Motorola MC68VZ328 - Manual (Page 212)

11-2 MC68VZ328 User’s Manual
RTC Overview
11.1 RTC Overview
The prescaler uses the CLK32 clock to create a 1 Hz clock used by all of the blocks in the RTC, as shown
in Figure 11-1 on page 11-2. The 1 Hz signal is used to increment the counters in the TOD clock. The TOD
clock is composed of second, minute, hour, and day counters. If enabled, the TOD alarm generates an RTC
interrupt when programmed alarm settings coincide with the TOD counters. The programmable real-time
interrupt timer is designed to support application software by providing a fully programmable event timer
that generates real-time interrupts to the interrupt controller. In addition, the RTC contains a 2-second
watchdog timer and a minute stopwatch.
The RTC can generate 15 event-related interrupts producing three level 4 interrupts to the interrupt
controller: a watchdog interrupt, a real-time interrupt, and an RTC interrupt. Each interrupt produced by
the RTC, both internally and externally, can be individually enabled or disabled in the real-time interrupt
enable register. The mapping of the RTC internal interrupts to the interrupt controller is shown in
Table 11-1.
The watchdog timer and the entire RTC can also be enabled and disabled. In the following descriptions it is
assumed that the real-time clock enable (RTCEN) bit in the real-time control register is set (default),
enabling the RTC.
11.1.1 Prescaler
The prescaler divides the CLK32 reference clock down to 1 pulse per second, resulting in a signal labeled
1HZ. After an initial power up, the CLK32 signal is always available, even when the unit is in a reduced
power mode. See Section 4.3.1, “CLK32 Clock Signal,” on page 4-4 for more information about the
CLK32; see Section 4.5, “Introduction to the Power Control Module,” on page 4-10 for detailed
information on the power modes of the MC68VZ328. The actual frequency of the CLK32 is determined by
the external crystal used as the crystal oscillator. The MC68VZ328 supports either a 32.768 kHz or a
38.4 kHz frequency crystal.
NOTE:
If a 38.4 kHz crystal is used as the crystal oscillator, the REFREQ bit in the
real-time control register (RTCCTL) must be set. Failure to set this bit will
make the RTC timing incorrect.
Table 11-1. RTC Interrupt Mapping
Internal Name Interrupt Controller Resolution
Real-time interrupt Real-time interrupt Eight different rates
Stopwatch Real-time clock Minutes
1HZ Real-time clock Seconds
MIN Real-time clock Minutes
HR Real-time clock Hours
DAY Real-time clock Days
ALM Real-time clock Seconds
Watchdog Watchdog Minutes