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The Microchip TCP/IP Stack software automatically controls the MDIX state (if present) by
randomly toggling the MDIX pin when no Ethernet link is detected. Random timing is used to
ensure that two identical devices powered up at an identical time will not toggle in lock-step
with each other, which would never converge on the correct routing.
If the target application does not need auto-crossover functionality, this feature may be dis-
abled by leaving the MDIX signal tri-stated or by actively driving it low. If complete hardware
removal of this feature is desired, unsolder and remove U8, and install 0 Ω resistors at
R10-R12 and R16.
The Fairchild FST3257 was used in this circuit due to its low-cost, low input/output capaci-
tance and low on resistance. Low capacitance and resistance are necessary to ensure that
the switches do not add a significant impedance discontinuity which would distort the high-
speed signaling waveform and increase return loss. This circuit was tested to ensure that
the transmit waveform in both 10Base-T and 100Base-TX speeds was not significantly
altered or in violation of the IEEE 802.3? standards (excluding the noted deviations in the
ENC624J600 silicon errata documentation). Functional receive performance was also tested
under worst case cable lengths of 100 meters.
If implementing a similar circuit in end products, take close note of the +5V power require-
ments. With 10Base-T signaling, the TPOUT+/- and TPIN+/- Common mode is forced, via
R6, to roughly 3V3ETH – (80mA * 10 Ω ) or 2.5V nominally, 2.8V max. The 10Base-T
differential signaling adds up to +/-1.4V on top of this Common mode, resulting in a peak
voltage of 4.2V that could be applied to the bus switches. The 100Base-TX signaling will
also result in peak voltages that exceed 3V3ETH, but not as significantly as 10Base-T
mode.
If 5V is not available in the target application, it is possible to create a crude voltage source
by making a change pump off of the ENCX24J600 CLKOUT output, as demonstrated here:
Voltage Source Using the ENCX24J600 CLKOUT Output
+ 3.3 V
~+5V
0.1 μF
10 k Ω
BAV99
0.1 μF
CLKOUT
Most bus switches have extremely low-current requirements, so despite the limited current
sourcing capability of CLKOUT, it can potentially be used to both generate ~5V required for
the bus switches and still be used as a clock source for other circuitry in the application.
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