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DP83825EVM: Looking for more info for PCB design

Part Number: DP83825EVM
Other Parts Discussed in Thread: DP83825I

Hello,

I am working on a PCB that will be using the DP83825I, connected to the MAC on a 6748 DSP, and connected to an Ethernet jack which has internal magnetics.  Regarding the EVM board, it is very hard to follow signals on the schematic, and the PCB is missing many reference designators, making it very challenging to use as a reference.  These are the documents I am looking at (please confirm they are correct):

Schematic - https://www.ti.com/lit/df/snlr041a/snlr041a.pdf?ts=1615570244407&ref_url=https%253A%252F%252Fwww.google.com%252F

PCB - https://www.ti.com/lit/df/snlr040/snlr040.pdf?ts=1615571873265

I have a number of questions I'm hoping can be answered:

- Can you provide a MFG P/N or details on type / case size / etc. for the 4700pF cap (C28) and 1M resistor (R37) that go from earth ground to system ground?  Where are they located on the board?

- Why are the series resistors going to J9 "DNP"? Don't they need to be populated for the RMII interface to work?

- Is C23 there for switch debounce, or would it be needed in a design where the RESET line is being controlled?

- Why are there 33 ohm resistors (R9, 10, 17) placed between various RMII signals?

- In my design, are series termination resistors needed on the RMII lines?

- Is an external pulldown resistor needed on CRS_DV?

Thanks in advance for you help with this.

Jan Switzky

  • Hi Jan,

    1. We recommend using a capacitor of size 0805 or bigger to avoid immunity issues, and the resistor should be the same size. It does not matter where the two components are located on the board.
    2. The series resistors are DNP because the default configuration on the EVM is RMII on board loopback. As a result, the series resistors and the 33 ohm resistors should not be populated at the same time. If you wish to use normal mode, then the RMII signals should be routed to J9 connector, and the 33 ohm resistors removed.
    3. The C23 is there for switch debounce. If the RESET line is connected to or controlled by a microcontroller or processor, then the capacitor is not necessary.
    4. See number 2. The 33 ohm resistors are there for RMII on board loopback.
    5. Series termination resistors may be needed. I would recommend placing at least 0 ohm resistors initially. The value of the series termination resistors will depend on your MAC device. It is possible your MAC connection needs 0 ohm or another value to work.
    6. An external pulldown resistor is not needed on CRS_DV. However, it is also a strap pin, so a pulldown resistor may be needed depending on which strap settings you need.

    Regards,

    Adrian Kam

  • Hi Adrian,

    Thanks for your quick reply.  You have answered most of my questions, but I am still unclear on the resistors and capacitors going from earth ground to system ground.  I found another very similar design on your website for a "TIDA-010046E1_PD_Side_Board_Schematic" which uses the DP83825.  In that schematic, they show 2 sets of Rs and Cs being populated.  The caps (C20 and 21) are 4700pF 1812 package and the resistors (R19 and 20) are 1M 2010 package.  You previously stated that the parts should be "0805 or bigger to avoid immunity issues".  Please clarify what package size should be used, as it makes a big difference for cost.  If 2 sets of R/C are needed, should they be placed in a specific location relative to the Ethernet jack?

    Also, does the use of those components change if we don't have a true earth ground in our product?  We are running off a 6V wall power adapter which uses a 2 prong AC plug.

    Regards,

    Jan

  • Hi Jan,

    1. From our experience, as long as the components are of package size 0805 or larger, it is good. What specific package size you should use depends on your cost and space requirements. In addition, one set of R/C should be good enough. If you use two sets, there is no specific location relative to the Ethernet jack they should be placed at.
    2. The components do not change.

    Regards,

    Adrian Kam