Page 132 - The Art of Designing Embedded Systems

Hardware Musings 1 19

With watchdog timers and other circuits connected to reset inputs, be
wary of small timing spikes. I spent several frustrating days working with
an AMD part that sometimes powered up oddly, running most instructions
fine but crashing on others. The culprit was a subnanosecond spike on the
reset input, one too fast to see on a 100-MHz scope.
Homemade battery-backed-up SRAh4 circuits often contain reset-
related design flaws. The battery should take over, maintaining a small bias
to the RAM’S Vcc pins, when main power fails. That’s not enough to avoid
corrupting the memory’s contents, though.
As power starts to ramp down, the processor may run crazy for a
while, possibly creating errant writes that destroy vast amounts of carefully
preserved data in the RAM. The solution is to clamp the chip’s reset input
as soon as power falls below the part’s minimum Vcc (typically 4.75 volts
on a 5-volt part).
With reset properly asserted, Vcc now at zero, and the battery pro-
viding a bit of RAM support, be sure that the chip select and write lines to
the RAM are in guaranteed “idle” states. You may have to use a small pull-
up resistor tied to the battery, but be wary of discharging the battery
through the resistor when the system is operating normally.
And be sure you can actually pull the line up despite the fact that the
driver will experience Vcc’s from +5 to zero as power fails. The cleanest
solution is to avoid the problem entirely by using a RAM with an active
high chip select, which you clamp to zero as soon as Vcc falls out of spec.
Despite our apparent digital world, the harsh reality is that every
component we use pushes electrons around. Electrical specifications are
every bit as important to us as to an analog designer. This field is still elec-
tronic engineering tilled with all of the tradeoffs associated with building
things electronic. Ignore those who would have you believe that designing
an embedded system is nothing more than slapping logic blocks together.

Small CPUs
Shhhh! Listen to the hum. That’s the sound of the incessant informa-
tion processing that subtly surrounds us, that keeps us warm, washes our
clothes, cycles water to the lawn, and generally makes life a little more tol-
erable. It’s so quiet and keeps such a low profile that even embedded de-
signers forget how much our lives are dominated by data processing. Sure,
we rail at the banks’ mainframes for messing up a credit report while the
fridge kicks into auto-defrost and the microwave spits out another meal.
The average house has some 40 to 50 microprocessors embedded in
appliances. There’s neither central control nor networking: each quietly