DS1558

· Integrated real-time clock (RTC), power- fail control circuit, and NV RAM controller
· Clock registers are accessed identically to the static RAM; these registers are resident in the 16 top RAM locations
· Century register
· Greater than 10 years of timekeeping and data retention in the absence of power with small lithium coin cell(s) and low-leakage SRAM
· Precision power-on reset
· Programmable watchdog timer and RTC alarm
· BCD-coded year, month, date, day, hours,minutes, and seconds with automatic leapyear compensation valid up to the year 2100
· Battery voltage- level indicator flag
· Power- fail write protection allows for ±10% V_CC power-supply tolerance
· Underwriters Laboratory (UL) recognized

Voltage Range on Any Pin Relative to Ground.......... -0.3V to +6.0V
Storage Temperature Range ..................................-55 to +125
Soldering Temperature Range ..............See IPC/JEDEC J-STD-020A

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

The DS1558 is a full function, year 2000-compliant (Y2KC), real-time clock/calendar with an RTC alarm, watchdog timer, power-on reset, battery monitor, and NV SRAM controller. User access to all registers within the DS1558 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 DS1558 maps the RTC registers into the SRAM address space and constantly monitors A0A18.When any of the upper 16 address locations are accessed, the DS1558 inhibits CER and OER to the SRAM, and redirects reads and writes to the RTC registers within the DS1558. The DS1558 can be used with SRAMs up to 524,272 addresses. Smaller SRAMs can be used, provided that the unused upper address lines on the DS1558 are connected to V_CC.

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; this occurs regardless of external register settings to guarantee that accurate RTC information is always maintained.

The DS1558 has interrupt ( IRQ /FT) and reset ( RST ) outputs that 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 it can be programmed to occur when in the battery-backed state to serve as a system wake-up. The IRQ /FT output can also be used as a CPU watchdog timer. CPU activity is monitored and an interrupt or reset output are activated if the correct activity is not detected within programmed limits. The DS1558 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 DS1558 also contains its own power-fail circuitry, which automatically protects the data in the clock and SRAM against out-of-tolerance V_CCI conditions by inhibiting the CE input when the V_CC supply enters an out-of-tolerance condition. When V_CCI goes below the level of VBAT, the external battery is switched on to supply energy to the clock and the external SRAM. This feature provides a high degree of data security during unpredictable system operation brought on by low V_CC levels.