AMD SC2200 5.5 Real-Time Clock (RTC)

AMD Geode™ SC2200 Processor Data Book 111

SuperI/O Module 32580B

5.5 Real-Time Clock (RTC)

The RTC provides timekeeping and calendar management

capabilities. The RTC uses a 32.768KHz 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

5.5.1 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.

5.5.2 RTC Clock Generation

The RTC uses a 32.768KHz clock signal as the basic

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

plied by the internal oscillator circuit, or by an external

oscillator (see Section 5.5.2.2 "External Oscillator" on page

112).

5.5.2.1 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

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

a listing of the circuit components. The oscillator may be

disabled in certain conditions. See Section 5.5.2.8 "Oscilla-

tor Activity" on page 115 for more details.

Figure 5-5. Recommended Oscillator External

Circuitry

VBAT

CFInternal

External

X32I X32O

C1C2

To other

modules

Battery

CF = 0.1 μF

Table 5-17. Crystal Oscillator Circuit Components

Component Parameters Values Tolerance

Crystal Resonance Frequency 32.768KHz Parallel mode User-defined

Type N-cut or XY-bar

Serial Resistance 40KΩMax

Quality Factor, Q 35000 Min

Shunt Capacitance 2pF Max

Load Capacitance, CL9-13pF

Temperature Coefficient User-defined

Resistor R1Resistance 20MΩ5%

Resistor R2Resistance 120KΩ5%

Capacitor C1Capacitance 3 to 10pF (Note) 5%

Capacitor C2Capacitance 3 to 10pF (Note) 5%

Note: When voltage is applied to the oscillator it may not start to oscillate immediately due to the balanced external circuit. In general

this is not a problem because the oscillator runs all the time (whether system is on or off). In systems where this is not the case, C1 and

C2 should be different by 50% to assure an unbalanced circuit