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1337CSRI Datasheet(PDF) 4 Page - Integrated Device Technology |
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1337CSRI Datasheet(HTML) 4 Page - Integrated Device Technology |
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4 / 24 page  IDT1337 REA LTIME CLOCK WITH SERIAL INTERFACE RTC IDT™ REAL-TIME CLOCK WITH I2C SERIAL INTERFACE 4 IDT1337 REV J 111009 Note: IDT1337CSRI integrates a standard 32.768 kHz crystal in the package and contributes an additional frequency error of 10ppm at nominal VCC (+3.3 V) and TA=+25°C. ESR (Effective Series Resistance) Choose the crystal with lower ESR. A low ESR helps the crystal to start up and stabilize to the correct output frequency faster compared to high ESR crystals. Frequency Tolerance The frequency tolerance for 32 KHz crystals should be specified at nominal temperature (+25°C) on the crystal manufacturer datasheet. The crystals used with IDT1337 typically have a frequency tolerance of +/-20ppm at +25°C. Specifications for a typical 32 kHz crystal used with our device are shown in the table below. PCB Design Consideration • Signal traces between IDT device pins and the crystal must be kept as short as possible. This minimizes parasitic capacitance and sensitivity to crosstalk and EMI. Note that the trace capacitances play a role in the effective crystal load capacitance calculation. • Data lines and frequently switching signal lines should be routed as far away from the crystal connections as possible. Crosstalk from these signals may disturb the oscillator signal. • Reduce the parasitic capacitance between X1 and X2 signals by routing them as far apart as possible. • The oscillation loop current flows between the crystal and the load capacitors. This signal path (crystal to CL1 to CL2 to crystal) should be kept as short as possible and ideally be symmetric. The ground connections for both capacitors should be as close together as possible. Never route the ground connection between the capacitors all around the crystal, because this long ground trace is sensitive to crosstalk and EMI. • To reduce the radiation / coupling from oscillator circuit, an isolated ground island on the GND layer could be made. This ground island can be connected at one point to the GND layer. This helps to keep noise generated by the oscillator circuit locally on this separated island. The ground connections for the load capacitors and the oscillator should be connected to this island. PCB Layout PCB Assembly, Soldering and Cleaning Board-assembly production process and assembly quality can affect the performance of the 32 KHz oscillator. Depending on the flux material used, the soldering process can leave critical residues on the PCB surface. High humidity and fast temperature cycles that cause humidity condensation on the printed circuit board can create process residuals. These process residuals cause the insulation of the sensitive oscillator signal lines towards each other and neighboring signals on the PCB to decrease. High humidity can lead to moisture condensation on the surface of the PCB and, together with process residuals, reduce the surface resistivity of the board. Flux residuals on the board can cause leakage current paths, especially in humid environments. Thorough PCB cleaning is therefore highly recommended in order to achieve maximum performance by removing flux residuals from the board after assembly. In general, reduction of losses in the oscillator circuit leads to better safety margin and reliability. Parameter Symbol Min Typ Max Units Nominal Freq. fO 32.768 kHz Series Resistance ESR 50 k Ω Load Capacitance CL 7pF 1337 |
Similar Part No. - 1337CSRI |
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Similar Description - 1337CSRI |
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