DS1374 Maxim, DS1374 Datasheet - Page 9

• Manufacturer Part Number: DS1374
• Description: The DS1374 is a 32-bit binary counter designed to continuously count time in seconds
• Manufacturer: Maxim

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 5. Oscillator Circuit Showing Internal Bias Network
The DS1374 is a real-time clock with an I
face. It provides elapsed seconds from a user-defined
starting point in a 32-bit counter (Figure 4). A 24-bit
counter can be configured as either a watchdog
counter or an alarm counter. An on-chip oscillator cir-
cuit uses a customer-supplied 32.768kHz crystal to
keep time. A power-control circuit switches operation
from V
cycled. The oscillator and counters continue to operate
when powered by either supply. If a rechargeable
backup supply is used, a trickle charger can be
enabled to charge the backup supply while V
Nominal
Frequency
Series
Resistance
Load
Capacitance
PARAMETER
CC
I
to V
DS1374
2
BACKUP
C, 32-Bit Binary Counter Watchdog RTC with
C
SYMBOL
L
1
ESR
X1
C
f
O
L
Detailed Description
and back when power on V
CRYSTAL
Trickle Charger and Reset Input/Output
MIN
X2
32.768
C
L
TYP
2
6
_____________________________________________________________________
COUNTDOWN
REGISTERS
CHAIN
2
RTC
C serial inter-
MAX
45
CC
is on.
UNITS
CC
kHz
kΩ
pF
is
The DS1374 uses 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. Figure 5 shows a
functional schematic of the oscillator circuit. The startup
time is usually less than 1 second when using a crystal
with the specified characteristics.
Clock accuracy is dependent upon the accuracy 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 tempera-
ture shifts. External circuit noise coupled into the oscilla-
tor circuit can result in the clock running fast. Figure 6
shows a typical PC board layout for isolating the crystal
and oscillator from noise. Refer to Application Note 58:
Crystal Considerations with Dallas Real-Time Clocks for
detailed information.
The DS1374C integrates a standard 32,768Hz crystal
into the package. Typical accuracy at nominal V
25°C is approximately 10ppm. See Application Note 58
for information about crystal accuracy vs. temperature.
Figure 6. Layout Example
LOCAL GROUND PLANE (LAYER 2)
NOTE: AVOID ROUTING SIGNALS IN THE CROSSHATCHED AREA (UPPER LEFT-HAND
QUADRANT) OF THE PACKAGE UNLESS THERE IS A GROUND PLANE BETWEEN THE
SIGNAL LINE AND THE PACKAGE.
CRYSTAL
Oscillator Circuit
Clock Accuracy
GND
X1
X2
DS1374C Only
CC
and
9