The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Clocks and Calendars
10
Clocks and Calendars
Many 8052-BASIC systems can make use of a real-time clock that keeps track of seconds,
minutes, hours, and even days, months, and years. You can use the clock to trigger operations
at specified intervals, such as every five minutes, hourly, daily, on the first of the month, or
whatever. Or, a data logger might record the time and date of each measurement it takes, or
the times when it detects selected events.
BASIC-52 includes its own real-time clock that counts in 5-millisecond increments. For
many timing tasks, this is all you’ll need. Another approach is to add a timekeeping chip
that automatically keeps track of time and calendar information. Many clocks perform
functions beyond simple timekeeping, such as generating periodic interrupts or acting as a
watchdog that resets the microprocessor in case of program crashes. Plus, using a separate
timekeeping chip means that you don’t have to devote any of the 8052-BASIC’s resources
to the task.
This chapter describes how to use both BASIC-52’s real-time clock and Dallas Semiconductor’s
DS1286 Watchdog Timekeeper chip.
BASIC-52’s Real-time Clock
The 8052-BASIC, like other computers, has a timing crystal or another frequency source
connected to its XTAL pins. In fact, the chip will do nothing at all without this input, since
it is what clocks instructions into the chip’s CPU for execution. While this clock provides
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