RTCs with Constant-Voltage Trickle Charger
5C/DS12R87
Pin Description (continued)
PIN
NAME
FUNCTION
SO
EDIP
BGA
Reset Input. The active-low RESET pin has no effect on the clock, calendar, or RAM. On
power-up, the RESET pin can be held low for a time to allow the power supply to
stabilize. The amount of time that RESET is held low is dependent on the application.
However, if RESET is used on power-up, the time RESET is low should exceed 200ms to
ensure that the internal timer that controls the DS12R885 on power-up has timed out.
When RESET is low and V is above V , the following occurs:
CC
PF
A. Periodic interrupt-enable (PIE) bit is cleared to 0.
B. Alarm interrupt-enable (AIE) bit is cleared to 0.
C. Update-ended interrupt-enable (UIE) bit is cleared to 0.
D. Periodic-interrupt flag (PF) bit is cleared to 0.
E. Alarm-interrupt flag (AF) bit is cleared to 0.
18
18
A2
RESET
F. Update-ended interrupt flag (UF) bit is cleared to 0.
G. Interrupt-request status flag (IRQF) bit is cleared to 0.
H. IRQ pin is in the high-impedance state.
I. The device is not accessible until RESET is returned high.
J. Square-wave output-enable (SQWE) bit is cleared to 0.
In a typical application, RESET can be connected to V . This connection allows the
CC
DS12R885 to go in and out of power fail without affecting any of the control registers.
Interrupt Request Output. The IRQ pin is an active-low output of the DS12R885 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-
19
19
A4
IRQ
drain output and requires an external pullup resistor to V
.
CC
Connection for Rechargeable Battery or Super Cap. This pin provides trickle charging
when V is greater than V . On the DS12CR887 and DS12R887, the V pin
is missing and is internally connected to a lithium cell.
20
21
—
—
—
V
BACKUP
CC
BACKUP
BACKUP
RAM Clear. The active-low RCLR pin is used to clear (set to logic 1) all 114 bytes of
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
A5
RCLR
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 3. The SQW signal can be turned on and
23
24
23
24
C4
SQW
off using the SQWE bit in Register B. The SQW signal is not available when V is less
CC
than V
.
PF
A6–A8,
B1–B8,
C6–C8
DC Power Pin for Primary Power Supply. When VCC is applied within normal limits, the
V
CC
device is fully accessible and data can be written and read. When V is below V
CC
PF
reads and writes are inhibited.
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