MC68HC908LJ12 Datasheet, PDF (211/413 Pages) Freescale Semiconductor, Inc – 8-bit microcontroller units

Real Time Clock (RTC)
12.4 Functional Description
The RTC module provides clock indications in seconds, minutes, and
hours; calendar indications in day-of-week, day-of-month, month, and
year; with automatic adjustment for month and leap year. Reading the
clock and calendar registers return the current time and date. Writing to
these registers set the time and date, and the counters will continue to
count from the new settings.
The alarm interrupt is set for the hour and minute. When the hour and
minute counters matches the time set in the alarm hour and minute
registers, the alarm flag is set. The alarm can be configured to generate
a CPU interrupt request.
A 1/100 seconds chronograph counter is provided for timing
applications. This counter can be independently enabled or disabled,
and cleared at any time.
RTC module interrupts include the alarm interrupt and seven periodic
interrupts from the clock counters.
For proper RTC module operation, one of the following oscillator
frequencies (CGMXCLK) must be used:
• 32.768 kHz
• 32.000 kHz
• 38.400 kHz
• 64.000 kHz
• 76.800 kHz
Configuring the XTL[2:0] bits in the RTC control register 2 selects the
appropriate prescalers and dividers to divide CGMXCLK down to the
basic 1Hz clock for driving the clock counters.
Figure 12-2 shows the structure of the RTC module.
Technical Data
212
Real Time Clock (RTC)
MC68HC908LJ12 — Rev. 2.1
Freescale Semiconductor