RTC with Constant-Voltage Trickle Charger
Pin Description (continued)
PIN
NAME
FUNCTION
SO
EDIP
BGA
Reset Input. The 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:
PF
CC
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-drain output and requires an
19
19
A4
IRQ
external pullup resistor to V
.
CC
Connection for Rechargeable Battery or Super Cap. This pin provides trickle charging
20
21
—
—
—
V
BACKUP
when V
is greater than V
.
BACKUP
CC
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 V
not applied. The RCLR function is designed to be used through a human interface
is
CC
A5
RCLR
(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
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 V
is applied within normal limits, the
CC
V
device is fully accessible and data can be written and read. When V is below V reads
CC PF
and writes are inhibited.
CC
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DALLAS [ DALLAS SEMICONDUCTOR ]
