DS1557

· Integrated NV SRAM, real time clock, crystal, power-fail control circuit and lithium energy source
· Clock registers are accessed identically to the static RAM; these registers are resident in the 16 top RAM locations
· Century byte register; i.e., Y2K complaint
· 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% V_CC power supply tolerance
· Lithium energy source is electrically disconnected to retain freshness until power is applied for the first time

Voltage on Any Pin Relative to Ground .........-5.0V to +6.0V
Operating Temperature ...................................0°C to 70°C
Storage Temperature ...............................-55°C to +125°C
Soldering Temperature .....................260°C for 10 seconds 

* This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operation sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods of time may affect reliability.

The DS1557 is a full function, year 2000-compliant (Y2KC), real-time clock/calendar (RTC) with a RTC alarm, watchdog timer, power-on reset, battery monitor, and 512k x 8 non-volatile static RAM. User access to all registers within the DS1557 is accomplished with a bytewide 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 DS1557 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 are continuously updated; this occurs regardless of external registers settings to guarantee that accurate RTC information is always maintained.

The DS1557 has interrupt ( IRQ /FT) and reset ( RST ) outputs which can be used to control CPU activity. The 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 IRQ /FT or 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 DS1557 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 RST output is used for this function.

The DS1557 also contains its own power-fail circuitry which automatically deselects the device when the V_CC supply enters an out of tolerance condition. This feature provides a high degree of data security during unpredictable system operation brought on by low V_CC levels.