AN499| Application Note

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Application Note 90 Using the Multiplex Bus RTC Extended Features

DESCRIPTION
This application note discusses the extended features for the following Real Time Clocks (RTCs): DS1685/87, DS1688/91, DS1689/93, DS17285/87, DS17485/87 and DS17885/87. Also included is an initialization flow chart. All of these devices incorporate these extended features unless otherwise noted.

EXTENDED FEATURES AUXILIARY BATTERY INPUT (VBAUX)
The auxiliary battery input, VBAUX, provides the power required to use the and the 32 kHz square wave in the absence of system power. The timekeeping and RAM data can also be maintained by this power source. The ABE bit, Bit 7 in control register 48h must be set to a logic 1, to enable VBAUX to power the 32 kHz square wave and kickstart input in the absence of VCC. If this input is not going to be used then ABE must be set to a logic 0 and the VBAUX input tied to ground.

KICKSTART INPUT ( KS )
The kickstart pin is intended to be used in the battery-backed mode in conjunction with the PWR signal providing systems with power management control. A signal providing a ground closure will force the PWR signal to transition low. The KS pin can also be used as an interrupt input while VCC is applied.

POWER ON OUTPUT ( PWR )
This output is open drain and requires an external pull up resistor for proper operation. This signal is intended to be used in conjunction with KS and the wakeup alarm for power management features.

32 KHz SQUARE WAVE OUTPUT (SQW)
The SQW output pin can provide a 32 kHz square waves for power management purposes. The DS1685/87, DS1688/91, DS17285/87, DS17485/87, and DS17885/87 will provide the 32 kHz each time the system power, VCC, is applied. The DS1689/93 must be programmed to output the 32 kHz after the system power is applied.

SILICON SERIAL NUMBER
A unique 64 bit silicon serial number is located in bank 1 registers 40h 47h. The serial number is divided into three separate parts. The first byte, location 40h, contains a model number to identify the device type and revision. The following is a list of the model numbers currently assigned for the devices mentioned above:

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