AMD SC1200 Computer Hardware User Manual


 
AMD Geode™ SC1200/SC1201 Processor Data Book 105
SuperI/O Module
32579B
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 V
SB
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
oscillator (see Section 5.5.2.2 "External Oscillator" on page
106).
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 109 for more details.
Figure 5-5. Recommended Oscillator External
Circuitry
V
BAT
C
F
Internal
External
R
2
X32I
X32O
C
1
C
2
R
1
Y
To other
modules
Battery
B
1
C
F
= 0.1 μF
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 KΩ Max
Quality Factor, Q 35000 Min
Shunt Capacitance 2 pF Max
Load Capacitance, C
L
9-13 pF
Temperature Coefficient User-defined
Resistor R
1
Resistance 20 MΩ 5%
Resistor R
2
Resistance 120 KΩ 5%
Capacitor C
1
Capacitance 3 to 10 pF (Note) 5%
Capacitor C
2
Capacitance 3 to 10 pF (Note) 5%
Note: When voltage is applied to the oscillator it may not start to oscillate immediately due to the balanced external circuit. In gen-
eral 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, C
1
and C
2
should be different by 50% to assure an unbalanced circuit.