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3.3V Single-Piece 16Mb Nonvolatile SRAM
with Clock
Using the Watchdog Timer
The watchdog timer can be used to detect an out-of-
control processor. The user programs the watchdog
timer by setting the desired timeout delay into the
WATCHDOG register. The five high-order WATCHDOG
register bits store a binary multiplier and the two lower-
order WATCHDOG bits select the resolution, where 00
= 1 / 16 second, 01 = 1 / 4 second, 10 = 1 second, and 11
= 4 seconds. The watchdog timeout value is then
determined by multiplication of the 5-bit multiplier value
with the 2-bit resolution value. (For example: writing
00001110 (0Eh) into the WATCHDOG register = 3 x 1
second, or 3 seconds.) If the processor does not reset
the timer within the specified period, the watchdog flag
(WF) is set to a 1 and a processor interrupt is generat-
ed and stays active until either WF is read or the
WATCHDOG register is read or written.
The MSB of the WATCHDOG register is the watchdog
steering bit (WDS). When WDS is set to a 0, the watch-
dog activates the IRQ /FT output when the watchdog
times out. WDS should not be written to a 1, and should
be initialized to a 0 if the watchdog function is enabled.
The watchdog timer resets when the processor per-
forms a read or write of the WATCHDOG register. The
timeout period then starts over. The watchdog timer is
disabled by writing a value of 00h to the WATCHDOG
register. The watchdog function is automatically dis-
abled upon power-up and the WATCHDOG register is
cleared to 00h.
Clock Accuracy
The DS3070W modules are trimmed at the factory to an
accuracy of 1 minute per month at +25 ° C.
Power-On Default States
Upon each application of power to the device, the fol-
lowing register bits are automatically set to 0:
WDS = 0, BMB0 – BMB4 = 0, RB0 = 0, RB1 = 0, AE = 0,
ABE = 0.
All other RTC bits are undefined.
Data-Retention Mode
The DS3070W provides full functional capability for V CC
greater than 3.0V and write-protects by 2.8V. Data is
maintained in the absence of V CC without additional
support circuitry. The NV SRAM constantly monitors
V CC . Should the supply voltage decay, the NV SRAM
automatically write-protects itself. All inputs become
don ’ t care, and all data outputs become high imped-
ance. As V CC falls below approximately 2.5V (V SW ), the
power-switching circuit connects the lithium energy
source to the clock and SRAM to maintain time and
retain data. During power-up, when V CC rises above
V SW , the power-switching circuit connects external V CC
to the clock and SRAM, and disconnects the lithium
energy source. Normal clock or SRAM operation can
resume after V CC exceeds V TP for a minimum duration
of t REC .
Battery Charging
When V CC is greater than V TP an internal regulator will
charge the battery. The UL-approved charger circuit
includes short-circuit protection and a temperature-sta-
bilized voltage reference for on-demand charging of
the internal battery. Typical data retention expectations
greater than 2 years per charge cycle are achievable.
A maximum of 96 hours of charging time is required to
fully charge a depleted battery.
System Power Monitoring
When the external V CC supply falls below the selected
out-of-tolerance trip point, the output RST is forced
active (low). Once active, the RST is held active until
the V CC supply has fallen below that of the internal bat-
tery. On power-up, the RST output is held active until
the external supply is greater than the selected trip
point and one reset timeout period (t RPU ) has elapsed.
This is sufficiently longer than t REC to ensure that the
RTC and SRAM are ready for access by the micro-
processor.
Freshness Seal and Shipping
The DS3070W is shipped from Dallas Semiconductor
with the RTC oscillator disabled and the lithium battery
electrically disconnected, guaranteeing that no battery
capacity has been consumed during transit or storage.
As shipped, the lithium battery is ~60% charged, and
no pre-assembly charging operations should be
attempted.
When V CC is first applied at a level greater than V TP ,
the lithium battery is enabled for backup operation. The
user is required to enable the oscillator (MSB of SEC-
ONDS register) and initialize the required RTC registers
for proper timekeeping operation. A 96 hour initial bat-
tery charge time is recommended for new system
installations.
Applications Information
Power-Supply Decoupling
To achieve the best results when using the DS3070W,
assure that all V CC and GND balls are connected and
decouple the power supply with a 0.1μF capacitor. Use a
high-quality, ceramic surface-mount capacitor if possible.
16
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