DS1558 Datasheet, PDF (3/18 Pages) Dallas Semiconductor – Watchdog Clock with NV RAM Control

DS1558
PIN DESCRIPTION (continued)
PIN NAME
FUNCTION
Battery Inputs for Any Standard +3V Lithium Cell or Other Energy Source. Battery
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V_{BAT1 voltage must be held between 2.5V and 3.7V for proper operation. UL recognized to}
ensure against reverse charging current when used with a lithium battery. If only one
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V_BAT2
battery is used, it should be attached to V_BAT1, and V_BAT2 should be grounded. See
“Conditions of Acceptability” at http://www.maxim-ic.com/TechSupport/QA/ntrl.htm.
Active-Low Power-On Reset Output (Open Drain). This pin is an output used to signal
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RST
that V_CC is out of tolerance. On power-up, RST is held low for a period of time to allow
the system to stabilize. The RTC and SRAM are not accessible while RST is active. This
pin is open drain and requires an external pullup resistor.
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V_CCO
V_CC Output to RAM. While V_CC is above V_BAT, the external SRAM is powered by V_CC.
When V_CC is below the battery level, the SRAM is powered by one of the V_BAT inputs.
Power-Supply Input. DC power is provided to the device on these pins. V_CC is the +5V
input. When 5V (or 3.3V for the 3.3V version) is applied within normal limits, the device
is fully accessible and data can be written and read. Reads and writes are inhibited when
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V_CC a 3V battery is connected to the device and V_CC is V_TP. However, the timekeeping
function continues unaffected by the lower input voltage. As V_CC falls below V_BAT, the
RAM and RTC are switched over to the external power supply (nominal 3.0V DC) at
V_BAT.
Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is
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X1
designed for operation with a crystal having a specified load capacitance (C_L) of 6pF. For
more information about crystal selection and crystal layout considerations, refer to
Application Note 58: Crystal Considerations with Dallas Real-Time Clocks. The DS1558
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X2 can also be driven by an external 32.768kHz oscillator. In this configuration, the X1 pin
is connected to the external oscillator signal and the X2 pin is floated.
TYPICAL OPERATING CIRCUIT
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