BQ4285LS datasheet(5/31 Pages) TI1

5 / 31 page

Programming the RTC

The time-of-day, alarm, and calendar bytes can be written

in either the BCD or binary format (see Table 2).

These steps may be followed to program the time, alarm,

and calendar:

Modify the contents of register B:

Write a 1 to the UTI bit to prevent trans-

fers between RTC bytes and user buffer.

Write the appropriate value to the data for-

mat (DF) bit to select BCD or binary format

for all time, alarm, and calendar bytes.

Write the appropriate value to the hour

format (HF) bit.

Write new values to all the time, alarm, and

calendar locations.

Clear the UTI bit to allow update transfers.

On the next update cycle, the RTC updates all 10 bytes

in the selected format.

Square-Wave Output

The bq4285E/L divides the 32.768kHz oscillator fre-

quency to produce the 1 Hz update frequency for the

clock and calendar. Thirteen taps from the frequency di-

vider are fed to a 16:1 multiplexer circuit. The output of

this mux is fed to the SQW output and periodic inter-

rupt generation circuitry. The four least-significant bits

of register A, RS0–RS3, select among the 13 taps (see

Table 3). The square-wave output is enabled by writing

a 1 to the square-wave enable bit (SQWE) in register B.

A 32.768kHz output may be selected by setting

OSC2–OSC0 in register A to 011 while SQWE=1and

32KE = 1.

bq4285E/L

Address

RTC Bytes

Range

Decimal

Binary

Binary-Coded

Decimal

Seconds

0–59

00H–3BH

00H–59H

Seconds alarm

0–59

00H–3BH

00H–59H

Minutes

0–59

00H–3BH

00H–59H

Minutes alarm

0–59

00H–3BH

00H–59H

Hours, 12-hour format

1–12

01H–OCH AM;

81H–8CH PM

01H–12H AM;

81H–92H PM

Hours, 24-hour format

0–23

00H–17H

00H–23H

Hours alarm, 12-hour format

1–12

01H–OCH AM;

81H–8CH PM

01H–12H AM;

81H–92H PM

Hours alarm, 24-hour format

0–23

00H–17H

00H–23H

Day of week (1=Sunday)

1–7

01H–07H

Day of month

1–31

01H–1FH

01H–31H

Month

1–12

01H–0CH

01H–12H

Year

0–99

00H–63H

00H–99H

Table 2. Time, Alarm, and Calendar Formats

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