Hardware Reference
In-Depth Information
chipset South Bridge or I/O Controller Hub (ICH) component, meaning no separate chip
is required.
The clock enables software to read the date and time and preserves the date and time
data even when the system is powered off or unplugged. The NVRAM portion of the
chip has another function: It is designed to store the basic system configuration, including
the amount of memory installed, types of disk drives installed, PnP device configuration,
power-on passwords, and other information. Although some chips have been used that
store up to 4KB or more of NVRAM, most motherboard chipsets with integrated RTC/
NVRAMincorporate256bytesofNVRAM,ofwhichtheclockuses14bytes.Thesystem
reads this information every time you power it on.
Modern CMOS Batteries
Motherboard NVRAM (CMOS RAM) batteries come in many forms. Most are of a lithi-
um design because they last 2-5 years or more. I have seen systems with conventional al-
kaline batteries mounted in a holder; these are much less desirable because they fail more
frequently and do not last as long. Also, they are prone to leak, and if a battery leaks on
the motherboard, the motherboard can be severely damaged. By far, the most commonly
usedbatteryformotherboardstodayisthecoincell,mountedinaholderthatispartofthe
motherboard.Twomaintypesofcoincellsareused,differingintheirchemistry.Mostuse
a manganese dioxide (Mn02) cathode, designated by a CR prefix in the part number; oth-
ersuseacarbonmonoflouride(CF)cathode,designatedbyaBRprefixinthepartnumber.
The CR types are more plentiful (and thus easier to get) and offer slightly higher capacity.
The BR types are useful for higher-temperature operation (above 60°C or 140°F).
Because the CR series is cheaper and easier to obtain, it is generally what you will find
in a PC. The other digits in the battery part number indicate the physical size of the bat-
tery. For example, the most common type of lithium coin cell used in PCs is the CR2032,
which is 20mm in diameter (about the size of a quarter) and 3.2mm thick and uses a man-
ganese dioxide cathode. These are readily available at electronics supply stores, camera
shops,andevendrugstores.
Figure18.41
showsacutawayviewofaCR2032lithiumcoin
cell battery.
Figure 18.41
Cutaway view of a CR2032 lithium coin cell.
