Intel 6300ESB ICH, Design Manual

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238
Intel
®
855GME Chipset and Intel
®
6300ESB ICH Embedded Platform Design Guide
Intel
®
6300ESB Design Guidelines
9.11.5 RTC External RTCRST# Circuit
The 6300ESB RTC requires some additional external circuitry. The RTCRST# signal is used to
reset the RTC well. The external capacitor and the external resistor between RTCRST# and the
RTC battery (VBAT) were selected to create an RC time delay, such that RTCRST# will go high
some time after the battery voltage is valid. The RC time delay should be in the range of
18-25 ms. When RTCRST# is asserted, bit 2 (RTC_PWR_STS) in the GEN_PMCON_3 (General
PM Configuration 3) register is set to 1, and remains set until software clears it. As a result of this,
when the system boots, the BIOS knows that the RTC battery has been removed.
The RTCRST# signal may also be used to detect a low battery voltage. RTCRST# will be asserted
during a power up from G3 state if the battery voltage is below 2 V. This sets the RTC_PWR_STS
bit as described above. When desired, BIOS may request that the user replace the battery.
This RTCRST# circuit is combined with the diode circuit (shown in Figure 130 ) to allow the RTC
well to be powered by the battery when the system power is not available. Figure 131 is an
example of this circuitry that is used in conjunction with the external diode circuit.
9.11.6 V
BIAS
DC Voltage and Noise Measurements
V
BIAS
is a DC voltage level necessary for biasing the RTC oscillator circuit. This DC voltage level
is filtered out from the RTC oscillation signal by the RC Network of R2 and C3 (see Figure 129 );
therefore, it is a self-adjusted voltage. Board designers should not manually bias the voltage level
on V
BIAS
. Checking V
BIAS
level is used for testing purposes only to determine the right bias
condition of the RTC circuit.
V
BIAS
should be at least 200 mV DC. The RC network of R2 and C3 will filter out most of AC
signal that exist on this ball, however, the noise on this ball should be kept minimal in order to
ensure the stability of the RTC oscillation.
Probing V
BIAS
requires the same technique as probing the RTCX1, RTCX2 signals (using
Op-Amp). See Application Note AP-728 for further details on measuring techniques.
Figure 131. RTCRST# External Circuit for the 6300ESB RTC
B1184-02
1 K
9
V
CC
RTC
V
CCSUS
3_3
1.0 µF
1.0 µF
20 K
9
RTCRST#
CIRCUIT
DIODE/
BATTERY
CIRCUIT
+
–
RTCRST#