23-2
MC9328MX1 Reference Manual
MOTOROLA
Real-Time Clock (RTC)
Figure 23-1. Real-Time Clock Block Diagram
23.1 Operation
The prescaler converts the incoming crystal reference clock to a 1 Hz signal which is used to increment the
seconds, minutes, hours, and days TOD counters. The alarm functions, when enabled, generate RTC
interrupts when the TOD settings reach programmed values. The sampling timer generates
fixed-frequency interrupts, and the minute stopwatch allows for efficient interrupts on minute boundaries.
23.1.1 Prescaler and Counter
The prescaler divides the reference clock down to 1 Hz. The reference frequencies of 32.768 kHz,
38.4 kHz and 32 kHz are supported. The prescaler stages are tapped to support the sampling timer.
The counter portion of the RTC module consists of four groups of counters that are physically located in
three registers:
•
The 6-bit seconds counter is located in the SECONDS register
•
The 6-bit minutes counter and the 5-bit hours counter are located in the HOURMIN register
•
The 9-bit day counter is located in the DAYR register
These counters cover a 24-hour clock over 512 days. All three registers can be read or written at any time.
Interrupts signal when each of the four counters increments, and can be used to indicate when a counter
rolls over. For example, each tick of the seconds counter causes the 1 Hz interrupt flag to be set. When the
seconds counter rolls from 59 to 00, the minute counter increments and the MIN interrupt flag is set. The
same is true for the minute counter with the HR signal, and the hour counter with the DAY signal.
SAMPLING
TIMER
PRESCALER
SECOND
MINUTE
INTERRUPT
CONTROL
INTERRUPT
ENABLE
CLOCK
CONTROL
INTERRUPT
STATUS
ALARM COMPARATOR
SECOND
LATCH
MINUTE
LATCH
HOUR
LATCH
DATA
BUS CONTROL
RTC_INT
RTC_SAM_INT
ADDRESS
MINUTE STOPWATCH
32.768K OR 32K OR 38.4K
1 PPM
IPBUS
DECODE
HOUR
DAY
TOD CLOCK
1 PPS
1 PPH
1 PPD
CLK2HZ
CLK32HZ
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