Page 142 - The Art of Designing Embedded Systems

Hardware Musings 129

PALS, FPGAs, and PLDs all ease this process to some extent. Many
changes are not much more difficult than editing and recompiling a file. It
is important to have the right tools available: your frustration level will
skyrocket if the PAL burner is not right at the bench.
FPGAs that are programmed at boot time via a ROM download usu-
ally have a debugging mechanism-a serial connection from the device to
your PC, so you can develop the logic in a manner analogous to using a
ROM emulator. Be sure to put the special connector on your design, and
buy the little adapter and cable. Burning ROMs on each iteration is a ter-
rible waste of time.
PLDs often come like EPROMs, in ceramic packages with quartz
erasure windows. These are great. . . if you were clever enough either to
socket the parts, or to have left room around the part for a socket.
On through-hole designs I generally have the technicians load sock-
ets for every part on the prototype. I want to replace suspected failed de-
vices quickly, without spending a lot of time agonizing over “Is it really
dead?’
Sockets also greatly ease making circuit modification. With an 8-
layer board it’s awfully hard to know where to cut a track that snakes be-
tween layers and under components. Instead, remove the pin from the
socket and wire directly to it.
You can’t lift pins on programmable parts, as the device programmer
needs all of them inserted when reburning the equations. Instead, stack
sockets. Insert a spare socket between the part and the socket soldered on
the board. Bend the pins up on this one. All too often the metal on the
upper socket will, despite the bent-out pin, still short to the socket on the
bottom. Squish the metal in the bottom socket down into the plastic to
eliminate this hard-to-find problem.
Surface-mount parts are much more problematic. Get a good set of
dental tools and a very fine soldering iron, so you can pry up pins as
needed. You’ll need a bright light with magnifier, a steady hand, and ab-
stinence from coffee. A decent surface-mount rework machine (such as
from Pace Electronics) is essential; get one that vectors hot air around the
IC’s pins. Don’t even try to use conventional solder on fine-pitch parts; use
solder paste instead, and keep it fresh (usually it’s best stored in a fridge).
Since SMT is so tough, I always make prototype boards with tracks
on the outer layers. Sure, the final version might reverse this (power and
ground outside to reduce emissions), but reverse the layering during
debug. It’s easy to cut tracks with an X-Acto knife.
Every engineer needs at least two X-Acto knives. One is for finger-
nail cleaning, cutting open envelopes, and tossing at the dartboard. The