DS17285_1 Datasheet, PDF (9/31 Pages) Maxim Integrated Products – Real-Time Clocks

Real-Time Clocks
PIN
24
28
21
3
22
4
23
5
24
6, 7
2, 3, 16,
20
(DS17x87
only)
11, 18
Pin Description (continued)
NAME
FUNCTION
RCLR
V_BAUX
SQW
V_CC
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.
N.C. No Connection
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