Page 152 - The Art of Designing Embedded Systems

Troubleshooting Tools 139

Time stumping-Emulators and logic analyzers often include time
information in the trace buffer. Time stamps usually eat up about 32 bits of
trace width. Combined with the trace system’s triggers, it’s easy to perform
quite involved timing measurements.

Emulators

In-Circuit Emulators (ICEs) have always been the choice weapons in
the war on bugs. Yet, for as long as I can remember pundits have been pre-
dicting their death. Though it seems as quaint as IBM’s 1950s prediction
that the worldwide market for computers was merely a couple of dozen, in
fact 20 years ago many people believed that the 4-MHz 280 would spell
doom for ICEs. “4 MHz is just too fast,” they proclaimed. “No one can run
those speedy signals down a cable.”
Time proved them wrong, of course. Today’s units run at 60+ MHz
on processors with single-clock memory cycles, an astonishing achieve-
ment.
Is an end yet in sight? I believe so, though the limiting frequency is a
bit hazy. Today’s approach of putting all or much of the ICE’S electronics
on the pod removes the cabling and bus driver problems, but electrons do
move at a finite speed and even the fastest of circuits have nonzero propa-
gation delays.
CPU vendors squeeze the last bit of clock rates from their creations
partly by tuning their chips ever more exquisitely to the rest of the system’s
memory and YO. Clearly, an intrusion by any sort of development tool will
at best be problematic. Yes, today’s Pentium emulators do work. Will to-
morrow’s units be able to handle the continued push into stratospheric
clock rates? I have doubts.
Packages are creating another sort of problem. Heat, speed, and size
constraints have yielded a proliferation of packaging styles that challenge
any sort of probing for debugging. If you’ve ever tried to use a scope on a
208-pin PQFP device or, worse, a 100-pin TQFP, you know what I mean.
Yes, some tremendously innovative probing systems exist-notably those
from Emulation Technology and HP. Despite these, it’s still difficult at
best to establish a reliable connection between a target CPU and any sort
of hardware debugger, from a voltmeter to an ICE.
Surface-mount devices have exposed pins that you at least have a
prayer of getting to. Newer devices don’t. The BGA (Ball Grid Array)
package, which is suddenly gaining favor, connects to a PC board via hun-
dreds of little bumps on the underside of the package-where they are
completely inaccessible. Other technologies bond the silicon itself under a