Smsc LAN9512 User Manual download pdf (Page 44)

USB Hub with Integrated 10/100 Ethernet Controller

Datasheet

Revision 1.0 (04-20-09) 44 SMSC LAN9512

DATASHEET

4.6 Clock Circuit

LAN9512 can accept either a 25MHz crystal (preferred) or a 25MHz 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 0-3.3V 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 4.6 for the recommended crystal specifications.

Note 4.12 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 4.13 Frequency Deviation Over Time is also referred to as Aging.

Note 4.14 The total deviation for the Transmitter Clock Frequency is specified by IEEE 802.3u as

+/- 50 PPM.

Note 4.15 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 4.6 LAN9512 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 F

fund

- 25.000 - MHz

Frequency Tolerance @ 25

tol

- - +/-50 PPM Note 4.12

Frequency Stability Over Temp F

temp

- - +/-50 PPM Note 4.12

Frequency Deviation Over Time F

age

- +/-3 to 5 - PPM Note 4.13

Total Allowable PPM Budget - - +/-50 PPM Note 4.14

Shunt Capacitance C

-7 typ-pF

Load Capacitance C

- 20 typ - pF

Drive Level P

300 - - uW

Equivalent Series Resistance R

--50Ohm

Operating Temperature Range 0 - +70

LAN9512 XI Pin Capacitance - 3 typ - pF Note 4.15

LAN9512 XO Pin Capacitance - 3 typ - pF Note 4.15

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