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251MI-XXT Datasheet(PDF) 2 Page - Integrated Device Technology |
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251MI-XXT Datasheet(HTML) 2 Page - Integrated Device Technology |
2 / 10 page FIELD PROGRAMMABLE SS VERSACLOCK SYNTHESIZER 2 REVISION E 05/19/14 ICS251 DATASHEET Pin Assignment 8-pin (150 mil) SOIC Output Clock Selection Table Pin Descriptions External Components The ICS251 requires a minimum number of external components for proper operation. Series Termination Resistor Clock output traces over one inch should use series termination. To series terminate a 50 trace (a commonly used trace impedance), place a 33 resistor in series with the clock line, as close to the clock output pin as possible. The nominal impedance of the clock output is 20 . Decoupling Capacitor As with any high-performance mixed-signal IC, the ICS251 must be isolated from system power supply noise to perform optimally. A decoupling capacitor of 0.01µF must be connected between VDD and the PCB ground plane. Crystal Load Capacitors The device crystal connections should include pads for small capacitors from X1 to ground and from X2 to ground. These capacitors are used to adjust the stray capacitance of the board to match the nominally required crystal load capacitance. Because load capacitance can only be increased in this trimming process, it is important to keep stray capacitance to a minimum by using very short PCB traces (and no vias) been the crystal and device. Crystal capacitors must be connected from each of the pins X1 and X2 to ground. The value (in pF) of these crystal caps should equal (CL -6 pF)*2. In this equation, CL= crystal load capacitance in pF. Example: For a crystal with a 16 pF load capacitance, each crystal capacitor would be 20 pF [(16-6) x 2] = 20. X1 / I C L K VD D G N D S0 S1 CL K X2 1 2 3 4 8 7 6 5 PD T S S1 S0 CLK (MHz) Spread Percentage 0 0 User Configurable User Configurable 01 User Configurable User Configurable 10 User Configurable User Configurable 11 User Configurable User Configurable Pin Number Pin Name Pin Type Pin Description 1 S0 Input Select pin 0 for frequency selection on CLK. Internal pull-up resistor. 2 VDD Power Connect to +3.3 V. 3 X1/ICLK XI Connect this pin to a crystal or external clock input. 4 X2 XO Connect this pin to a crystal, or float for clock input. 5 CLK Output Clock output. Weak internal pull-down when tri-state. 6 S1 Input Select pin 1 for frequency selection on CLK. Internal pull-up resistor. 7 GND Power Connect this to Ground. 8PDTS Input Powers down entire chip. Tri-states CLK outputs when low. No internal pull-up resistor. The pin must be tied either directly or through the external resistor to VDD or GND. External resistor value must be less than 15kOhm. |
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