DS17885 datasheet(17/38 Pages) DALLAS

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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 VBAUX 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 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 the auxiliary battery on and off for the above functions in the absence of VCC. When set to a 1, VBAUX

battery power is enabled; when cleared to 0, VBAUX 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

VBAT pin is at a lower voltage than VBAUX. If the DS17885 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 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 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

DS17885/DS17887 must have an auxiliary battery connected to the VBAUX 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

A wake-up sequence occurs as follows: When wake-up is enabled through 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). 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 VCC 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.

DS17885 数据表(PDF) 17 Page - Dallas Semiconductor

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