AMD SC2200 User Manual

32580B

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

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.

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
112).

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.

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

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, C

L

9-13 pF

Temperature Coefficient

User-defined

Resistor R

1

Resistance

20 M

Ω

5%

Resistor R

2

Resistance

120 K

Ω

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 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