Real-Time Clock
DS12885/DS12887/DS12887A/DS12C887/DS12C887A
Pin Description (continued)
PIN
SO,
PDIP
EDIP
PLCC
TQFP
NAME
FUNCTION
Active-Low Interrupt Request Output. The
IRQ
pin is an active-low output of the
device that can be used as an interrupt input to a processor. The
IRQ
output
remains low as long as the status bit causing the interrupt is present and the
corresponding interrupt-enable bit is set. The processor program normally
reads the C register to clear the
IRQ
pin. The
RESET
pin also clears pending
interrupts. When no interrupt conditions are present, the
IRQ
level is in the high-
impedance state. Multiple interrupting devices can be connected to an
IRQ
bus, provided that they are all open drain. The
IRQ
pin is an open-drain output
and requires an external pullup resistor to V
CC
.
Connection for a Primary Battery. (DS12885 Only.) Battery voltage must be held
between the minimum and maximum limits for proper operation. If a backup
supply is not supplied, V
BAT
must be grounded. Connect the battery directly to
the V
BAT
pin. Diodes in series between the V
BAT
pin and the battery may
prevent proper operation. UL recognized to ensure against reverse charging
when used with a lithium battery.
Active-Low RAM Clear. The
RCLR
pin is used to clear (set to logic 1) all the
general-purpose RAM, but does not affect the RAM associated with the RTC. To
clear the RAM,
RCLR
must be forced to an input logic 0 during battery-backup
mode when V
CC
is not applied. The
RCLR
function is designed to be used
through a human interface (shorting to ground manually or by a switch) and not
to be driven with external buffers. This pin is internally pulled up. Do not use an
external pullup resistor on this pin.
Square-Wave Output. The SQW pin can output a signal from one of 13 taps
provided by the 15 internal divider stages of the RTC. The frequency of the
SQW pin can be changed by programming Register A, as shown in Table 1.
The SQW signal can be turned on and off using the SQWE bit in Register B. The
SQW signal is not available when V
CC
is less than V
PF
.
DC Power Pin for Primary Power Supply. When V
CC
is applied within normal
limits, the device is fully accessible and data can be written and read. When
V
CC
is below V
PF
reads and writes are inhibited.
19
19
23
21
IRQ
20
—
24
22
V
BAT
21
21
(DS12887A/
DS12C887A)
25
24
RCLR
23
23
27
26
SQW
24
24
28
28
V
CC
10
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