Page 134 - The Art of Designing Embedded Systems

Hardware Musings 1 2 1

the haggard designer trying to ship a 68030-based controller. The micro-
controller is easy to use simply because it is stuffed into easy applications.
L.A. Gear sells sneakers that blink an LED when you walk. A
PIC16CSx powers these for months or years without any need to replace
the battery. Scientists tag animals in the wild with expendable subcuta-
neous tracking devices powered by these parts. In Chapter 4 I mentioned
the benefit of adding small CPUs just to partition the code. There are other
compelling reasons as well.
A friend developing instruments based on a 32-bit CPU discovered
that his PLDs don’t always properly recover from brown-out conditions.
He stuffed a $2 controller on the board to properly sequence the PLD’s
reset signals, ensuring recovery from low-voltage spikes. The part cost
virtually nothing, required no more than a handful of lines of code, and oc-
cupied the board space of a small DIP. Though it may seem weird to use a
full computer for this trivial function, it’s cheaper than a PAL.
Not that there’s anything wrong with PALs. Nothing is faster or bet-
ter at dealing with complex combinatorial logic. Modem super-fast ver-
sions are cheap (we pay $12 in singles for a 7-nanosecond 22V10) and
easy to use, and their reprogrammability is a great savior of designs that
aren’t quite right. PALs, though, are terrible at handling anything other
than simple sequential logic. The limited number of registers and clocking
options means you can’t use them for complicated decision making. PLDs
are better, but when speed is not critical a computer chip might be the sim-
plest way to go.
As the industry matures, lots of parts we depend on become obsolete.
One acquaintance found the UART his company depended on no longer
available. He built a replacement in a PIC16C74, which was pin-compati-
ble with the original UART, saving the company expensive redesigns.
In the good old days of microcomputing, hardware engineers also
wrote and debugged all of the system’s code. Most systems were small
enough that a single, knowledgeable designer could take the project from
conception to final product. In the realm of small, tractable problems like
those just described, this is still the case. Nothing measures up to the pride
of being solely responsible for a successful product; I can imagine how the
designer’s eyes must light up when he sees legions of kids skipping down
the sidewalk flashing their L.A. Gears at the crowds.
Part of the recent success of these parts comes from the aggressive
use of Flash and One-Time Programmable (OTP) program memory. OTP
memory is simply good old-fashioned EPROM, though the parts come
without an erasure window. That small quartz opening typical of EPROMs
and many PLDs is very expensive to manufacture. You can program the