Rainbow Electronics DS17887 - Manual (Page 17)

DS17885/DS17887
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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
BAUX
input is provided to supply power from an auxiliary battery for the DS17885/DS17887
kickstart, wake-up, and SQW output features in the absence of V
CC
. This power source must be available
in order to use these auxiliary features when no V
CC
is applied to the device.
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
CC
. When set to a 1, V
BAUX
battery power is enabled; when cleared to 0, V
BAUX
battery power is disabled to these functions.
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
BAT
pin is at a lower voltage than V
BAUX
. If the DS17885 is to be backed-up using a single battery with
the auxiliary features enabled, then V
BAUX
should be used and V
BAT
should be grounded. If V
BAUX
is not
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
PWR output pin. In addition, the kickstart feature allows the
system to be powered up in response to a low-going transition on the KS pin, without operating voltage
applied to the V
CC
pin. As a result, system power can be applied upon such events as a key closure or
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
BAUX
pin, the oscillator must be
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
PWR pin is not driven low in response to a kickstart or
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
CC
voltage), the clock/calendar monitors the current date for a match condition with
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
PWR pin is automatically driven low.
This output can be used to turn on the main system power supply that provides V
CC
voltage to the
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.