Real-Time Clock
Detailed Description
The DS12885 family of RTCs provide 14 bytes of real-
time clock/calendar, alarm, and control/status registers
and 114 bytes (113 bytes for DS12C887 and
DS12C887A) 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 devices also operate in
either 24-hour or 12-hour format with an AM/PM indica-
tor. A precision temperature-compensated circuit moni-
tors the status of V
CC
. If a primary power-supply failure
is detected, the devices automatically switch to a back-
up supply. The backup supply input supports a primary
battery, such as lithium coin cell. The devices are
accessed through a multiplexed address/data bus that
supports Intel and Motorola modes.
DS12885/DS12887/DS12887A/DS12C887/DS12C887A
Table 1. Crystal Specifications*
PARAMETER
Nominal
Frequency
Series
Resistance
Load
Capacitance
SYMBOL
f
O
ESR
C
L
6
MIN
TYP
32.768
50
MAX UNITS
kHz
kΩ
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.
Oscillator Circuit
The DS12885 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
DS12885. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is floated.
COUNTDOWN
CHAIN
C
L
1
C
L
2
RTC REGISTERS
DS12885
X1
CRYSTAL
X2
Figure 1. Oscillator Circuit Showing Internal Bias Network
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