DESIGN CYCLE
■ PHOTO 7. We are deploying the LMX-ISM-242
development board as a receive only node. The
development board can participate in a mixed LMX-
ISM-242-SR/LMX-ISM-242-LR network as a receiving
node, a transmitting node, or a sensor node.
ear shot of the Linksys router — we can
contact it remotely. The construction of
a bridge to the gateway is complete.
ASSEMBLING THE PAN
The bridge we just activated will be
used to transfer sensor data from the most
remote node of a two-node PAN. The receiving node of
the PAN will be in RF contact with the remote PAN node
and in serial port contact with the RN-XV bridge. Data
from the remote PAN node will ultimately cross the bridge
onto the Internet. The LMX-LR you saw in Photo 1 is a
powerful data radio. Regardless of its RF prowess, it can
only do what it is told. The LMX-LR support hardware can
be seen in Photo 5. It’s linked to the PIC18F46J13 as an
SPI slave. The PIC18F46J13 support hardware includes a
32.768 kHz crystal to drive its internal RTCC. Power
delivery is under the control of a Microchip MCP1703
LDO voltage regulator. When the LMX-LR and PIC are
sleeping, the total power consumption drops into the µA
range. The LMX-LR circuitry is graphically depicted in
Schematic 3.
I’m going to share a sensor with you that I wrote
about in the January 2013 issue of SERVO Magazine. The
Atlas Scientific ENV-TMP temperature sensor featured in
Photo 6 is a rugged, very low power temperature probe.
The ENV-TMP’s power consumption is 6 µA, which means
this temperature sensor can be powered by a PIC I/O pin.
The raw temperature data captured by the ENV-TMP is
easily processed by the PIC18F46J13.
Over in SERVO, we looked at how to use the ENV-TMP and wrote a firmware driver for it. We’ll take it one
step farther here. We’ll power it up and transport the
collected temperature data over the Internet to my laptop.
Our PAN’s receiving node will be realized with the LMX-
ISM-242 development board. As you can see in Photo 7,
the development board is designed to interface directly
with a PC via its USB portal. We will tap into the
PIC18F46J13’s UART by removing resistors R7 and R8,
which are in series with the UART’s transmit and receive
lines. Removing the resistors disconnects the PIC’s UART
from the FTDI USB-to-UART bridge IC. This leaves the
PIC’s UART transmit and receive lines free to be
connected to the RN-XV. Resistors R7 and R8 are obvious
in Photo 7 as they are located just below pin 1 of the
USB-to-UART bridge IC. You can also locate the resistors
with the help of Schematic 4.
TESTING, 0 1 2 3 ...
Okay. We have a remote LMX-LR squawking at our
■ PHOTO 6. When I come across
something that works really well, I like to
tell everyone I can about it. Some of you
are double-dippers and also read SERVO.
I don't like to "assume" anything, so if
this temperature probe interests you and
you're not a SERVO subscriber, get a copy
of the January 2013 issue and check out
what we did with it over there.
February 2013 19
