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FM31L272-G Datasheet(PDF) 8 Page - Ramtron International Corporation |
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FM31L272-G Datasheet(HTML) 8 Page - Ramtron International Corporation |
8 / 25 page FM31L278/L276/L274/L272 - 3V I2C Companion Rev. 3.0 Feb. 2009 Page 8 of 25 Crystal Oscillator The crystal oscillator is designed to use a 12.5pF crystal without the need for external components, such as loading capacitors. The FM31L27x device has built-in loading capacitors that match the crystal. If a 32.768kHz crystal is not used, an external oscillator may be connected to the FM31L27x. Apply the oscillator to the X1 pin. Its high and low voltage levels can be driven rail-to-rail or amplitudes as low as approximately 500mV p-p. To ensure proper operation, a DC bias must be applied to the X2 pin. It should be centered between the high and low levels on the X1 pin. This can be accomplished with a voltage divider. In the example, R1 and R2 are chosen such that the X2 voltage is centered around the X1 oscillator drive levels. If you wish to avoid the DC current, you may choose to drive X1 with an external clock and X2 with an inverted clock using a CMOS inverter. Figure 8. External Oscillator Layout Requirements The X1 and X2 crystal pins employ very high impedance circuits and the oscillator connected to these pins can be upset by noise or extra loading. To reduce RTC clock errors from signal switching noise, a guard ring must be placed around these pads and the guard ring grounded. SDA and SCL traces should be routed away from the X1/X2 pads. The X1 and X2 trace lengths should be less than 5 mm. The use of a ground plane on the backside or inner board layer is preferred. See layout example. Red is the top layer, green is the bottom layer. Layout for Surface Mount Crystal Layout for Through Hole Crystal (red = top layer, green = bottom layer) (red = top layer, green = bottom layer) X1 X2 Vdd FM31L27x R1 R2 VDD SCL SDA X2 X1 PFI VBAK VDD SCL SDA X2 X1 PFI VBAK |
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