BQ3285ESS datasheet(6/29 Pages) TI

6 / 29 page

Square-Wave Output

The bq3285E/L divides the 32.768kHz oscillator fre-

quency

to produce the 1Hz 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.

Interrupts

The bq3285E/L allows three individually selected inter-

rupt events to generate an interrupt request.

These

three interrupt events are:

The periodic interrupt, programmable to occur once

every 122

µs to 500ms.

The alarm interrupt, programmable to occur once per

second to once per day, is active in battery-backup

mode, providing a “wake-up” feature.

The update-ended interrupt, which occurs at the end

of each update cycle.

Each of the three interrupt events is enabled by an indi-

vidual interrupt-enable bit in register B. When an event

occurs, its event flag bit in register C is set. If the corre-

sponding event enable bit is also set, then an interrupt

request is generated.

The interrupt request flag bit

(INTF) of register C is set with every interrupt request.

Reading register C clears all flag bits, including INTF,

and makes INT high-impedance.

Two methods can be used to process bq3285E/L interrupt

events:

Enable interrupt events and use the interrupt request

output to invoke an interrupt service routine.

Do not enable the interrupts and use a polling routine

to periodically check the status of the flag bits.

The individual interrupt sources are described in detail

in the following sections.

Square Wave

Periodic Interrupt

OSC2

OSC1

OSC0

RS3

RS2

RS1

RS0

Frequency

Units

Period

Units

0100000

None

0100001

256

3.90625

0100010

128

7.8125

0100011

8.192

kHz

122.070

µs

0100100

4.096

kHz

244.141

µs

0100101

2.048

kHz

488.281

µs

0100110

1.024

kHz

976.5625

µs

0100111

512

1.95315

0101000

256

3.90625

0101001

128

7.8125

0101010

15.625

0101011

31.25

0101100

62.5

0101101

125

0101110

250

0101111

500

XXXX

32.768

kHz

same as above defined

by RS3–RS0

Table 3. Square-Wave Frequency/Periodic Interrupt Rate

bq3285E/L

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