PC87591L datasheet(371/437 Pages) NSC

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Host-Controlled Modules and Host Interface (Continued)

Revision 1.07

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Century Register (CEN)

Location: Programmable Index

Type: R/W

6.2.16 BCD and Binary Formats

6.2.17 Usage Hints

1. Read bit 7 of CRD register at each system power-up to validate the contents of the RTC registers and the CMOS RAM.

When this bit is 0, the contents of these registers 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 minimum recommended battery voltage,

2.4V. Although the RTC oscillator may function properly and the register contents may be correct at lower than 2.4V, this

bit is reset since correct functionality cannot be guaranteed. System BIOS may use a checksum method to revalidate

the contents of the CMOS-RAM. The checksum 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

3. A rechargeable NiCd battery may be used instead of a non-rechargeable Lithium battery. This is the preferred solution

for portable systems, where small components are essential.

4. A supercap capacitor may be used instead of the normal Lithium battery. In a portable system, typically, the VSB voltage

is always present because the power management stops the system before its voltage falls to low. A supercap capacitor

in the range of 0.047F-0.47F should be able to supply the power during a battery replacement.

6.2.18 RTC General-Purpose RAM Map

Table 68. Standard RAM Map

Table 69. Extended RAM Map

Bit

76543210

Name

Century Data

Reset

00000000

Bit

Description

7-0

Century Data. Values may be 00 to 99 in BCD format or 00 to 63 in binary format.

Parameter

BCD Format

Binary Format

Seconds

00 to 59

00 to 3B

Minutes

00 to 59

00 to 3B

Hours

12-hour mode:01 to 12 (a.m.); 81 to 92 (p.m.)

24-hour mode:00 to 23

12-hour mode:01 to 0C (a.m.); 81 to 8C (p.m.)

24-hour mode:00 to 17

Day

01 to 07 (Sunday = 01)

01 to 07

Date

01 to 31

01 to 1F

Month

01 to 12 (January = 01)

01 to 0C

Year

00 to 99

00 to 63

Century

00 to 99

00 to 63

Index

Description

0E16 -7F16

1. Battery-backed 111-byte RAM (114

− 3 overlaid registers).

Battery-backed general-purpose 111-byte RAM.

Index

Description

0016 -7F16

Battery-backed general-purpose 128-byte RAM.

PC87591L Datasheet(PDF) 371 Page - National Semiconductor (TI)

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