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There are not many people who
doubt that one of the next big growth
areas in the electronics industry will
be wearables. Some predictions
claim that the market for wearables
could increase to $10bn by 2020.
Reaching that target will depend on
the user experience of the wearable,
and one of the main attributes that
users use to judge portable products
is battery life. Squeezing every
last drop of available life from the
batteries is vital for designers of
wearable devices. Users are looking
for batter life measured in months,
rather than hours.
To meet these exacting targets
requires the designer to look into
every aspect of the design, from
the start up time of clocks, to the
MOSFET switching times. This article
will look at some of the methods
that designers can use to save every
joule of energy possible. Some
advice will look at larger savings,
and some on small. After all multiple
small savings, soon add up to an
overall larger one.
Sleep Mode
Sleep mode is a natural first step for
any designer looking for a low power
usage design. This will be especially
important for wearable designs
as they usually won’t be powered
totally down. Most wearables will
take periodic sensor readings
and either store the reading till it
can be sent, or send the reading
immediately. It makes sense to put
the design into sleep mode between
active periods. The device can be
brought out of sleep mode by an
interrupt, or by a physical input,
such as a button push. How often
the device is awake will depend
on the application. Even within the
application, sleep times can vary
quite considerably. Dynamic sleep
intervals are often used to allow the
device to judge how often it needs
to make measurements. In the case
of a fitness device, it can wake and
check for movement. If there is
movement, it will intelligently narrow
the time between measurements.
In contrast, if there is a lack of
movement, it can extend the time till
the next measurement, prolonging
battery life.
Communications
between ICs
The communications protocol
between devices can be important
for energy saving. I2C uses pull-up
resistors, which dissipate energy.
SPI doesn’t have pull-up resistors, so
may prove a better choice. Another
way that energy can be lost in
communications is pin capacitance.
To minimize this figure, reduce the
Power Efficient Design for Wearable
Electronics
Mark Patrick, Mouser Electronics
22 l New-Tech Magazine Europe