Page 140 - The Art of Designing Embedded Systems

Hardware Musings 127

Cheap autorouting software means any engineer can design a PCB in
a matter of a couple of days-and you’ll have to do this eventually any-
way, so it’s not wasted time. Dozens of outfits will convert your design to
a couple of PCBs in under a week for a very reasonable price. How much?
Figure $looCrl500 for a 50-square-inch 4- to 6-layer board, with one-
week turnaround.
It’s magic. Modem your board design to the vendor, and days later
FedEx delivers your custom design, ready for assembly and test.
PCBs are much quieter, electrically, than their wire-wrapped
brethren. With fast rise times and high clock rates, noise is a significant
problem even in small embedded designs. I’ve seen far too many cases of
“Well, it doesn’t work reliably, but that’s probably due to the wire wrap.
It’ll probably get better when we go to PC.” These are clearly cases where
the prototype does not accomplish its prime objective: identify and fix all
risk factors.
Always build your prototype on a PCB, never on wirewrap or other
impedance-challenged technologies. And figure on using a multilayer de-
sign, with unadulterated power and ground planes. Modem logic is just too
fast, too noisy, and too intolerant of ground bounce and other impedance
issues to try and mix power and signals on any PCB layer.
The best source for information about speed and noise issues on PC
boards is High Speed Digital Design-A Handbook of Black Magic, by
Howard Johnson and Martin Graham (1993, PTR Prentice Hall, NJ). This
is a must-read for all digital engineers. If you felt that your college elec-
tromagnetics was a flunk-out course, one you squeaked through, fear not.
The authors do use plenty of math, but their prose descriptions are so lucid
you’ll gain a lot of insight by just reading the words and shpping over the
equations.
Design your prototype PCB with room for mistakes. Designing a
pure surface-mount board? These usually use tiny vias (the holes between
layers) to increase the density. Think about what happens during the pro-
totyping phase: you’ll make design changes, inevitably implemented by a
maze of wires. It’s impossible to run insulated wire through the tiny holes!
Be sure to position a number of unusually large vias (say, 0.03 I ”) around
the board that can act as wiring channels between the component and cir-
cuit sides of the board.
Add pads for extra chips; there’s a good chance you’ll have to
squeeze another PAL in somewhere. My latest design was so bad I had to
glue on five extra chips. Guess who felt like an idiot for a few days. . . .
Always build at least two copies of each prototype PCB. One may lag