PC87413 datasheet(142/257 Pages) NSC

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142

Revision 1.2

8.0

Real-Time Clock (RTC)

8.1

OVERVIEW

The RTC provides timekeeping and calendar management capabilities. It uses a 32.768 KHz signal as the basic clock for

timekeeping. The RTC also includes 242 bytes of battery-backed RAM for general-purpose use.

The RTC provides the following functions:

Accurate timekeeping and calendar management.

Alarm at a predetermined time and/or date.

Three programmable interrupt sources.

Valid timekeeping during power-down by utilizing external battery backup.

242 bytes of battery-backed RAM.

RAM lock schemes to protect its content.

Internal oscillator circuit (the crystal itself is off-chip) or external clock supply for the 32.768 KHz clock.

A century counter.

PnP support:

— Relocatable index and data registers

— Module access enable/disable option

— Host interrupt enable/disable option

Additional low-power features such as:

— Automatic switching from VBAT to VSB

— Internal power monitoring on the VRT bit

— Oscillator disabling to conserve battery power during storage

Software compatible with the DS1287 and MC146818.

8.2

FUNCTIONAL DESCRIPTION

8.2.1

Bus Interface

The RTC function is initially mapped to the default ServerI/O locations at indexes 70h to 73h (two Index/Data pairs). These

locations may be reassigned in compliance with Plug and Play requirements.

8.2.2

RTC Clock Generation

The RTC uses a 32.768 KHz clock signal as the basic clock for timekeeping. The 32.768 KHz clock is supplied by either the

internal oscillator circuit or by an external oscillator (see Sections 8.2.3 and 8.2.4).

8.2.3

Internal Oscillator

The internal oscillator employs an external crystal connected to the on-chip amplifier. The on-chip amplifier is accessible on

the 32KX1 input pin and 32KX2 output pin. See Figure 39 for the recommended external circuit; see Table 35 on page 143

for a listing of the circuit components. The oscillator may be disabled in certain conditions. See Section 8.2.12 on page 147

for more details.

Figure 39. Recommended Oscillator External Circuitry

VBAT

Internal

External

32KX1 /

32KX2

To other

modules

Battery

CF = 0.1

µF

32KCLKIN

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