FIGURE 3. The MRF24J40MB can stuff up to 250 kbps of data
through an 802.15.4 link. A typical SPI portal can run as fast as its
MASTER node can reliably clock data in and out of it.
milliwatt signal. The higher transmit power rating allows it
to push a signal out to a maximum of 4,000 feet with no
need for a license. The MRF24J40MB construction includes
a PIC24FJ128GA006 (power amplifier) and LNA (low noise
amplifier) that allows it to outperform the MRF24J40MA on
both the transmit and receive levels. If you want to know
all of the pointy hat details, I’ll leave reading the datasheet
up to you. I’m interested in moving some data using the
Earth’s magnetic field and as you’re reading this you are
most likely in that mindset, as well. So, let’s only look at
the physics of the MRF24J40MB that interest us. For
instance, what do all of its pins do?
The active-low RESET pin performs the obvious function
of a global reset input. The WAKE input implies that the
MRF24J40MB can be put to sleep to conserve power. Most
microcontrollers have ample time to poll external
communications resources like the MRF24J40MB. However,
it’s always a good thing when the microcontroller can go
about its business and only service a device when required
to do so. That’s where the INT output pin comes into play.
The INT pin is an output that is tied to one of the host
microcontroller’s external interrupt inputs. When the
MRF24J40MB needs to perform data I/O or do something
the host microcontroller needs to know about, the
MRF24J40MB activates its INT pin. The activation of the INT
output forces the host microcontroller to run an interrupt
handler routine to service the INT-initiated request. The
four-wire SPI portal is comprised of the SDO, SDI, SCK, and
CS I/O pins. Rather than ramble through how all of these
MRF24J40MB signals hook up to a host microcontroller, a
quick perusal of Figure 3 makes it perfectly clear.
Installing an MRF24J40MB
Normally, we would design and fabricate a specialized
PCB for this project. However, our initial inclusion of the
high-quality Twin Industries breadboard in the Trainer
Expansion project allows us to mount and connect our
MRF24J40MB just as its datasheet recommends. The
MRF24J40MB is designed to be mounted over a ground
plane with its printed circuit antenna hanging off the edge
PHOTO 3. The MRF24J40MB is easily mounted SMT style or via
pins and headers.
PHOTO 4. As Spock would say, “Random chance seems to have
operated in our favor.” There is NO WAY that I purposely designed
these socket areas to overlap so perfectly.
of the host PCB. The Twin Industries 8100-45-LF breadboard
is equipped with plated through holes and a single copper
plane. We will ground the breadboard plane to meet the
MRF24J40MB installation requirements.
As you can see in Photo 4, the MRF24J40MB footprint
puts its mounting pads just inside of the shade-tree
engineered XBee-Pro mounts. The actual electrical hookups
are shown in Schematic 1.
Customizing the MiWi Stack
Building a Mi Wi definition file (Mi WiDefs.h) is as easy
as starting up a ZENA Mi Wi stack configuration session.
Basically, the stack configuration utility wants to know what
demo board or microcontroller you will be using, how fast
it will be clocked, and what microcontroller I/O and
interrupt pins you would like to assign to the MRF24J40MB
SERVO 06.2010 69
