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I/O Subsystem
11.8.
RTC
The Intel 82801DBM ICH4-M 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 ICH4-M uses a crystal circuit to generate a low-swing 32 kHz input sine wave. This input is
amplified and driven back to the crystal circuit via the RTCX2 signal. Internal to the ICH4-M, the
RTCX1 signal is amplified to drive internal logic as well as generate a free running full swing clock
output for system use. This output ball of the ICH4-M is called SUSCLK. This is illustrated in Figure
107.
Figure 107. RTCX1 and SUSCLK Relationship in ICH4-M
Low-Swing 32.768 kHz
Sine Wave Source
Full-Swing 32.768 kHz
For further information on the RTC, please consult Application Note AP-728 ICH Family Real Time
Clock (RTC) Accuracy and Considerations Under Test Conditions. This application note is valid for the
ICH4-M.
Even if the ICH4-M internal RTC is not used, it is still necessary to supply a clock input to RTCX1 of
the ICH4-M because other signals are gated off that clock in suspend modes. However, in this case, the
frequency accuracy (32.768 kHz) of the clock inputs is not critical; a cheap crystal can be used or a
single clock input can be driven into RTCX1with RTCX2 left as no connect; Figure 108 illustrates the
connection. This is not a validated feature on the ICH4-M. Please note that the peak-to-peak swing
on RTCX1 cannot exceed 1.0 V.
Figure 108. External Circuitry for the ICH4-M Where the Internal RTC is Not Used
218
Output Signal
RTCX1
32 KHz
5M
RTCX2
Internal
External
No Connection
®
Intel
855GM/855GME Chipset Platform Design Guide
RTCX1
Internal
ICH4-M
Oscillator
SUSCLK
R
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