DS1390 Maxim, DS1390 Datasheet - Page 13

• Manufacturer Part Number: DS1390
• Description: The low-voltage serial-peripheral interface (SPI™) DS1390/DS1391/DS1394 and the low-voltage 3-wire DS1392/DS1393 real-time clocks (RTCs) are clocks/calendars that provide hundredths of a second, seconds, minutes, hours, day, date, month, and ye
• Manufacturer: Maxim

The power-control function is provided by a precise,
temperature-compensated voltage reference and a
comparator circuit that monitors the V
device is fully accessible and data can be written and
read when V
V
blocked from any access. If V
the device power is switched from V
when V
V
V
Timekeeping operation and register data are main-
tained from the V
nominal levels (Table 1). After V
read and write access is allowed after RST goes high
(Figure 5).
Table 1. Power Control
Figure 7. Layout Example
CC
BACKUP
BACKUP
V
V
V
V
CC
CC
CC
CC
CONDITION
V
V
V
V
SUPPLY
falls below V
CC
CC
CC
CC
< V
> V
< V
> V
CC
< V
< V
> V
> V
BACKUP
BACKUP
BACKUP
BACKUP
, the device power is switched from V
PF
PF
PF
PF
drops below V
when V
,
,
,
,
CC
BACKUP
is greater than V
PF
, the internal clock registers are
READ/WRITE
CC
ACCESS)
source until V
LOCAL GROUND PLANE (LAYER 2)
Yes
Yes
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.
No
No
drops below V
PF
PF
. If V
Power Control
CC
is less than V
Low-Voltage SPI/3-Wire RTCs with
PF
PF
returns above V
. However, when
____________________________________________________________________
CC
CC
is greater than
CRYSTAL
POWERED BY
CC
is returned to
V
to V
BACKUP
V
V
V
level. The
CC
CC
CC
BACKUP
BACKUP
BACKUP
CC
PF
to
,
.
,
All five devices use an external 32.768kHz crystal. The
oscillator circuit does not require any external resistors
or capacitors to operate. Table 2 specifies several crys-
tal parameters for the external crystal. 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 7 shows a typical PC board layout
for isolation of the crystal and oscillator from noise.
Refer to Application Note 58: Crystal Considerations
with Maxim Real-Time Clocks for detailed information.
Table 2. Crystal Specifications*
* The crystal, traces, and crystal input pins should be isolated
from RF generating signals. Refer to Application Note 58:
Crystal Considerations for Maxim Real-Time Clocks for addi-
tional specifications.
Nominal Frequency
Series Resistance
Load Capacitance
PARAMETER
GND
X1
X2
Trickle Charger
SYMBOL
ESR
C
f
O
L
Oscillator Circuit
Clock Accuracy
MIN
32.768
TYP
6
MAX
55
UNITS
kHz
kΩ
pF
13