The DS1556 is a full-function, year-2000-compliant (Y2KC), real-time clock/calendar (RTC) with an
RTC alarm, watchdog timer, power-on reset, battery monitor, and 128k x 8 nonvolatile static RAM. User
access to all registers within the DS1556 is accomplished with a byte-wide interface as shown in Figure 1.
The RTC registers contain century, year, month, date, day, hours, minutes, and seconds data in 24-hour
BCD format. Corrections for day of month and leap year are made automatically.
The RTC registers are double-buffered into an internal and external set. The user has direct access to the
external set. Clock/calendar updates to the external set of registers can be disabled and enabled to allow
the user to access static data. Assuming the internal oscillator is turned on, the internal set of registers is
continuously updated, which occurs regardless of external registers settings to guarantee that accurate
RTC information is always maintained.
The DS1556 has interrupt (active-low IRQ/FT) and reset (active-low RST) outputs which can be used to control CPU activity.
The active-low IRQ/FT interrupt output can be used to generate an external interrupt when the RTC register values
match user programmed alarm values. The interrupt is always available while the device is powered from
the system supply and can be programmed to occur when in the battery-backed state to serve as a system
wake-up. Either the active-low IRQ/FT or active-low RST outputs can also be used as a CPU watchdog timer, CPU activity is
monitored and an interrupt or reset output will be activated if the correct activity is not detected within
programmed limits. The DS1556 power-on reset can be used to detect a system power down or failure
and hold the CPU in a safe reset state until normal power returns and stabilizes; the active-low RST output is used
for this function.
The DS1556 also contains its own power-fail circuitry, which automatically deselects the device when the
VCC supply enters an out of tolerance condition. This feature provides a high degree of data security
during unpredictable system operation brought on by low VCC levels.
Key Features
Integrated NV SRAM, Real-Time Clock
(RTC), Crystal, Power-Fail Control Circuit,
and Lithium Energy Source
Clock Registers are Accessed Identically to
the Static RAM; These Registers Reside in
the 16 Top RAM Locations
Century Byte Register (i.e., Y2K Compliant)
Totally Nonvolatile with Over 10 Years of
Operation in the Absence of Power
Precision Power-On Reset
Programmable Watchdog Timer and RTC
Alarm
BCD-Coded Year, Month, Date, Day, Hours,
Minutes, and seconds with Automatic Leap-
Year Compensation Valid Up to the Year
2100
Battery Voltage-Level Indicator Flag
Power-Fail Write Protection Allows for ±10%
VCC Power-Supply Tolerance
Lithium Energy Source is Electrically
Disconnected to Retain Freshness Until
Power is Applied for the First Time
Also Available in Industrial Temperature
Range: -40°C to +85°C
Notes: **This pricing is BUDGETARY, for comparing similar parts. Prices are in
U.S. dollars and subject to change. Quantity pricing may vary
substantially and international prices may differ due to local
duties, taxes, fees, and exchange rates. For volume-specific prices
and delivery, please see the price and availability page
or contact an authorized distributor.
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