SC2200 datasheet(114/433 Pages) NSC

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114

Revision 3.0

SuperI/O Module (Continued)

4.5

REAL-TIME CLOCK (RTC)

The RTC provides timekeeping and calendar management

capabilities. The RTC uses a 32.768 KHz signal as the

basic clock for timekeeping. It 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 VSB

— Internal power monitoring on the VRT bit

— Oscillator disabling to save battery during storage

• Software compatible with the DS1287 and MC146818

4.5.1

Bus Interface

The RTC function is initially mapped to the default SuperI/O

locations at Indexes 70h to 73h (two Index/Data pairs).

These locations may be reassigned, in compliance with

Plug and Play requirements.

4.5.2

RTC Clock Generation

The RTC uses a 32.768 KHz clock signal as the basic

clock for timekeeping. The 32.768 KHz clock can be sup-

plied by the internal oscillator circuit, or by an external

oscillator (see Section 4.5.2.2 "External Oscillator" on page

115).

4.5.2.1

Internal Oscillator

The internal oscillator employs an external crystal con-

nected to the on-chip amplifier. The on-chip amplifier is

accessible on the X32I input and X32O output. See Figure

4-5 for the recommended external circuit and Table 4-17 for

a listing of the circuit components. The oscillator may be

disabled in certain conditions. See Section 4.5.2.8 "Oscilla-

tor Activity" on page 118" for more details.

Figure 4-5. Recommended Oscillator External

Circuitry

VBAT

Internal

External

X32I

X32O

To other

modules

Battery

CF =0.1 µF

Table 4-17. Crystal Oscillator Circuit Components

Component

Parameters

Values

Tolerance

Crystal

Resonance Frequency

32.768 KHz Parallel mode

User-defined

Type

N-cut or XY-bar

Serial Resistance

40 K

Ω

Max

Quality Factor, Q

35000

Min

Shunt Capacitance

2 pF

Max

Load Capacitance, CL

9-13 pF

Temperature Coefficient

User-defined

Resistor R1

Resistance

20 M

Ω

Resistor R2

Resistance

120 K

Ω

Capacitor C1

Capacitance

3 to 10 pF

Capacitor C2

Capacitance

3 to 10 pF

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