SC1100 datasheet(91/348 Pages) NSC

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SuperI/O Module (Continued)

4.5.4.1

Usage Hints

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 min-

imum 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.

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 VBAT, the battery may be

changed in backup mode.

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.

A supercap capacitor may be used instead of the nor-

mal Lithium battery. In a portable system usually the

VSB voltage is always present since the power man-

agement stops the system before its voltage falls too

low. The supercap capacitor in the range of 0.047-

0.470 F should supply the power during the battery

replacement.

4.5.5

RTC General-Purpose RAM Map

Table 4-22. Standard RAM Map

Index

Description

0Eh-7Fh

Battery-backed general-purpose 111-

byte RAM.

Table 4-23. Extended RAM Map

Index

Description

00h-7Fh

Battery-backed general-purpose 128-

byte RAM.

SC1100 Datasheet(PDF) 91 Page - National Semiconductor (TI)

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