Page 125 - The Art of Designing Embedded Systems

1 12 THE ART OF DESIGNING EMBEDDED SYSTEMS

nents because they are, well, boring. Who can get worked up over
the lowly carbon resistor? You can’t even buy them one at a time any
more. At Radio Shack they come paired in bright decorator packages for
an outrageous sum.
Back when I was in the emulator business we dealt with a lot of user
target systems that, because of poor resistor choices, drove the tools out of
their minds. Consider one typical example: a unit based on an 8-MHz
80188, memory and VO all connected in a carefully thought-out manner.
Power and ground distribution were well planned; noise levels were satis-
fyingly low. And yet . . . the only tool that seemed to work for debugging
code was a logic analyzer. Every emulator the poor designer tested failed
to run the code properly. Even a ROM emulator gave erratic results.
Though the emulator wouldn’t run the user’s code, it did show an im-
mediate service of the non-maskable interrupt-which wasn’t used in the
system. (Note: When things get weird, always turn to your emulator’s
trace feature, which will capture weirdness like no other tool.)
A little further investigation revealed that the NMI input (which is ac-
tive high on the 188) was tied low through a 47k resistor.
Now, the system ran fine with a ROM and processor on the board. I
suppose the 47k pull-down was at least technically legitimate. A few
microamps of leakage current out of the input pin through 47k yields a nice
legal logic zero. Yet this 47k was too much resistance when any sort of
tool was installed, because of the inevitable increase in leakage current.
Was the design correct because it violated none of Intel’s design
specs? I maintain that the specs are just the starting point of good design
practice. Never, ever, violate one. Never, ever, assume that simply meet-
ing spec is adequate.
A design is correct only if it reliably satisfies all intended applica-
tions-including the first of all applications, debugging hardware and soft-
ware. If something that is technically correct prevents proper debugging,
then there is surely a problem.
Pull-down resistors are often a source of trouble. It’s practically im-
possible to pull down an LS input (leakage is so high the resistor value must
be frighteningly low). Though CMOS inputs leak very little, you must be
aware of every potential application of the circuit, including that of plug-
ging tools in. The solution is to avoid pull-downs wherever possible.
In the case of a critical edge-triggered (read “really noise sensitive”)
input such as NMI, you simply should never pull it low. Tie it to ground.
Otherwise, switching noise may get coupled into the input. Even worse,
every time you lay out the PC board, the magnitude of the noise problem
can change as the tracks move around the board.