The Microcontroller Idea Book - Jan Axelson's Lakeview Research

If you want an alarm frequency other than daily, hourly, or on the minute, there are a couple
of ways to achieve it. For an alarm every 10 minutes, you could generate an interrupt once
per minute and ignore 9 out of 10 interrupts.
If this seems wasteful, you can update the alarm minutes to the next desired value after each
interrupt. Using the example of an interrupt every 10 minutes, you would set the mask bits
for when minutes match (0-1-1), and start out by storing 0 in register 3, which will cause an
interrupt to occur on the hour. When the interrupt occurs, you would add 10 to register 3 to
schedule the next interrupt for 10 minutes after the hour. By continuing to add 10 to register
3 after each interrupt, and returning to 0 on a count of 60, you end up with an interrupt every
10 minutes.
As Table 10-1 shows, many of the DS1286’s registers have multiple functions. In register
9, bits 0-4 store the current month, bit 6 enables the square-wave output, bit 7 enables the
clock, and bit 5 is always 0. For situations like this, you can create mask bytes and use
BASIC-52’s logical operators to read and write to selected bits while ignoring other bits in
a register. For example, assume that you want to store a month in bits 0-4 of the 1286’s
register 9, without affecting the settings of bits 5-7.
To do so, follow these steps:
(1) Read the current value of the byte. In our example, with a current month of December,
the clock enabled, and the square wave disabled, register 9 will hold these values:
0101 0010
(2) Create a mask byte by setting all bits to be masked, or unchanged, to 1, and clearing the
other bits. To alter only the month’s value, bits 5-7 are masked:
1110 0000
(3) Logically AND the current value with the mask byte, with this result:
0100 0000
(4) Place the new month’s value in bits 0-4 of the byte. To change the month to June (6th
month), logically OR the above byte with this:
0000 0110
which results in:
0100 0110
Clocks and Calendars
The Microcontroller Idea Book 179