DS1689_05 Datasheet, PDF (7/36 Pages) Dallas Semiconductor – 3V/5V Serialized Real-Time Clocks with NV RAM Control

DS1689/DS1693
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 3V or 5V operation is
determined based on the voltage on V_CCI. Selection of 5V operation is automatically invoked when V_CCI
rises above 4.5V for a minimum of t_REC. However, 3V operation is automatically selected if V_CCI does not
rise above the level of 4.25V. Selection of the power supply input levels requires 150ms of input stability
before operation can commence.
When 5V operation is selected, the device is fully accessible and data can be written and read only when
V_CCI is greater than 4.5V. When V_CCI is below 4.5V, read and writes are inhibited. However, the
timekeeping function continues unaffected by the lower input voltage. As V_CC falls below the greater of
V_BAT and V_BAUX, the RAM and timekeeper are switched over to a lithium battery connected either to the
V_BAT pin or V_BAUX pin.
When 3V operation is selected and applied within normal limits, the device is fully accessible and data
can be written or read. When V_CCI falls below V_PF, access to the device is inhibited. If V_PF is less than
V_BAT and V_BAUX, the power supply is switched from V_CCI to the backup supply (the greater of V_BAT and
V_BAUX) when V_CCI drops below V_PF. If V_PF is greater than V_BAT and V_BAUX, the power supply is switched
from V_CCI to the backup supply when V_CCI drops below the larger of V_BAT and V_BAUX.
When V_CC falls below V_PF, the chip is write-protected. With the possible exception of the KS, PWR, and
SQW pins, all inputs are ignored and all outputs are in a high impedance state.
RTC ADDRESS MAP
The address map for the RTC registers of the DS1689/DS1693 is shown in Figure 2. The address map
consists of the 14-clock/calendar registers. Ten registers contain the time, calendar, and alarm data, and
four bytes are used for control and status. All registers can be directly written or read except for the
following:
1. Registers C and D are read-only.
2. Bit 7 of Register A is read-only.
3. The high order bit of the seconds byte is read-only.
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