Page 155 - The Art of Designing Embedded Systems

142 THE ART OF DESIGNING EMBEDDED SYSTEMS

ICES can be finicky beasts to tame. With a hundred or more con-
nections to your target hardware, the smallest bit of dirt, vibration,
or bad luck can cause erratic operation that will drive your devel-
opers out of their minds. For this reason I always recommend sol-
dering the emulator to an SMT part, rather than using a clip-on
connection. Find a reliable hook-up scheme early, to avoid infinite
frustration later.

BDMs

CPU cores hidden away inside ASICs give fabulously small systems,
yet that buried processor is all but impossible to probe. Couple bus cycles
within fractions of a nanosecond to a peripheral and you leave no margin
for your tools. One-off CPUs, whether from burying a VHDL virtual
processor inside a high-integration part, or from the huge explosion of de-
rivatives of popular parts, are often tool orphans. Tool vendors, after all,
won’t invest huge sums in developing products for a particular CPU unless
they see a large, healthy market for their offerings.
Even seemingly boring issues such as device packaging further iso-
late us from the processor. If we can’t probe it, we can’t see what’s going
on. We lose the visibility needed to find bugs.
The trend is to separate run control from real-time trace. “Run
control” means those simple debugging features that we’d expect even in
nonembedded work: simple breakpoints, single-stepping, and access
to processor resources, memory, and peripherals. Probably 95% of all
debugging uses nothing more than these relatively simple features. Trace,
though, demands real-time access to the entire data, address, and control
busses, and so is generally a rather thorny and expensive part of any
emulator.
But the promise of a serial debugger remains seductive, given that
just a few wires replace the hundreds of connections used by an emulator
or logic analyzer. Motorola recognized this early on and created the Back-
ground Debug Mode (BDM), a feature first found on the 683xx and
68HC 16 processors, since extended and incorporated on many other chips.
BDM is a bit of specialized debugging hardware built right into the
chip (Figure 7-2). Transistors are so cheap it makes sense to build a debug
interface into even production chips. Clearly this overcomes one major ob-
jection of bond-outs: the “stepping level” of the production IC is always
identical to the debug part. , . because they are one and the same.
BDMs eliminate all speed and packaging issues. As part of the sili-
con, the debugger runs as fast as the chip; the interface to the outside world