Rainbow Electronics DS1543 - Manual (Page 4)

DS1543
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active provided that care is taken with the data bus to avoid bus contention. If OE is low prior to WE
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 5-volt 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
(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 3.3-volt 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
BAT
, 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
BAT
, the device power is switched from V
CC
to the internal backup lithium battery when
V
CC
drops below V
BAT
. 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 DS1543 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 DS1543 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 DS1543 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. Actual life
expectancy of the DS1543 will be much longer than 10 years since no internal battery energy is
consumed when V
CC
is present. In fact, in most applications, the life expectancy of the DS1543 will be
approximately equal to the shelf life (expected useful life of the internal lithium battery with no load
attached) of the battery which may prove to be as long as 20 years.
INTERNAL BATTERY MONITOR
The DS15433 constantly monitors the battery voltage of the internal batter. The Battery Low Flag (BLF)
bit of the Flags register (B4 of 1FF0h) is not writable and should always be a 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 level of V
CC
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 a period of 40 ms to 200 ms. The power-on reset function is
independent of the RTC oscillator and thus is operational whether or not the oscillator is enabled.
CLOCK OPERATIONS
Table 2 and the following paragraphs describe the operation of RTC, Alarm, and Watchdog functions.