Hardware Reference
In-Depth Information
up to a week or more. If the system remains unplugged for longer than that, the NVRAM
informationislost.Inthatcase,thesesystemscanreloadtheNVRAMfromabackupkept
in a special flash ROM chip contained on the motherboard. The only pieces of informa-
tion that are actually missing when you repower the system are the date and time, which
you have to re-enter. By using the capacitor combined with an NVRAM backup in flash
ROM, these systems have a reliable solution that lasts indefinitely.
Manyoldersystemsuseaseparatebatterythatplugsinviaacableorthatcanevenbedir-
ectly soldered into the motherboard (mostly older, obsolete systems). For those older sys-
tems with the battery soldered in, a spare battery connector usually exists on the mother-
board where you can insert a conventional plug-in battery if the original ever fails.
CMOS Battery Troubleshooting
Symptomsthatindicatethatthebatteryisabouttofailincludehavingtoresettheclockon
your PC every time you shut down the system (especially after moving it) and problems
duringthesystem'sPOST,suchasdrive-detectiondifficulties.Ifyouexperienceproblems
such as these, you should make note of your system's CMOS settings and replace the bat-
tery as soon as possible.
Caution
When you replace a PC battery, be sure you get the polarity correct; otherwise, you will
damage the RTC/NVRAM (CMOS) chip, which is normally integrated into the motherboard
chipset. Because the chip is soldered onto most motherboards, this can be an expensive mis-
take! The coin cell battery holder on the motherboard is normally designed so that the pos-
itive of the battery should be facing up. Older motherboards may use a plug-in battery, the
connections for which are normally keyed.
When you replace a battery, in most cases the existing data stored in the NVRAM is
lost. Sometimes, however, the data remains intact for several minutes (I have observed
NVRAM retain information with no power for an hour or more), so if you make the bat-
tery swap quickly, the information in the NVRAM might be retained. Just to be sure, I
recommendthatyourecordallthesystemconfigurationsettingsstoredintheNVRAMby
your system Setup program. In most cases, you should run the BIOS Setup program and
copy or print out all the screens showing the various settings. Some Setup programs offer
the capability to save the NVRAM data to a file for later restoration if necessary.
Tip
If your system BIOS is password-protected and you forget the password, one possible way
to bypass the block is to remove the battery for a few minutes and then replace it. This resets
the BIOS to its default settings, removing the password protection.
