The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 10
Listing 10-2. Toggles a port bit and displays the result.
10 REM toggles P1.7 once per second
20 TIME=0
30 CLOCK 1
40 DO
50 ONTIME 1,100
60 WHILE 1=1
70 END
100 REM reset time
110 TIME=0
120 REM toggle Port 1, bit 7
130 PORT1=PORT1.XOR.80H
140 PRINT “Port 1, bit 7 = ”,(PORT1.AND.80H)/80H
150 RETI
But in reality, because of the way that BASIC-52 calculates time, small variations in XTAL
usually do not effect the real-time clock. Although BASIC-52 will store a XTAL value as
precise as 12000001, it uses the same time base for all XTAL values from 11963191 to
12039877. If your crystal frequency is within this range, small adjustments to XTAL won’t
make the real-time clock more accurate. The value that controls the time base is stored at
4Ah in internal data memory. At 12 Mhz, it’s 64h. If you want to experiment, change the
value of XTAL, then type PH0. DBY(4AH) to find out if the time base has changed.
A Watchdog Timekeeper
Dallas Semiconductor’s DS1286 Watchdog Timekeeper, shown in Figure 10-1, is another
way to keep track of time. The chip is easy to use because it contains its own quartz-crystal
timing reference, plus a lithium cell for backup power. Once you initialize the clock and
calendar and start the oscillator, the clock keeps time for ten years or more, whether or not
an external power source is present. You don’t have to reset the clock every time you power
up.
The DS1286 can be especially useful in battery-powered systems. Since it continues to keep
time when the main power supply is off, you can use its interrupt output to power circuitry
at programmed times or intervals. For example, by adding circuits to control a power supply,
the DS1286’s interrupt could trigger a data logger or other instrument to power up at a
programmed time. After taking data or performing other operations, the instrument could
power itself down until the next interrupt from the DS1286. The longer the time between
readings, the greater the power savings.
174 The Microcontroller Idea Book