DS1689 Datasheet, PDF (5/32 Pages) Dallas Semiconductor – 3-Volt/5-Volt Serialized Real-Time Clock with NV RAM Control

DS1689/DS1693
KS (Kickstart Input; active low) - When V_CC is removed from the DS1689/DS1693, the system can be
powered on in response to an active low transition on the KS pin, as might be generated from a key
closure. V_BAUX must be present and Auxiliary Battery Enable bit (ABE) must be set to 1 if the kickstart
function is used, and the KS pin must be pulled up to the V_BAUX supply. While V_CC is applied, the KS pin
can be used as an interrupt input.
RCLR (RAM Clear Input; active low) - If enabled by software, taking RCLR low will result in the
clearing of the 114 bytes of user RAM. When enabled, RCLR can be activated whether or not V_CC is
present.
V_BAUX - Auxiliary battery input required for kickstart and wake-up features. This input also supports
clock/calendar and External NVRAM if V_BAT is at lower voltage or is not present. A standard +3-volt
lithium cell or other energy source can be used. Battery voltage must be held between +2.5 and +3.7 volts
for proper operation. If V_BAUX is not going to be used it should be grounded and auxiliary battery enable
bit bank 1, register 4BH, should=0.
DS1689 ONLY
X1, X2 - Connections for a standard 32.768 kHz quartz crystal. For greatest accuracy, the DS1689 must
be used with a crystal that has a specified load capacitance of either 6 pF or 12.5 pF. The Crystal Select
(CS) bit in Extended Control Register 4B is used to select operation with a 6 pF or 12.5 pF crystal. The
crystal is attached directly to the X1 and X2 pins. There is no need for external capacitors or resistors.
Note: X1 and X2 are very high impedance nodes. It is recommended that they and the crystal by guard-
ringed with ground and that high frequency signals be kept away from the crystal area.
For more information on crystal selection and crystal layout considerations, please consult Application
Note 58, “Crystal Considerations with Dallas Real Time Clocks.” The DS1689 can also be driven by an
external 32.768 kHz oscillator. In this configuration, the X1 pin is connected to the external oscillator
signal and the X2 pin is floated.
V_BAT - Battery input for any standard 3-Volt lithium cell or other energy source. Battery voltage must be
held between 2.5 and 3.7 volts for proper operation.
POWER-DOWN/POWER-UP CONSIDERATIONS
The real-time clock function will continue to operate and all of the RAM, time, calendar, and alarm
memory locations remain nonvolatile regardless of the level of the V_CCI input. When V_CCI is applied to
the DS1689/DS1693 and reaches a level of greater than V_PF (power fail trip point), the device becomes
accessible after t_REC, provided that the oscillator is running and the oscillator countdown chain is not in
reset (see Register A). This time period allows the system to stabilize after power is applied.
When PSEL is floating or logic 0, the DS1689 is in autosense mode and 3-volt or 5-volt operation is
determined based on the voltage on V_CCI. Selection of 5-volt operation is automatically invoked when
V_CCI rises above 4.5 volts for a minimum of t_REC. However, 3-volt operation is automatically selected if
V_CCI does not rise above the level of 4.25 volts. Selection of the power supply input levels requires
150 ms of input stability before operation can commence.
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