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DS1340 Datasheet(PDF) 7 Page - Maxim Integrated Products |
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DS1340 Datasheet(HTML) 7 Page - Maxim Integrated Products |
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7 / 16 page  I2C RTC with Trickle Charger Maxim Integrated 7 DS1340 when VCC drops below VBACKUP. The registers are maintained from the VBACKUP source until VCC is returned to nominal levels (Table 1). After VCC returns above VPF, read and write access is allowed tREC. Oscillator Circuit The DS1340 uses 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. Figure 3 shows a functional schematic of the oscillator circuit. If using a crystal with the specified characteristics, the startup time is usually less than one second. Clock Accuracy The initial clock accuracy depends on the accuracy of the crystal and the accuracy of the match between the capacitive load of the oscillator circuit and the capaci- tive 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 isolating the crystal and oscillator from noise. Refer to Application Note 58: Crystal Considerations with Dallas Real-Time Clocks (www.maximintegrated.com/RTCapps) for detailed information. DS1340C Only The DS1340C integrates a standard 32,768Hz crystal into the package. Typical accuracy with nominal VCC and +25°C is approximately +15ppm. Refer to Application Note 58 for information about crystal accu- racy vs. temperature. Operation The DS1340 operates as a slave device on the serial bus. Access is obtained by implementing a START condition and providing a device identification code fol- lowed 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 VBACKUP, the device power is switched from VCC to VBACKUP when VCC drops below VPF. If VPF is greater than VBACKUP, the device power is switched from VCC to VBACKUP when VCC drops below VBACKUP. The regis- ters are maintained from the VBACKUP source until VCC is returned to nominal levels. The functional diagram (Figure 5) shows the main elements of the serial RTC. PARAMETER SYMBOL MIN TYP MAX UNITS Nominal Frequency fO 32.768 kHz Series Resistance ESR 80 k Ω Load Capacitance CL 12.5 pF 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 Dallas Real-Time Clocks for addi- tional specifications. COUNTDOWN CHAIN RTC X1 X2 CL1 CL2 CRYSTAL RTC REGISTERS Figure 3. Oscillator Circuit Showing Internal Bias Network CRYSTAL X1 X2 GND LOCAL GROUND PLANE (LAYER 2) Figure 4. Layout Example |
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