Page 133 - The Art of Designing Embedded Systems

120 THE ART OF DESIGNING EMBEDDED SYSTEMS

goes about its business, ably taking care of just one little function. This is
distributed processing at its best.
Billions and billions of 4- to 16-bit micros find their way into our
lives every year, yet mostly we hear of the few tens of millions that reside
on our desktops.
Now, I’d never give up that zillion-MIP little beauty I’m hunched
over at the moment. We all crave more horsepower to deal with Micro-
soft’s latest cycle-consuming application. I’m just getting tired of 32-bit
hype for embedded applications. Perhaps that 747 display controller or
laser printer needs the power. Surely, though, the vast majority of applica-
tions do not.
A 4-bit controller that formed the basis for a calculator started this in-
dustry, and in many ways we still use tiny processors in these minimal ap-
plications. That is as it should be: use appropriate technology for the job at
hand.
Derivatives of some of the earliest embedded CPUs still dominate the
market. Motorola’s 6805 is a scaled up 6800 which competed with the
8080 back in the embedded Dark Ages. The 805 1 and its variants are based
on the almost 20-year-old 8048.
8051s, in particular, have been the glue of this industry, corre-
sponding to the analog world’s old 741 op amp or the 555 timer. You find
them everywhere. Their price, availability, and on-board EPROM made
them the natural choice for applications requiring anywhere from just a
hint of computing power to fairly substantial controllers with limited user
interfaces.
Now various vendors have migrated this architecture to the 16-bit
world. I can’t help but wonder if this makes sense, as scaling a CPU, while
maintaining backward compatibility, drags lots of unpleasant baggage
along. Applications written in assembly may benefit from the increased
horsepower; those coded in C may find that changing processor families
buys the most bang for the buck.
Microchip, Atmel, and others understand that the volume part of the
embedded industry comes from tiny little CPUs scattered with reckless
abandon into every corner of the world. These are cool parts! The smaller
members offer a minimum amount of compute capability that is ideal for
simple, cost-sensitive systems. Higher-end versions are well suited for
more complicated control applications.
Designers seem to view these CPUs as something other than com-
puters. “Oh, yeah, we tossed in a couple of PIC16s to handle the mi-
croswitches,” the engineer relates, as if the part were nothing more than a
PAL. This is a bit different from the bloodied, battered look you’ll get from