DS1557 4Meg, Nonvolatile, Y2K-Compliant Timekeeping RAM
2 of 17
ORDERING INFORMATION
PART TEMP RANGE
VOLTAGE
(V)
PIN-PACKAGE TOP MARK**
DS1557P-70+ 0°C to +70°C 5.0 34 PowerCap* DS1557P+70
DS1557P-70IND+ -40°C to +85°C 5.0 34 PowerCap* DS1557P+70 IND
DS1557WP-120+ 0°C to +70°C 3.3 34 PowerCap* DS1557WP+120
DS1557WP-120IND+ -40°C to +85°C 3.3 34 PowerCap* DS1557WP+120 IND
+ Denotes a lead(Pb)-free/RoHS-compliant package.
*DS9034PCX+ or DS9034I-PCX+ (PowerCap) required. Must be ordered separately.
*An “IND” on the top mark denotes an industrial temperature grade device.
DESCRIPTION
The DS1557 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 512k x 8 nonvolatile static RAM. User
access to all registers within the DS1557 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; 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
wakeup. 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.
