Hacking_and_Penetration_Testing_with_Low_Power_Devices

BeagleBone capes
139
If you don’t plan on using the XBee sleep and reset functionality and you have
another cape that would like to use pins P8-7 and P8-9, the script above should be
modified. Change the total number of pins from b’\x00\x04\’ to b’\x00\x02’ and the
line for setting P8-7 and P8-9 from b’\xc0\x0f\xc0\x0f’ to b’\x00’ * 4.
A basic two-sided circuit board layout for the XBee cape is shown in Figure 6.9.
The cape is shown superimposed atop the outline of the BeagleBone. If you are reading
the print version of this book, it may be somewhat difficult to decipher this figure
as it is printed in gray scale. The heavier traces are printed on the bottom of the board
and lighter traces on the top.
The full cape can be manufactured using the same techniques as the mini-cape.
There is one complication, however. This board is double-sided. This requires etching
on both sides, which in and of itself is not difficult. Traces on each side of the
board must line up properly. If you choose to make this board yourself, ensure that
you have carefully aligned your traces on the top and bottom of the board. Also be
careful to ensure header pins are soldered to traces on both the top and bottom of the
board as your board will not be plated through like a commercially produced board.
There are a couple of choices when it comes to the connections between your
cape and the BeagleBone expansion headers. Nonstacking headers can be used if
you don’t intend to stack additional capes atop your cape. Nonstacking headers
are essentially just male header pins. Given that an XBee modem plugs into this cape,
the use of nonstacking headers makes sense as it is not practical to put another cape
on top of the XBee cape. The System Reference Manual lists several compatible
headers both stacking and nonstacking.
Incidentally,ifacapeisnonstackable,thereisnostrictrequirementtousetheheaders
listed in the System Reference Manual. Standard breakaway headers such as those
employed with the mini-cape may also be used. In addition, if you don’t wish to drill
all 92 holes in the cape board, you could likely install headers only on pins actually used
and on the ends of each expansion header (for alignment).
Single-sided XBee cape
As previously mentioned, producing a double-sided board yourself is a bit more complicated.
Realizing that this might be a little much for an electronics novice, I have
also designed a single-sided version of the XBee cape. The schematic for this simplified
board is shown in Figure 6.10.
The single-sided cape schematic is nearly identical to the double-sided cape schematic
with two notable exceptions. The most obvious change is that the connector for
the FTDI cable has been removed. This was done because there was no practical way
to have this connector on a single-sided board as it would require crossing signals.
This problem is exacerbated by having the P8 and P9 expansion headers on the very
edge of the board.
The second change is not as obvious. A zero ohm resistor, R8, has been added
inline to the XBee modem power connection. This is somewhat of a kludge to make
the single-sided board work. This zero ohm resistor was necessary in order to hop