RTCs with Constant-Voltage Trickle Charger
Please consult related application notes for detailed
Detailed Description
information on battery lifetime versus depth of dis-
The DS12R885 is a drop-in replacement for the
charge, and expected product lifetime based upon
DS12885 RTC. The device provides 14 bytes of real-
battery cycles.
time clock/calendar, alarm, and control/status registers
and 114 bytes of nonvolatile, battery-backed static
Oscillator Circuit
RAM. A time-of-day alarm, three maskable interrupts
The DS12R885 uses an external 32.768kHz crystal. The
with a common interrupt output, and a programmable
oscillator circuit does not require any external resistors
square-wave output are available. The DS12R885 also
or capacitors to operate. Table 1 specifies several crys-
operates in either 24-hour or 12-hour format with an
tal parameters for the external crystal. Figure 1 shows a
AM/PM indicator. A precision temperature-compensat-
functional schematic of the oscillator circuit. An enable
ed circuit monitors the status of V . If a primary
CC
bit in the control register controls the oscillator.
Oscillator startup times are highly dependent upon
crystal characteristics, PC board leakage, and layout.
High ESR and excessive capacitive loads are the major
contributors to long startup times. A circuit using a
crystal with the recommended characteristics and
proper layout usually starts within one second.
power-supply failure is detected, the device automati-
cally switches to a backup supply. The backup supply
input supports either a rechargeable battery or a super
cap, and includes an integrated trickle charger. The
trickle charger is always enabled. The DS12R885 is
accessed through a multiplexed address/data bus that
supports Intel and Motorola modes.
The DS12R887 is a surface-mount package using the
DS12R885 die, a 32.768kHz crystal, and a recharge-
able battery. The device provides a real-time clock/cal-
endar, one time-of-day alarm, three maskable interrupts
with a common interrupt output, a programmable
square wave, and 114 bytes of nonvolatile, battery-
backed static RAM. The date at the end of the month is
automatically adjusted for months with fewer than 31
days, including correction for leap years. It also oper-
ates in either 24-hour or 12-hour format with an AM/PM
indicator. A precision temperature-compensated circuit
Table 1. Crystal Specifications*
PARAMETER
SYMBOL MIN
TYP
MAX UNITS
Nominal
Frequency
f
32.768
kHz
O
Series
Resistance
ESR
50
kꢀ
Load
Capacitance
C
L
12.5
pF
monitors the status of V . If a primary power failure is
CC
*The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations with Dallas Real-Time Clocks (RTCs) for
additional specifications.
detected, the device automatically switches to a back-
up battery included in the package. The device is
accessed through a multiplexed byte-wide interface,
which supports both Intel and Motorola modes.
5C/DS12R87
The DS12CR887 EDIP integrates a DS12R885 die with
a crystal and battery. The charging circuit on the
DS12R885 die is disabled. The battery has sufficient
capacity to power the oscillator and registers for five
COUNTDOWN
CHAIN
years in the absence of V
at +25°C.
CC
The DS12R887 BGA includes a crystal and a recharge-
able battery. A fully charged battery can power the oscil-
lator and registers (typical current at +25°C) in the
C 1
L
C 2
L
RTC REGISTERS
absence of V
for approximately 11 days (10% of
CC
capacity consumed) or 98 days (90% capacity con-
sumed). When the discharge depth is 10% of capacity,
the battery can be recharged up to 1,000 times. If the dis-
charge depth is 90% of capacity, the battery can be
recharged up to 30 times. Thus, the life of the device
would be approximately 30 years (11 days X 1,000
cycles) or 8 years (98 days x 30 cycles). Charging time to
DS12R885
X1
X2
CRYSTAL
Figure 1. Oscillator Circuit Showing Internal Bias Network
full capacity is approximately two days with V applied.
CC
12
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