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Some MCUs integrate a wide variety
of low-power active modes. These
modes provide the option to turn
off or reduce the speed of the core
processor, while selectively keeping
the system clock active for the on-chip
peripherals.
One frequently heard statement is
“the higher the performance of the
core, the faster the execution of the
tasks, then the sooner it can return to
sleep mode.” While this might be true
in some cases, there is a flaw to this
logic. We have to remember that the
core consumes more power than any
other module in the MCU. Additionally,
all of the tasks that require the core
must be executed sequentially (FIFO),
regardless of the speed. Therefore,
the core can’t be turned off until
the last task is completed. When a
microcontroller can perform some of
the required tasks in parallel, using
integrated peripherals that can operate
independently of the core, then it
makes the speed of the core irrelevant
while significantly reducing the overall
power usage. Core independent
peripherals are fully functional while
the MCU’s core is in sleep mode.
Designing
battery-powered
applications has become more
complex, due to their increasing
functionality. Engineers should
analyze and fully understand the
current-consumption profile of each
component in different power and
activity modes, in order to achieve
the highest battery usage efficiency.
The core independent peripheral set
found in the next generation of 8-bit
microcontrollers enable engineers to
be creative with their designs, without
sacrificing performance.
Note: PIC is a registered trademark
of Microchip Technology Incorporated
in the U.S.A. and other countries. All
other trademarks mentioned herein
are the property of their respective
companies.
corresponding to 200ns to 1us), in a
32-bit architecture, employing deep-
sleep techniques to limit leakage
currents, it becomes a matter of tens
of microseconds, often voiding any/all
benefits resulting from the subsequent
typical faster execution speed.
While we would like to do everything
in sleep mode, certain tasks must
be performed in active mode where
the MCU core consumes the highest
amount of power relative to all other
modules. This is where things can get
a bit tricky. Figure 2 is a simplified
graphic representation of the system
current consumption over time. The
area under the current-consumption
line represents the total discharge over
time, measured in Coulombs. If the
sum of all the areas under the sleep-
mode period is much greater than the
active mode, then the sleep-current
value is more critical since most of the
energy consumption takes place in a
low-power mode. Vice versa, if the
sum of the area under the active-mode
period is significantly higher, then the
sleep current value and the time spent
in sleep mode become irrelevant.
Applications
with
wireless
communication, such as Wi-Fi
®
or Bluetooth
®
LE, are particularly
challenging systems in which to reduce
power consumption. Designers of
these systems must consider how
much data is transmitted or received,
since this will directly impact the
overall current consumption. Wireless
modules can be used in “Beacon
Mode,” to wake up periodically and
search for signals; or they can go into
standby mode when not in use.
In such wireless systems the MCU
processing speed is actually irrelevant
as the application is most often I/O
bound, but the MCU wake up time
impacts significantly the application
profile as the radio circuitry power
consumption (typically 10-20 mA) is
extended and ends up dominating the
application budget.
Analog sensors require the use of the
MCU’s on-chip ADC module. Typically,
the time needed for ADC sampling
is much longer than the conversion
time. The more time spent in active
mode, the more current is consumed.
However, some MCUs have ADC
modules that allow conversions in
sleep mode, which saves power by
minimizing the time spent in active
mode.
Figure 2: A graphic representation of a microcontroller’s current
consumption over time