
AMD Geode SC1200/SC1201 Processor Data Book
125
SuperI/O Module
Revision 7.1
5.5
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
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 32.768 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
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
Figurea listing of the circuit components. The oscillator may be
Figure 5-5. Recommended Oscillator External
Circuitry
VBAT
CF
Internal
External
R2
X32I
X32O
C1
C2
R1
Y
To other
modules
Battery
B1
CF = 0.1 F
Table 5-17. 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
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
5%
Resistor R2
Resistance
120 K
5%
Capacitor C1
Capacitance
3 to 10 pF
5%
Capacitor C2
Capacitance
3 to 10 pF
5%