DS1685/DS1687 3V/5V Real-Time Clocks
SILICON SERIAL NUMBER
A unique 64-bit lasered serial number is located in bank 1, registers 40h to 47h. This serial number is divided into
three parts. The first byte in register 40h contains a model number, 47h, to identify the device type. Registers 41h
to 46h contain a unique binary number. Register 47h contains a CRC byte used to validate the data in registers
40h to 46h. All 8 bytes of the serial number are read-only registers.
The DS1685/DS1687 is manufactured such that no two devices contain an identical number in locations 41h to
47h.
CENTURY COUNTER
A register has been added in bank 1, location 48H, to keep track of centuries. The value is read in either binary or
BCD according to the setting of the DM bit.
AUXILIARY BATTERY
The VBAUX 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 VCC. This power source must be available in order to use these
auxiliary features when no VCC is applied to the device.
The auxiliary-battery enable (ABE; bank 1, register 04BH) bit in extended control register 4B is used to turn on and
off the auxiliary battery for the above functions in the absence of VCC. When set to a 1, VBAUX battery power is
enabled, and when cleared to 0, VBAUX 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 VBAT pin is at a lower voltage
than VBAUX. If the DS1685 is to be backed-up using a single battery with the auxiliary features enabled, then VBAUX
should be used and VBAT should be grounded. If VBAUX 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 VCC 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 4B (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 4B (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 VCC 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 VCC 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 tKSPW. 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.
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DALLAS [ DALLAS SEMICONDUCTOR ]
