■ SCHEMATIC 1. This little circuit is capable of
communicating using the 802.15.4 protocol,
Microchip's Mi Wi or ZigBee.
ground current, the TC1262 adds a mere 80 µA to our
power budget. The fruits of our design become physical in
Running the PIC18F46J50 at full tilt boogie ( 48 MHz)
pulls 33. 7 mA. Turning off the PIC18F46J50’s PLL and
dividing the 12 MHz CPU clock by three reduces the
current drain to 24. 17 mA. Here’s where
you have to make a decision. Do you really
want to save that few milliamps and take
longer to send your message? Or, do you
want to take less time to send the message
and run the higher clock speed? Personally,
I would choose to run as fast as necessary
and keep my messages very short. In the
long run, you’ll use more energy in a
shorter amount of time by taking the extra
time to compute and send your data.
PIC18F46J50-based MRF24J40MA 802.15.4 node sleeps
at 83. 7 µA. The next thing to consider is how do we wake
it up? The answer lies in the 32.768 kHz crystal and its
supporting 12 pF capacitors.
The PIC is endowed with an internal RTCC which
happens to be clocked by the 32.768 kHz crystal. The
PIC’s internal RTCC can do a bit more than just keep the
time of day. It can also act as a CPU alarm clock. All of us
know that alarm clocks are not all-knowing and need to
be set. The same is true for the microcontroller’s RTCC.
Regardless of the clock speed, our
■ PHOTO 1. One 25AA02E48, one PIC18F46J50, and one MRF24J40MA equals
one Mi Wi P2P node.
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