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AMD Geode™ SC2200 Processor Data Book 121
SuperI/O Module
32580B
5.5.3.1 Usage Hints
1) Read bit 7 of CRD at each system power-up to vali-
date the contents of the RTC registers and the CMOS
RAM. When this bit is 0, the contents of these regis-
ters and the CMOS RAM are questionable. This bit is
reset when the backup battery voltage is too low. The
voltage level at which this bit is reset is below the mini-
mum recommended battery voltage, 2.4V. Although
the RTC oscillator may function properly and the regis-
ter contents may be correct at lower than 2.4V, this bit
is reset since correct functionality cannot be guaran-
teed. System BIOS may use a checksum method to
revalidate the contents of the CMOS-RAM. The check-
sum byte should be stored in the same CMOS RAM.
2) Change the backup battery while normal operating
power is present, and not in backup mode, to maintain
valid time and register information. If a low leakage
capacitor is connected to V
BAT
, the battery may be
changed in backup mode.
3) A rechargeable NiCd battery may be used instead of a
non-rechargeable Lithium battery. This is a preferred
solution for portable systems, where small size com-
ponents is essential.
4) A supercap capacitor may be used instead of the nor-
mal Lithium battery. In a portable system usually the
V
SB
voltage is always present since the power man-
agement stops the system before its voltage falls to
low. The supercap capacitor in the range of 0.047-
0.47 F should supply the power during the battery
replacement.
5.5.4 RTC General-Purpose RAM Map
Table 5-24. Standard RAM Map
Index Description
0Eh - 7Fh Battery-backed general-purpose 111-
byte RAM.
Table 5-25. Extended RAM Map
Index Description
00h - 7Fh Battery-backed general-purpose 128-
byte RAM.