DS1371

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PIN DESCRIPTION

PIN

NAME

FUNCTION

1, 2

X1, X2

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

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.

3

WDS

Watchdog Input. A positive edge-triggered hardware interrupt input that restarts the

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

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.