DS17885 Dallas, DS17885 Datasheet - Page 17

• Manufacturer Part Number: DS17885
• Description: (DS17886 / DS17887) 3V/5V Real-Time Clock
• Manufacturer: Dallas

can be used to determine if and how many RTC writes have occurred since the last time this register was
read.
Auxiliary Battery
The V
kickstart, wake-up, and SQW output features in the absence of V
in order to use these auxiliary features when no V
The auxiliary-battery-enable (ABE; bank 1, register 04BH) bit in extended control register 4B is used to
turn the auxiliary battery on and off for the above functions in the absence of V
battery power is enabled; when cleared to 0, V
In the DS17885/DS17887, this auxiliary battery can be used as the primary backup power source for
maintaining the clock/calendar, user RAM, and extended external RAM functions. This occurs if the
V
the auxiliary features enabled, then V
to be used, it should be grounded and ABE should be cleared to 0.
Wake-Up/Kickstart
The DS17885/DS17887 incorporates a wake-up feature that powers on the system at a predetermined
date and time through activation of the
system to be powered up in response to a low-going transition on the
applied to the V
modem-ring-detect signal. In order to use either the wake-up or the kickstart features, the
DS17885/DS17887 must have an auxiliary battery connected to the V
running, and the countdown chain must not be in reset (Register A DV2, DV1, DV0 = 01X). If DV2,
DV1, and DV0 are not in this required state, the
wake-up condition, while in battery-backed mode.
The wake-up feature is controlled through the wake-up-interrupt-enable bit in extended control register
4B (WIE, bank 1, 04BH). Setting WIE to 1 enables the wake-up feature, clearing WIE to 0 disables it.
Similarly, the kickstart feature is controlled by the kickstart-interrupt-enable bit in extended control
register 4B (KSE, bank 1, 04BH).
A wake-up sequence occurs as follows: When wake-up is enabled through WIE = 1 while the system is
powered down (no V
the date alarm register (bank 1, register 049H). With the date alarm register, the hours, minutes, and
seconds alarm bytes in the clock/calendar register map (bank 0, registers 05H, 03H, and 01H) are also
monitored. As a result, a wake-up occurs at the date and time specified by the date, hours, minutes, and
seconds alarm register values. This additional alarm occurs regardless of the programming of the AIE bit
(bank 0, register B, 0BH). When the match condition occurs, the
This output can be used to turn on the main system power supply that provides V
DS17885/DS17887 as well as the other major components in the system. Also at this time, the wake-up
flag (WF, bank 1, register 04AH) is set, indicating that a wake-up condition has occurred.
BAT
pin is at a lower voltage than V
BAUX
input is provided to supply power from an auxiliary battery for the DS17885/DS17887
CC
pin. As a result, system power can be applied upon such events as a key closure or
CC
voltage), the clock/calendar monitors the current date for a match condition with
BAUX
BAUX
. If the DS17885 is to be backed-up using a single battery with
PWR
should be used and V
BAUX
output pin. In addition, the kickstart feature allows the
CC
PWR
battery power is disabled to these functions.
is applied to the device.
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pin is not driven low in response to a kickstart or
BAT
CC
. This power source must be available
PWR
should be grounded. If V
KS
BAUX
pin is automatically driven low.
pin, without operating voltage
pin, the oscillator must be
CC
. When set to a 1, V
CC
voltage to the
BAUX
is not
BAUX