Intel
82801BA ICH2 Datasheet
5-63
Functional Description
5.10.5
Clearing Battery-Backed RTC RAM
Clearing CMOS RAM in an ICH2-based platform can be done by using a jumper on RTCRST# or
GPI or using the SAFEMODE strap. Implementations should not attempt to clear CMOS by using
a jumper to pull VccRTC low.
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
the 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 the 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.
Table 5-31
shows which bits are set to their default state
when RTCRST# is asserted.
Using a GPI to clear CMOS
A jumper on a GPI can also be used to clear CMOS values. BIOS detects 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
detects 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 and SAFEMODE strap techniques to clear CMOS require multiple steps to
implement. The system is booted with the jumper in a 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.
Table 5-31. Configuration Bits Reset By RTCRST# Assertion
Bit Name
Default State
Register
Location
Bit(s)
CPU_BIST_EN
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
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