Rainbow Electronics DS1687 - Manual (Page 20)

DS1685/DS1687
20 of 38
AUXILIARY BATTERY
The V
BAUX
input is provided to supply power from an auxiliary battery for the DS1685/DS1687 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 B is used to
turn on and off the auxiliary battery for the above functions in the absence of V
CC
. When set to a 1, V
BAUX
battery power is enabled, and when cleared to 0, V
BAUX
battery power is disabled to these functions.
In the DS1685/DS1687, 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 DS1685 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 DS1685/DS1687 incorporates a wake-up feature that can power the system on 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
DS1685/DS1687 must have an auxiliary battery connected to the V
BAUX
pin and 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 does not drive 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 B
(WIE, bank 1, 04BH). Setting WIE to 1 enables the wake-up feature, clearing WIE to 0 disables it.
Similarly, the kick-start feature is controlled through the kickstart-interrupt-enable bit in extended control
register B (KSE, bank 1, 04BH).
A wake-up sequence occurs as follows: When wake-up is enabled by 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). In conjunction 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 automatically
drives low. This output can be used to turn on the main system power supply, which provides V
CC
voltage
to the DS1685/DS1687 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.
A kickstart sequence occurs when kickstarting is enabled by KSE = 1. While the system is powered
down, the
KS input pin is monitored for a low-going transition of minimum pulse width t
KSPW
. When
such a transition is detected, the PWR line pulls low, as it does for a wake-up condition. Also at this time,
the kickstart flag (KF, bank 1, register 04AH) is set, indicating that a kickstart condition has occurred.
The timing associated with both the wake-up and kickstarting sequences is illustrated in the “Wake-