Page 159 - The Art of Designing Embedded Systems

146 THE ART OF DESIGNING EMBEDDED SYSTEMS

The ROM monitor will not work if the hardware is broken.
Real-time instrumentation is weak. You just won’t find trace or
timing data in any ROM monitor product.

ROM Emulators

A significant problem with conventional emulators is that they are
CPU-specific. Change from a 68332 to a 68340 and, even though the
processor’s architecture doesn’t change, you’ll need a new emulator-r at
least a new multi-thousand-dollar pod. ROM emulators, instead, connect to
your target system via a memory socket. They consist of a RAM array that
mimics the ROM chip . . . while allowing you to download new code in a
heartbeat. The serial port is built into the unit itself.
ROM emulators are so inexpensive that even when using some other
debugging tool I keep a few around for those unexpected problems that al-
ways seem to surface.
ROM emulators continue to play an important role in embedded de-
velopment for the following reasons:
As ROM replacements they offer convenient overlay RAM. Espe-
cially in smaller systems, this may be critical so you can download
code, rather than bum a dozen ROMs an hour.
Most are very inexpensive-some go for just a few hundred dol-
lars. This means every developer can have a reasonable debugging
tool at hand.
ROM emulators are processor-independent. The source debugger
may change as you move from a 68000 to a 186, but the hardware
element remains unchanged.
Few, if any, target resources are required.
Problems include:

Just as with an ICE, speed is an ever-increasing concern.
The physical connection to the target system might be difficult if
you’re emulating SMT ROM devices. As with ICES, many ven-
dors do offer innovative connection strategies, but bear in mind
that making a reliable connection may be difficult.
The ROM socket does not provide any convenient way to set
breakpoints! About half of the vendors do offer a breakpoint strat-
egy; be sure the one you select won’t leave you breakpoint-
starved.