ADVANCED TECHNIQUES FOR DESIGN ENGINEERS
■ BY FRED EADY
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RIDING 802.15.4 RADIO WAVES
Wire is wonderful. Wire doesn't need external power or batteries to operate.
Wire can be used as a conductor, a resistor, a thermometer, a tie wrap, and
a fuse. Wire keeps the pigs and chickens in, and the dogs out. Wire can also
keep the dogs in, and the chickens and pigs out. Wire supports tomato plants
and gives string beans something to climb on.
The 802.15.4 protocol rides on a radio wave. It does require external power or
batteries to be radiated; 802.15.4 cannot be used as a conductor, a resistor, a
thermometer, a tie wrap, or a fuse. By itself, 802.15.4 cannot keep animals in
their place or provide a walking stick for tomatoes or beans. However, when
it comes to transferring data, 802.15.4 radios are just as good as wire.
NOTHING IS FREE
When properly supported, an 802.15.4 radio can tell
you how hot or how cold it is. An 802.15.4 radio with the
right stuff can tell you if a fuse has blown or if the pigs are
in the garden instead of in their pen. These super powers
come with a price tag called energy. Usually, that energy
is a product of a battery as AC mains power is normally
not installed at the location of every tomato plant in a
field. So, to make that 802.15.4 radio emulate a wire for a
very long time, we must figure out how to conserve the
energy provided by that battery.
BEGINNING A LOW POWER
50 September 2012
The first and most important step in a low power
radio design is to carefully choose the components. Our
reference design is depicted in Schematic 1. As you can
see (in Schematic 1), we are basing the design around a
PIC18F46J50. The PIC18F46J50 is a member of the
nano Watt XLP (Extreme Low Power) family. Ridiculously
low current consumption can be realized using the
PIC18F46J50’s sleep and deep sleep modes.
The 25AA02E48 EEPROM provides our low power
radio node with a unique write protected 48-bit 802.15.4
radio address. The 25AA02E48 also allows us to store up
to 192 bytes of data in its read/write EEPROM array. Like
its host, the 25AA02E48 can be easily forced to conserve
energy. A standby current consumption of 1.0 µA is
obtained by simply driving the 25AA02E48’s active-low CS
line logically high. When powered by a 5. 5 volt power
supply, the worst case current consumption for the
25AA02E48 is 5.0 mA. The EEPROM draws a paltry 3.0
mA with a 2. 5 volt power rail. We can count on a current
consumption figure of approximately 4.0 mA with our 3. 3
volt supply voltage.