Page 131 - The Art of Designing Embedded Systems

1 18 THE ART OF DESIGNING EMBEDDED SYSTEMS

nored only to find massive troubles getting the CPU to start. I think every
single 2280 design in the world suffered from this particular ill at one time
or another.
Sometimes slew rate is an issue. The old RC startup circuit generates
a long ramp that some processors cannot tolerate. You might want to feed
it into a circuit with hysteresis, like a Schmidt Trigger, to clean up the
ramp.
The more complex CPUs require a long time after power-up to sta-
bilize their internal logic. Reset cannot be unasserted until this interval
goes by. Further complicating this is the ramp-up time of the system power
supply, as the CPU will not start its power-up sequence until the supply is
at some predefined level. The 386, for example, requires 219 clock cycles
if the self-test is initiated before it is ready to run.
Think about it: in a 386 system four events are happening at once.
The power supply is coming up. The CPU is starting its internal power-up
sequence. The clock chip is still stabilizing. The reset circuit is getting
ready to unassert reset. How do you guarantee that everything happens
to spec?
The solution is a long time delay on reset, using a circuit that doesn’t
start timing out until the power supply is stable. Motorola, Dallas, and oth-
ers sell wonderful little reset devices that clamp until the supply hits 4.5
volts or so. Use these in conjunction with a long time constant so the
processor, power supply, and clocks are all stable before reset is released.
When Intel released the 188XL they subtly changed the timing re-
quirements of reset from that of the 188. Many embedded systems didn’t
function with this “compatible” part simply because they weren’t compliant
with the new chip’s reset spec. The easy solution is a three-pin reset clamp.
The moral? Always read the data sheets. Don’t skip over the electri-
cal specifications with a mighty yawn. Those details make the difference
between a reliable production product and a life of chasing mysterious
failures.
One of my favorite bumper stickers reads “Question Authority.” It’s
a noble sentiment in almost all phases of life . . . but not in designing em-
bedded systems, Obey the specifications listed in the chip vendors’
datasheets !
If you’ve read many annual reports from publicly held companies,
you know that the real meat of their condition is contained in the notes.
This is just as true in a chip’s data sheet. It seems no one specifies sink and
source current for a microprocessor’s output, but the specification of the
device’s Vol and Voh will always reference a note that gives the test con-
dition. This is generally a safe maximum rating.