BQ4285_14 Datasheet, PDF (6/29 Pages) Texas Instruments – Real-Time Clock (RTC) With NVRAM Control

bq4285
Square-Wave Output
The bq4285 divides the 32.768kHz oscillator frequency
to produce the 1 Hz update frequency for the clock and
calendar. Thirteen taps from the frequency divider are
fed to a 16:1 multiplexer circuit. The output of this mux
is fed to the SQW output and periodic interrupt genera-
tion 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.
Interrupts
The bq4285 allows three individually selected interrupt
events to generate an interrupt request. These three in-
terrupt events are:
s The periodic interrupt, programmable to occur once
every 122 µs to 500 ms
s The alarm interrupt, programmable to occur once per
second to once per day
s 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 bq4285 interrupt
events:
s Enable interrupt events and use the interrupt
request output to invoke an interrupt service routine.
s 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
Register A Bits
Square Wave
Periodic Interrupt
RS3
RS2
RS1
RS0
Frequency Units
Period
Units
0
0
0
0
None
None
0
0
0
1
256
Hz
3.90625
ms
0
0
1
0
128
Hz
7.8125
ms
0
0
1
1
8.192
kHz
122.070
µs
0
1
0
0
4.096
kHz
244.141
µs
0
1
0
1
2.048
kHz
488.281
µs
0
1
1
0
1.024
kHz
976.5625
µs
0
1
1
1
512
Hz
1.95315
ms
1
0
0
0
256
Hz
3.90625
ms
1
0
0
1
128
Hz
7.8125
ms
1
0
1
0
64
Hz
15.625
ms
1
0
1
1
32
Hz
31.25
ms
1
1
0
0
16
Hz
62.5
ms
1
1
0
1
8
Hz
125
ms
1
1
1
0
4
Hz
250
ms
1
1
1
1
2
Hz
500
ms
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