DS1371 Datasheet, PDF (6/14 Pages) Maxim Integrated Products – 2-Wire, 32-Bit Binary Counter Watchdog Clock

DS1371
PIN DESCRIPTION
PIN NAME
FUNCTION
These signals are connections for a standard 32.768kHz quartz crystal. The internal
oscillator circuitry is designed for operation with a crystal having a specified load
capacitance (C_L) of 6pF.
1, 2
X1, X2 _{For more information about crystal selection and crystal layout considerations, refer to}
Application Note 58: Crystal Considerations with Dallas Real-Time Clocks. The
DS1371 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.
Watchdog Input. A positive edge-triggered hardware interrupt input that restarts the
3
WDS watchdog counter when this signal transitions from a low to a high. While WDS remains
at a static low or high, the watchdog counter continues to decrement.
4
GND Ground
5
SDA 2-Wire Serial Data. Input/output for 2-wire data.
6
SCL 2-Wire Serial Clock. Input for 2-wire clock.
7
SQW/INT
Square-Wave/Interrupt Output. This pin is used to output the programmable square-
wave or alarm interrupt signal. It is open drain and requires an external pullup resistor.
8
V_CC
Supply Voltage Terminal
Detailed Description
The DS1371 is a real-time clock (RTC) with a 2-wire serial interface that provides elapsed seconds from
a user-defined starting point in a 32-bit counter (Figure 2). A 24-bit counter can be configured as either a
watchdog counter or as an alarm counter. An on-chip oscillator circuit uses a customer-supplied
32.768kHz crystal to keep time.
Oscillator Circuit
The DS1371 uses an external 32.768kHz crystal. The oscillator circuit does not require any external
resistors or capacitors to operate. Table 1 specifies several crystal parameters for the external crystal;
Figure 3 shows a functional schematic of the oscillator circuit. Using a crystal with the specified
characteristics, the startup time is usually less than one second.
Clock Accuracy
The accuracy of the clock 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 temperature shifts. External
circuit noise coupled into the oscillator circuit can result in the clock running fast. Figure 4 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.
6 of 14