INTEL82801 Datasheet, PDF (92/414 Pages) Intel Corporation – 82801AB (ICH0) I/O Controller Hub

Functional Description
Using RTCRST# to clear CMOS
A jumper on RTCRST# can be used to clear CMOS values, as well as reset to default, the state of
those configuration bits that reside in the RTC power well. When the RTCRST# is strapped to
ground, the RTC_PWR_STS bit (D31:F0:A4h bit 2) is set and those configuration bits in the RTC
power well are set to their default state. BIOS can monitor the state of this bit, and manually clear
the RTC CMOS array once the system is booted. The normal position would cause RTCRST# to be
pulled up through a weak pull-up resistor. The following table shows which bits are set to their
default state when RTCRST# is asserted
.
Table 5-26. Configuration Bits Reset By RTCRST# Assertion
Bit Name
Default State
Register
Location
Bit(s)
CPU_BIST_EN
(ICH 82801AA only)
0
General Status
D31:F0:D7-D4
12
FREQ_STRAP[3:0]
0Fh
General Status
D31:F0:D7-D4
11:8
undefined RTC Register A
I/O space
ALL
PIE, UIE, SQWE
0, 0, 0
RTC Register B
I/O space
6,4,3
IRQF, PF, AF, UF
0, 0, undef, 0 RTC Register C
I/O space
7,6,5,4
VRT, Date Alarm
1, undefined RTC Register D
I/O space
7, 5:0
RTC_PWR_STS
0
General PM Configuration 3
D31:F0:A4
2
PWR_FLR
0
General PM Configuration 3
D31:F0:A4
1
AFTERG3_EN
0
General PM Configuration 3
D31:F0:A4
0
PRBTNOR_STS
0
Power Management 1 Status
D31:F0 PMBase+0h
11
PME_EN, RI_STS
0, 0
General Purpose Event 0 Enable D31:F0 PMBase+2Ah 11,8
NEW_CENTURY_STS
0
TCO1 Status
D31:F0 TCOBase+04h 7
INTRD_DET
0
TCO2 Status
D31:F0 TCOBase+06h 0
Using a GPI to Clear CMOS
A jumper on a GPI can also be used to clear CMOS values. BIOS would detect the setting of this
GPI on system boot-up, and manually clear the CMOS array.
Using the SAFEMODE Strap to Clear CMOS
A jumper on AC_SDOUT (SAFEMODE strap) can also be used to clear CMOS values. BIOS
would detect the setting of the SAFE_MODE status bit (D31:F0: Offset D4h bit 2) on system boot-
up, and manually clear the CMOS array.
Note:
Both the GPI & SAFEMODE strap techniques to clear CMOS require multiple steps to implement.
The system is booted with the jumper in new position, then powered back down. The jumper is
replaced back to the normal position, then the system is rebooted again. The RTCRST# jumper
technique allows the jumper to be moved and then replaced, all while the system is powered off.
Then, once booted, the RTC_PWR_STS can be detected in the set state.
Note: Clearing CMOS, using a jumper on VCCRTC, must NOT be implemented.
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82801AA and 82801AB Datasheet