Page 130 - The Art of Designing Embedded Systems

Hardware Musings 1 17

A better solution is to use clock-shaping logic near the processor it-
self. If the clock is generated a long way away, use a CMOS hysteresis cir-
cuit (such as a 74HCT14) to clean it up. The extra logic adds delay,
though. If your system requires clock synchronization, then use a special
low-skew clock driver made for that purpose.
In slower systems-under 20 MHz or so-I prefer to design circuits
that don’t depend on a synchronous clock. What happens if you change to
a second sourced processor with slightly different timing? Keep lots of
margin.
Never drive a critical signal such as clock off board without buffer-
ing. There are a very few absolutely critical signals in any system that must
be noise-free. Examine your design and determine what these are, and take
appropriate steps. Clock, of course, is the first that comes to mind. Another
is ALE (Address Latch Enable), used on processors with a multiplexed ad-
dresddata bus. A tiny bit of noise on ALE can cause your address register
to latch in the middle of a data cycle, driving an incorrect address to the
memories.
OK-so now your voltage levels are right. Go back to the data sheet
and make sure the clock’s timing is in spec.
The 8088 requires a 33% clock duty cycle. Sure, it’s a little odd, but
this is a fundamental rule of nature to 8088 designers. Other chips have
tight duty cycle requirements as well.
Rise and fall times are just as important, though difficult to design
for. Some chips have minimum rise/fall time requirements! It’s awfully
hard to predict the rise/fall time for a track routed all over the board. That’s
one attraction of microprocessors with a clock-out signal. Provide a decent
clock-input to the chip, connect nothing to this line other than the proces-
sor, and then drive clock-out all over the board.
Motorola’s 68HC16 pulls a really neat trick. You can use a 32,768-
Hz standard watch crystal to clock the device. An internal PLL multiplies
this to 16 MHz or whatever, and drives a clock output to feed to the rest of
the board. This gets around many of the clock problems and gives a “free”
accurate time-of-day clock source.

Reset

The processor’s reset input is another source of trouble. Like clock.
some processors have unusual input voltage requirements for reset. Be
wary.
Other chips require synchronous circuits. The old 2280 had a very
odd timing spec, clearly spelled out in the documentation, that everyone ig-