The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 10
an essential timing reference, by itself it doesn’t keep track of real-world time measured in
seconds, minutes, and hours. But if you know the crystal’s frequency, you can measure
seconds by counting the oscillations of the crystal. This is what BASIC-52’s real-time clock
does.
A CLOCK1 statement starts the real-time clock, which causes the TIME operator to
increment every 5 milliseconds. Reading the TIME operator tells you the number of seconds
that have passed since the clock was enabled. CLOCK0 stops the clock and freezes TIME
at its current value. TIME resets to 0 when the count reaches 65536 seconds (18 hours, 12.3
minutes), or when the statement TIME=0 executes. If you stop the clock and then then restart
it, TIME will continue counting from where it left off, unless you first reset it to 0.
The ONTIME instruction jumps to a subroutine whenever TIME reaches the value you
specify. Because the ONTIME subroutine is an interrupt routine, you use RETI, not
RETURN, to end it.
Listing 10-1 is a program that counts seconds, minutes, and hours, and displays the current
reading once per second. For accurate timekeeping, the XTAL operator must match the
value of your timing crystal.
You can also use ONTIME to trigger periodic operations. Listing 10-2 is a program that
toggles bit 7 of Port 1 once per second and displays the logic state of the bit after each toggle.
Clock Accuracy
The more accurate your timing reference, the more accurate your clock will be. You can tune
the frequency of a crystal slightly by varying the value of one of the capacitors that connects
from the crystal to ground.
Temperature variations will cause a crystal’s frequency to drift. Crystal accuracy is rated in
parts per million per degree Celsius (often shortened to ppm). Over time, a crystal rated at
±10 ppm should vary no more than 0.001 percent per degree Celsius, or 0.86 seconds per
day, if the temperature varies no more than ±1 degree Celsius. If your clock must be
super-accurate, choose the most stable crystal you can find and and avoid temperature
fluctuations.
You might think that you can get a more accurate real-time clock by adjusting XTAL to
match your crystal’s actual frequency, rather than its rated value. You could measure the
crystal’s frequency with a frequency counter, or experiment by varying the value of XTAL
and monitoring the real-time clock to find the best match. For example, if your 12Mhz crystal
actually oscillates at 11.97 Mhz, you could set XTAL equal to 11970000.
172 The Microcontroller Idea Book