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DS1558
3 of 18
PIN
CRIP
tin
NAME
ON
DES
TION (con
ued)
PIN
FUNCTI
35
VBAT1
37
VBAT2
puts fo
y Sta
el
Energy Source. Battery
ust be
betw
r p
ation. UL recognized to
ainst re
e cha
ed
ium battery. If only one
used, it should b
nd
uld be grounded. See
ns of Acceptabil
c.com/TechSupport/QA/ntrl.htm
battery is
Battery In
r An
ndard +3V Lithium C l or Other
voltage m
held
een 2.5V and 3.7V fo roper oper
ensure ag
vers
rging current when us
with a lith
e attached to VBAT1, a
VBAT2 sho
“Conditio
.
38
RST
CC is out of tolerance. On power-up,
the system to stabilize. The
Active-Low Power-On Reset Output (Open Drain). This pin is an output used to signal
RST is held low for a period of time to allow
RTC and SRAM are not accessible while
RST is active. This
pin is open drain and requires an external pullup resistor.
that V
40
VCCO
VCC Output to RAM. While VCC is above VBAT, the external SRAM is powered by VCC.
When VCC is below the battery level, the SRAM is powered by one of the VBAT inputs.
42
VCC
Power-Supply Input. DC power is provided to the device on these pins. VCC 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
a 3V battery is connected to the device and VCC is VTP. However, the timekeeping
function continues unaffected by the lower input voltage. As VCC falls below VBAT, the
RAM and RTC are switched over to the external power supply (nominal 3.0V DC) at
VBAT.
46
X1
47
X2
Connections for Standard 32.768kHz Quartz Crystal. The internal oscillator circuitry is
designed for operation with a crystal having a specified load capacitance (CL) 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
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