PC87413 datasheet(159/257 Pages) NSC

159 / 257 page

8.0 Real-Time Clock (RTC) (Continued)

Revision 1.2

159

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8.3.20

BCD and Binary Formats

8.4

USAGE HINTS

1. Read bit 7 of the 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 below the minimum specified battery voltage, VBATMIN. Although the RTC oscillator may func-

tion properly and the register contents may be correct at lower voltages than VBATMIN, this bit is reset because correct

functionality cannot be guaranteed. System BIOS may use a checksum method to revalidate the contents of the CMOS-

RAM. The checksum byte must be stored in the CMOS RAM.

2. To maintain valid time and register information, change the backup battery while normal operating power is on and not

while in Backup mode; however, if a low leakage capacitor is connected to VBAT, the battery can also be changed in

Backup mode.

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 size components is essential.

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

present because the power management stops the system before its voltage falls. The supercap capacitor in the range

of 0.047-0.47F will supply the power during the battery replacement.

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 (AM)

81 to 92 (PM)

24-Hour mode: 00 to 23

12-Hour mode: 01 to 0C (AM)

81 to 8C (PM)

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

PC87413 데이터시트(PDF) 159 Page - National Semiconductor (TI)

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