I/O Controller Hub
132 Design Guide
9.6.5 RTC External RTCRST# Circuit
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 10 ms – 20 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. Because of this, when the system boots, the BIOS knows that the RTC battery has been
removed.
This RTCRST# circuit is combined with the diode circuit (shown in Figure 9-12) whose purpose is
to allow the RTC well to be powered by the battery when the system power is not available.
Figure 9-13 is an example of RTCRST# circuitry that is used in conjunction with the external diode
circuit.
9.6.6 VBIAS DC Voltage and Noise Measurements
VBIAS is a DC voltage level that is 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 9-10); therefore, it is a self-adjusted voltage. Board designers should not manually bias the
voltage level on VBIAS. Checking VBIAS level is used for testing purposes only to determine the
right bias condition of the RTC circuit.
VBIAS should be at least 200 mV DC. The RC network of R2 and C3 filters out most of the AC
signals that exist on this ball. However, the noise on this ball should be kept to a minimum to
guarantee the stability of the RTC oscillation.
Probing VBIAS requires the same technique as probing the RTCX1 and RTCX2 signals (using
Op-Amp). See application note AP-728, Intel
®
ICH Family Real Time Clock (RTC) Accuracy and
Considerations Under Test Conditions, for further details on measuring techniques.
Note: VBIAS is also very sensitive to environmental conditions.
Figure 9-13. RTCRST# External Circuit
VCC_3.3SBY
VCCRTC
1.0 µF
1 kΩ
2.2 µF
8.2 kΩ
RTCRST#
RTCRST#
CIRCUIT
DIODE /
BATTERY
CIRCUIT
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