DS1390-DS1393 MAXIM [Maxim Integrated Products], DS1390-DS1393 Datasheet - Page 12

• Manufacturer Part Number: DS1390-DS1393
• Description: Low-Voltage SPI/3-Wire RTCs with Trickle Charger
• Manufacturer: MAXIM [Maxim Integrated Products]

However, when V
registers are blocked from any access. If V
than V
to V
greater than V
from V
V
V
See the Functional Diagram for the main elements of
these serial RTCs.
Low-Voltage SPI/3-Wire RTCs with
Trickle Charger
Table 1. Crystal Specifications*
*The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Dallas Real-Time Clocks for addi-
tional specifications.
Figure 7. Oscillator Circuit Showing Internal Bias Network
12
Nominal Frequency
Series Resistance
Load Capacitance
BACKUP
BACKUP
PARAMETER
BACKUP
____________________________________________________________________
BACKUP
CC
C
source until V
. The registers are maintained from the
L
1
to V
when V
, the device power is switched from V
X1
BACKUP
CRYSTAL
BACKUP
CC
SYMBOL
falls below V
CC
ESR
C
, the device power is switched
f
CC
O
X2
L
drops below V
is returned to nominal levels.
when V
C
DS139x
L
2
MIN
RTC REGISTERS
PF
COUNTDOWN
32.768
CC
, the internal clock
TYP
CHAIN
6
drops below
PF
MAX
55
. If V
PF
is less
UNITS
PF
kHz
k
pF
CC
is
All four devices use an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 1 specifies several crys-
tal parameters for the external crystal, and Figure 7
shows a functional schematic of the oscillator circuit. If
a crystal is used with the specified characteristics, the
startup time is usually less than one second.
The accuracy of the clock is dependent upon the accu-
racy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and
the capacitive load for which the crystal was trimmed.
Additional error is added by crystal frequency drift
caused by temperature shifts. External circuit noise
coupled into the oscillator circuit can result in the clock
running fast. Figure 8 shows a typical PC board layout
for isolation of the crystal and oscillator from noise.
Refer to Application Note 58: Crystal Considerations
with Dallas Real-Time Clocks for detailed information.
Figure 8. Layout Example
LOCAL GROUND PLANE (LAYER 2)
NOTE: AVOID ROUTING SIGNAL LINES
IN THE CROSSHATCHED AREA
(UPPER LEFT QUADRANT) OF
THE PACKAGE UNLESS THERE IS
A GROUND PLANE BETWEEN THE
SIGNAL LINE AND THE DEVICE PACKAGE.
CRYSTAL
Oscillator Circuit
Clock Accuracy
GND
X1
X2