Page 171 - The Art of Designing Embedded Systems

158 THE ART OF DESIGNING EMBEDDED SYSTEMS

lously solve most problems. It just ain’t so. Buy the right tools, but under-
stand their inherent limitations.
Overcome limitations with clever designs, using a deep understand-
ing of where the problems come from. Here’s a collection of ideas drawn
from bitter experience:

Reliable Connections
In the good old days microprocessors came in only a few packages.
DIP, PGA, or PLCC, these parts were designed for through-hole PC boards
with the expectation that, at least for prototyping, designers would socket
the processor. Isolating or removing the part for software development re-
quired nothing more than the industry-standard chip puller (a bent paper
clip or small screwdriver).
Now tiny PQFP and TQFP packages essentially cannot be removed
for the convenience of the software group. Once you reflow a 100-pin de-
vice onto the board, it’s essentially there forever.
Part of the drive toward TQFP is the increasing die complexity. That
tiny device is far more than a microprocessor; it’s a pretty big chunk of
your system. The CPU core is surrounded with a sea of peripherals-and
sometimes even memory. Replace the device with a development system,
and the tool will have to replace both the core and all of those high-inte-
gration devices.
Take heart! Most semiconductor vendors are aware of the problem
and take great pains to provide work-arounds.
There’s no cheap cure for the purely mechanical problem of con-
necting a tool to those whisker-thin pins, but at least the industry’s con-
nector folks sell clips that snap right over the soldered-on processor. The
clip translates those SMT leads to a PC board with a PGA or header array
that your tools can plug into. Before starting any design, get a copy of Em-
ulation Technology’s catalog. Though their products are horrifically ex-
pensive, they offer a very wide range of adapters and connection strategies.
Another good source for connection ideas is the logic analyzer arena.
Both HP and Tektronix are starting to standardize their analyzer cables on
AMP’s “Mictor” connector, a very small, very high-density, controlled
impedance device. If you surround your CPU with Mictors (being careful
to match the pinouts used by the analyzer vendors), then probing becomes
trivial: just plug the analyzer cables in directly. If you’re frustrated with
logic analysis because of the agony of connecting 50 or 100 little clip leads
(half of which pop off at inconvenient times), take heart, as the Mictor goes
directly into the main analyzer cables, bypassing the clips altogether.