DS1553 Datasheet, PDF (4/19 Pages) Maxim Integrated Products – 64kB, Nonvolatile, Year-2000-Compliant Timekeeping RAM

DS1553 64kB, Nonvolatile, Year-2000-Compliant Timekeeping RAM
transitioning low, the data bus can become active with read data defined by the address inputs. A low
transition on WE will then disable the outputs t_WEZ after WE goes active.
DATA RETENTION MODE
The 5V device is fully accessible, and data can be written and read only when V_CC is greater than V_PF.
However, when V_CC is below the power-fail point (V_PF)—the point at which write protection occurs—the
internal clock registers and SRAM are blocked from any access. When V_CC falls below the battery switch
point V_SO (battery supply level), device power is switched from the V_CC pin to the internal backup lithium
battery. RTC operation and SRAM data are maintained from the battery until V_CC is returned to nominal
levels.
The 3.3V device is fully accessible and data can be written and read only when V_CC is greater than V_PF.
When V_CC falls below V_PF, access to the device is inhibited. If V_PF is less than V_SO, the device power is
switched from V_CC to the internal backup lithium battery when V_CC drops below V_PF. If V_PF is greater
than V_SO, the device power is switched from V_CC to the internal backup lithium battery when V_CC drops
below V_SO. RTC operation and SRAM data are maintained from the battery until V_CC is returned to
nominal levels.
All control, data, and address signals must be powered down when V_CC is powered down.
BATTERY LONGEVITY
The DS1553 has a lithium power source that is designed to provide energy for the clock activity and
clock and RAM data retention when the V_CC supply is not present. The capability of this internal power
supply is sufficient to power the DS1553 continuously for the life of the equipment in which it is
installed. For specification purposes, the life expectancy is 10 years at +25°C with the internal clock
oscillator running in the absence of V_CC. Each DS1553 is shipped from Dallas Semiconductor with its
lithium energy source disconnected, guaranteeing full energy capacity. When V_CC is first applied at a
level greater than V_PF, the lithium energy source is enabled for battery backup operation.
INTERNAL BATTERY MONITOR
The DS1553 constantly monitors the battery voltage of the internal battery. The Battery Low Flag (BLF)
bit of the Flags register (B4 of 1FF0h) is not writeable and should always be 0 when read. If a 1 is ever
present, an exhausted lithium energy source is indicated, and both the contents of the RTC and RAM are
questionable.
POWER-ON RESET
A temperature-compensated comparator circuit monitors the V_CC level. When V_CC falls to the power-fail
trip point, the RST signal (open drain) is pulled low. When V_CC returns to nominal levels, the RST signal
continues to be pulled low for 40ms to 200ms. The power-on reset function is independent of the RTC
oscillator and is therefore operational whether or not the oscillator is enabled.
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