DS12CR885 Datasheet, PDF (12/22 Pages) Maxim Integrated Products – RTC with Constant-Voltage Trickle Charger

RTC with Constant-Voltage Trickle Charger
Detailed Description
The DS12R885 is a drop-in replacement for the
DS12885 RTC. The device provides 14 bytes of real-
time clock/calendar, alarm, and control/status registers
and 114 bytes of nonvolatile, battery-backed static
RAM. A time-of-day alarm, three maskable interrupts
with a common interrupt output, and a programmable
square-wave output are available. The DS12R885 also
operates in either 24-hour or 12-hour format with an
AM/PM indicator. A precision temperature-compensat-
ed circuit monitors the status of V_CC. If a primary
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
monitors the status of V_CC. If a primary power failure is
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.
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
years in the absence of V_CC at +25°C.
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
absence of V_CC for approximately 11 days (10% of
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
full capacity is approximately two days with V_CC applied.
Please consult related application notes for detailed
information on battery lifetime versus depth of dis-
charge, and expected product lifetime based upon
battery cycles.
Oscillator Circuit
The DS12R885 uses an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 1 specifies several crys-
tal parameters for the external crystal. Figure 1 shows a
functional schematic of the oscillator circuit. An enable
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.
An external 32.768kHz oscillator can also drive the
DS12R885. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is floated.
Table 1. Crystal Specifications*
PARAMETER
Nominal
Frequency
SYMBOL MIN TYP MAX UNITS
f_O
32.768
kHz
Series
Resistance
ESR
50 kΩ
Load
Capacitance
C_L
12.5
pF
*The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Dallas Real-Time Clocks for
additional specifications.
COUNTDOWN
CHAIN
C_L1
C_L2
RTC REGISTERS
DS12R885
X1
X2
CRYSTAL
Figure 1. Oscillator Circuit Showing Internal Bias Network
12 ____________________________________________________________________