AMD SC2200 manual Real-Time Clock RTC, Bus Interface, RTC Clock Generation, Internal Oscillator

SuperI/O Module 32580B

5.5Real-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

5.5.1Bus 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.

5.5.2RTC 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 112).

5.5.2.1Internal 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 5-5for the recommended external circuit and Table 5-17for 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.

VBAT				To other
				modules
CF				Internal
	X32I			External
			R1	X32O
				R2
B1	C	1	Y	C
				2

Battery CF = 0.1 μF

Figure 5-5. Recommended Oscillator External

Circuitry

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, CL	9-13 pF
	Temperature Coefficient	User-defined
Resistor R1	Resistance	20 MΩ	5%
Resistor R2	Resistance	120 KΩ	5%
Capacitor C1	Capacitance	3 to 10 pF (Note)	5%
Capacitor C2	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
AMD Geode™ SC2200 Processor Data Book			111