Real-Time Clocks
Pin Description (continued)
PIN
24
28
NAME
FUNCTION
Active-Low RAM Clear Input. This pin is used to clear (set to logic 1) all the 114 bytes of
general-purpose RAM but does not affect the RAM associated with the real time clock or
extended RAM.
RCLR
may be invoked while the part is powered from any supply. The
RCLR
function is designed to be used via 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.
Auxiliary Battery Input. Required for kickstart and wake-up functions. This input also supports
clock/calendar and user RAM if V
BAT
is at lower voltage or is not used. A standard +3V lithium
cell or other energy source can be used. Diodes placed in series between V
BAUX
and the
battery may prevent proper operation. UL recognized to ensure against reverse charging
current when used with a lithium battery (
www.maxim-ic.com/qa/info/ul/
). For 3V V
CC
operation, V
BAUX
must be held between +2.5V and +3.7V. For 5V V
CC
operation, V
BAUX
must
be held between +2.5V and +5.2V. If V
BAUX
is not used it should be grounded and the
auxiliary-battery-enable bit bank 1, register 4BH, should = 0.
Square-Wave Output. When V
CC
rises above V
PF
, bits DV1 and E32k are set to 1. This
condition enables a 32kHz square-wave output. A square wave is output if either SQWE = 1 or
E32k = 1. If E32k = 1, then 32kHz is output regardless of the other control bits. If E32k = 0,
then the output frequency is dependent on the control bits in Register A. 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 off using the SQWE bit in Register B or the E32k bit in
extended register 4Bh. A 32kHz square wave is also available when V
CC
is less than V
PF
if
E32k = 1, ABE = 1, and voltage is applied to the V
BAUX
pin. When disabled, SQW is high
impedance when V
CC
is below 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.
DS17285/DS17287/DS17485/DS17487/DS17885/DS17887
21
3
RCLR
22
4
V
BAUX
23
5
SQW
24
6, 7
V
CC
2, 3, 16,
20
(DS17x87
only)
11, 18
N.C.
No Connection
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9
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
