings
 Alarm is raised after confirmation
 Device reverts to normal operation
Application Examples
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Normal consumption @ 4MIPS is 4mA
Current consumption in sleep mode with 32kHz watch crystal running on TIMER1 is
500nA+100nA static
Acquiring 8 ADC samples and deciding if values over threshold takes less than 500us
Duty cycle estimated to 0.05% (500us out of 1s)
Average current consumption is 4000uA*0.05%+0.6uA*99.95%=2.5997uA
Excluding leakage, device may run 22 years using a pair of 500mAh AAA batteries
Thank you!