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Error in IMXRT1050-EVKB ( and 1060 ) schematic, ethernet

Question asked by George Pontis on Oct 7, 2018
Latest reply on Oct 11, 2018 by Aldo Gutierrez

There appears to be a design error in the ethernet circuit on the IMXRT1050-EVKB and MIMXRT1060-EVK. Both show 49.9 ohm terminating resistors R126 through R129 on RXP, RXN, TXP, and TXN. However, the KSZ8081RNB PHY has internal termination. The Microchip hardware design guide for this chip ( DS00002779A ) says of all four of these lines:

 

It has an internal terminator and a bias, so it can be directly connected to the transmit channel of the magnetics and no external terminator and bias are needed

 

Adding the resistors would double-terminate the lines, but it would not be surprising if ethernet still worked under good conditions. Problems would be most likely to show up with longer cables.

 

On a related note, the eval boards also show places to optionally install 4 capacitors, one per line. These are marked DNP. I cannot think of any situation where these caps would be desired, so better to save the PCB board area and leave them off.

 

George

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