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LAN8820 Datasheet(PDF) 75 Page - Microchip Technology |
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LAN8820 Datasheet(HTML) 75 Page - Microchip Technology |
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75 / 83 page  2009-2015 Microchip Technology Inc. DS00001871B-page 75 LAN8820/LAN8820I 5.6 Clock Circuit The device can accept either a 25MHz crystal (preferred) or a 25 MHz single-ended clock oscillator (+/- 50ppm) input. If the single-ended clock oscillator method is implemented, XO should be left unconnected and XI should be driven with a nominal 0V-VDD25IO clock signal. The input clock duty cycle is 40% minimum, 50% typical and 60% maximum. It is recommended that a crystal utilizing matching parallel load capacitors be used for the crystal input/output signals (XI/XO). See Table 5-20 for the recommended crystal specifications. Note 5-21 The maximum allowable values for Frequency Tolerance and Frequency Stability are application dependant. Since any particular application must meet the IEEE +/-50 PPM Total PPM Budget, the combination of these two values must be approximately +/-45 PPM (allowing for aging). Note 5-22 Frequency Deviation Over Time is also referred to as Aging. Note 5-23 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as +/- 50 PPM. Note 5-24 0oC for commercial version, -40oC for industrial version. Note 5-25 +70oC for commercial version, +85oC for industrial version. Note 5-26 This number includes the pad, the bond wire and the lead frame. PCB capacitance is not included in this value. The XO/XI pin and PCB capacitance values are required to accurately calculate the value of the two external load capacitors. These two external load capacitors determine the accuracy of the 25.000 MHz frequency. TABLE 5-20: CRYSTAL SPECIFICATIONS Parameter Symbol Min Nom Max Units Notes Crystal Cut AT, typ Crystal Oscillation Mode Fundamental Mode Crystal Calibration Mode Parallel Resonant Mode Frequency Ffund - 25.000 - MHz Frequency Tolerance @ 25oCFtol - - +/-50 PPM Note 5-21 Frequency Stability Over Temp Ftemp - - +/-50 PPM Note 5-21 Frequency Deviation Over Time Fage - +/-3 to 5 - PPM Note 5-22 Total Allowable PPM Budget - - +/-50 PPM Note 5-23 Shunt Capacitance CO -7 pF Load Capacitance CL -18 pF Drive Level PW 300 - - uW Equivalent Series Resistance R1 -- 50 Ohm Operating Temperature Range Note 5-24 - Note 5-25 oC XI Pin Capacitance - 3 typ - pF Note 5-26 XO Pin Capacitance - 3 typ - pF Note 5-26 |
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