Motorola MC68VZ328 11.1 RTC Overview

11-2 MC68VZ328 User’s Manual 

RTC Overview

11.1 RTC Overview

The prescaler uses the CLK32 clock to create a 1Hz clock used by all of the blocks in the RTC, as shown

in Figure11-1 on page 11-2. The 1 Hz signal is used to incr ement 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 i ndividually enabled or disabled in the real-time interrupt

enable register. The mapping of the RTC interna l 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 f ollowing d escript ions it is

assumed that the real-time clock enable (RTCEN) bit in the real-time control register is set (default),

enabling the RTC.

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 Section4.3.1, “CLK32 Clock Signal,” on page 4-4 for more information about the

CLK32; see Section4.5, “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 determi ned by

the external crystal used as the crystal oscillator. The MC68VZ328 supports either a 32.768kHz 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 Re al-time clock Hours

DAY Real-time clock Days

ALM Real-time clock Seconds

Watchdog Watchdog Minutes