Page 129 - The Art of Designing Embedded Systems

1 16 THE ART OF DESIGNING EMBEDDED SYSTEMS

the electrical specs page-you know, the section without coffee spills or
solder stains. Skip over those 300 tattered pages about programming in-
ternal peripherals, bypass the pizza-smeared pinout section, and really look
at those one or two pristine pages of DC specifications.
Most CPUs accept TTL-level data and control inputs. Few are happy
with TTL on the clock and/or reset inputs. Each chip has different re-
quirements, but in a quick look through the data books I came up with the
following:
8086: Minimum Vih on clock: Vcc - 0.8
386: Minimum Vih on clock: Vcc - 0.8 at 20 MHz, 3.7 volts at 25
and 33 MHz
280: Minimum Vih on clock: Vcc - 0.6
805 1: Minimum Vih on clock and reset: 2.5 volts
In other words, connect your clock and maybe reset input to a normal
TTL driver, and the CPU is out of spec. The really bad news is that these
chips are manufactured to behave far better than the specs, so often they’ll
run fine despite illegal inputs. If only they failed immediately on any vio-
lation of specifications! Then, we’d find these elusive problems in the lab,
long before shipping a thousand units into the field.
Fully 75% of the systems I see that use a clock oscillator (rather than
a crystal) violate the clock minimum high-voltage requirement. It’s scary
to think we’re building a civilization around embedded systems that, well,
may be largely misdesigned.
If you drive your processor’s clock with the output of a gate or flip-
flop, be sure to use a device with true CMOS voltage levels. 74HCT or
74ACTECT are good choices. Don’t even consider using 74LS without at
least a heavy-duty pull-up resistor.
Those little 14-pin silver cans containing a complete oscillator are a
good choice . . . if you read the data sheet first. Many provide TTL levels
only. I’m not trying to be alarmist here, but look in the latest DigiKey cat-
alog-they sell dozens of varieties of CMOS and TTL parts.
Clocks must be clean. Noise will cause all sorts of grief on this most
important signal. It’s natural to want to use a Thevenin termination to more
or less match impedance on a clock routed over a long PCB trace or even
off board. Beware! Thevenin terminations (typically a 220-ohm resistor
to +5 and a 270 to ground) will convert your carefully crafted CMOS level
to TTL.
Use series damping resistors to reduce the edge rate if noise is a prob-
lem. A pull-up might help with impedance matching if the power supply
has a low impedance (as it should).