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DP83848 low impedance

jaya lakshmi64
Prodigy 90 points

Hi

PCB has been designed using lpc2388, Phy chip Dp83848, Transformer Hx1188nl.

In PCB , 2 Channels are used separetely

channel 1 -  lpc2388-phy chip - dp83848 

channel 2 - lpc2388 - phy chip - dp83848

both channels while routing in PCB, impedance trace was not used for differential lines TD+,TD- & RD+, RD-.

In Channel 1, routing from  phy chip 4 lines to transformer in one layer, it is working fine.

In channel 2, routing from phy chip 4 lines to transformer are in 3 different layers, ethernet does not works.

i have checked the voltage levels, resistance levels at phy chip are ok. i can configure the phy chip as 10 or 100Mbps through software in lpc2388.

LED LINK , LED ACT leds are glowing but LED LINK not glowing when connected to PC. In PC, network connection is shown and continuous packets are transmitted from PC to Board. In PC, pinging the Board IP address is unsuccessful.

My Question

1.Whether impedance is problem for ethernet failure.

 2. but how it works for channel 1 for same mismatch impedance.

3. shall i go relayout PCB

Please suggest the solution

  • 0
    TI__Mastermind 31600 points
    Impedance matching is required for the MDI traces (TD+/-, RD+/-). The traces should be 50 Ohms to ground or 100 Ohms differential. Uncontrolled impedances on these traces can introduce signal integrity problems that can impact compliance to the IEEE specification and overall functionality. I would recommend changing the layout to correct the trace impedance.

    Patrick