DS1338(2004) 查看數據表（PDF） - Maxim Integrated

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DS1338
(Rev.:2004)

I2C RTC with 56-Byte NV RAM

Maxim Integrated 

DS1338 I2C RTC with 56-Byte NV RAM

OPERATION

The DS1338 operates as a slave device on the serial bus. Access is obtained by implementing a START condition

and providing a device identification code, followed by data. Subsequent registers can be accessed sequentially

until a STOP condition is executed. The device is fully accessible and data can be written and read when VCC is

greater than VPF. However, when VCC falls below VPF, the internal clock registers are blocked from any access. If

VPF is less than VBAT, the device power is switched from VCC to VBAT when VCC drops below VPF. If VPF is greater

than VBAT, the device power is switched from VCC to VBAT when VCC drops below VBAT. The registers are maintained

from the VBAT source until VCC is returned to nominal levels. The block diagram (Figure 3) shows the main elements

of the DS1338.

An enable bit in the seconds register controls the oscillator. Oscillator startup times are highly dependent upon

crystal characteristics, PC board leakage, and layout. High ESR and excessive capacitive loads are the major

contributors to long start-up times. A circuit using a crystal with the recommended characteristics and proper layout

usually starts within 1 second.

OSCILLATOR CIRCUIT

The DS1338 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 4 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.

Table 1. Crystal Specifications*

PARAMETER

SYMBOL MIN

TYP MAX UNITS

Nominal Frequency

Series Resistance

Load Capacitance

fO

ESR

CL

32.768

kHz

45

kW

12.5

pF

*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 additional specifications.

Figure 4. Oscillator Circuit Showing Internal Bias Network

RTC

COUNTDOWN

CHAIN

C L1

C L2

X1

X2

CRYSTAL

RTC

REGISTERS

8 of 15

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