Intel Xeon Processor and Intel E7500/E7501Chipset Compatible Platform Design Guide

Intel
®
Xeon™ Processor and Intel
®
E7500/E7501 Chipset Compatible Platform Design Guide 153
I/O Controller Hub 3 (Intel
®
ICH3-S)
9.6 Real Time Clock (RTC)
The ICH3-S contains a real time clock (RTC) with 256 bytes of battery-backed SRAM. The
internal RTC module provides two key functions: keeping date and time, and storing system data in
its RAM when the system is powered down.
The ICH3-S uses a crystal circuit that generates a low-swing 32 kHz input sine wave. The RTCX1
input is amplified and driven back to the crystal circuit via the RTCX2 signal. Internal to the
ICH3-S, the RTCX1 signal is amplified to drive internal logic as well as generate a free running
full swing clock output for system use illustrated in Figure 9-9. This ICH-S output ball is called
SUSCLK.
For further information on the RTC, consult Intel application note AP-728 Intel
®
ICH Family Real
Time Clock (RTC) Accuracy and Considerations Under Test Conditions
(http://developer.intel.com/design/chipsets/applnots/292276.htm).
This section presents the recommended hookup for the RTC circuit for the ICH3-S.
Even if the ICH3-S internal RTC is not used, it is still necessary to supply clock inputs to RTCX1
and RTCX2 pins of the ICH3-S because other signals are gated with that clock in suspend modes.
However, in this case the frequency (32.768 kHz) of the clock inputs is not critical. A lower-cost
crystal can be used, or a single clock input can be driven into the RTCX1 pin with the RTCX2 pin
left as no connect; Figure 9-10 illustrates this. This is not a validated configuration with ICH3-S.
Figure 9-9. RTCX1 and SUSCLK Relationship
Intel
®
ICH3-S
Internal
Oscillator
RTCX1
SUSCLK
Low-Swing 32.768kHz
Sine Wave Source
Full-Swing 32.768kHz
Output Signal
Figure 9-10. RTC Connection When Not Using Internal RTC
5 MΩ
No
Connection
Internal
External
32 KHz
RTCX1 RTCX2