Programming Embedded Systems in C and C++

Variations on the Theme 7

Digital Watch

At the end of the evolutionary path that began with sundials, water clocks, and

hourglasses is the digital watch. Among its many features are the presentation of

the date and time (usually to the nearest second), the measurement of the length

of an event to the nearest hundredth of a second, and the generation of an annoy-

ing little sound at the beginning of each hour. As it turns out, these are very sim-

ple tasks that do not require very much processing power or memory. In fact, the

only reason to employ a processor at all is to support a range of models and fea-

tures from a single hardware design.

The typical digital watch contains a simple, inexpensive 8-bit processor. Because

such small processors cannot address very much memory, this type of processor

usually contains its own on-chip ROM. And, if there are sufficient registers avail-

able, this application may not require any RAM at all. In fact, all of the electron-

ics—processor, memory, counters and real-time clocks—are likely to be stored in

a single chip. The only other hardware elements of the watch are the inputs (but-

tons) and outputs (LCD and speaker).

The watch designer’s goal is to create a reasonably reliable product that has an

extraordinarily low production cost. If, after production, some watches are found

to keep more reliable time than most, they can be sold under a brand name with ...