BQ3285E Datasheet, PDF (6/29 Pages) Texas Instruments – Real-Time Clock RTC

bq3285E/L
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 = 1 and
32KE = 1.
Interrupts
The bq3285E/L allows three individually selected inter-
rupt events to generate an interrupt request. These
three interrupt events are:
n The periodic interrupt, programmable to occur once
every 122µs to 500ms.
n The alarm interrupt, programmable to occur once per
second to once per day, is active in battery-backup
mode, providing a “wake-up” feature.
n 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:
n Enable interrupt events and use the interrupt request
output to invoke an interrupt service routine.
n 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.
Table 3. Square-Wave Frequency/Periodic Interrupt Rate
OSC2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
OSC1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
Register A Bits
OSC0 RS3 RS2
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
0
0
1
0
0
1
0
1
0
0
1
0
0
1
0
0
1
0
0
1
1
0
1
1
0
1
1
0
1
1
0
1
1
X
X
RS1
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
X
Square Wave
RS0 Frequency Units
0
None
1
256
Hz
0
128
Hz
1
8.192
kHz
0
4.096
kHz
1
2.048
kHz
0
1.024
kHz
1
512
Hz
0
256
Hz
1
128
Hz
0
64
Hz
1
32
Hz
0
16
Hz
1
8
Hz
0
4
Hz
1
2
Hz
X
32.768
kHz
Periodic Interrupt
Period
Units
None
3.90625
ms
7.8125
ms
122.070
µs
244.141
µs
488.281
µs
976.5625
µs
1.95315
ms
3.90625
ms
7.8125
ms
15.625
ms
31.25
ms
62.5
ms
125
ms
250
ms
500
ms
same as above defined
by RS3–RS0
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