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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DALLAS [ DALLAS SEMICONDUCTOR ]
