PC87591L datasheet(356/437 Pages) NSC

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Host-Controlled Modules and Host Interface (Continued)

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356

Revision 1.07

6.2

REAL-TIME CLOCK (RTC)

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 battery to VCC

— Internal power monitoring on the VRT bit

— Oscillator disabling to save battery during storage

●

Software compatible with the DS1287 and MC146818

6.2.1

Bus Interface

The RTC function is initially mapped to the default SuperI/O locations at indexes 7016 to 7316 (two Index/Data pairs). These

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

6.2.2

RTC Clock Generation

The RTC uses a 32.768 KHz clock signal as the basic clock for timekeeping. The 32.768 KHz clock is generated by the in-

ternal oscillator circuit or by an external oscillator (see Sections 6.2.3 and 6.2.4).

6.2.3

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 the 32KX2 output pin. See Figure 113 for the recommended external circuit and Table 65 for a list-

ing of the circuit components. The oscillator may be disabled in certain conditions. See Section 6.2.12 on page 362 for more

details.

Figure 113. Recommended Oscillator External Circuitry

VBAT

Internal

External

32KX1 /

32KX2

To other

modules

Battery

CF = 0.1

µF

32KCLKIN

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