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Branching High speed singals

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  • Branching High speed singals

    Hello,
    I am working on the Hardware design part of Texas Instruments DP83620 Ethernet Phy
    This IC provides a common output TX+/- and RX+/- for both Copper and Fiber mode and there are some strapping options of adding and removing the resistors to configure the IC in either modes.
    My application requires both these modes to be designed on the PCB and should be field configurable. Now based on the evaluation kit it contains a set of 0 ohms resistor which can be stuffed or removed to use either of Copper or fiber mode however this cannot be done in the fields once the product is designed.
    One way is using a Jumper or switch to achieve this where I am skeptical about the performance since this is a high speed part they signals are bound to get disturbed with these solution.
    I am looking for an efficient solution which could be field configurable.

    Thank you.


  • #2
    The Best way of optimizing and best routing is to follow the datahsheet. Especially, TI gives best application notes for Specific packages ICs. They give optimum placement and Power and Supply splitting Planes.

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    • #3
      Thank you mohsin_qau .
      However my question is related to usage of Resistor or switch in the High speed track,

      The datasheet specifies for either of the modes and does not expect both modes to be designed simultaneously.It is my application that requires this to be done,
      Both modes to be designed with suitable selecting options which can be configured on the field.

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      • #4
        Would be your PCB install in Temperature environment?

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        • #5
          Using resistors has no effect in high speed design, if its in the range of MHz. ( Whats your effective frequency Range). And for Analog/Digital switch, must consider its Response time from Low to High Transition and vice versa.

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          • #6
            chitransh92, once we used switch on our TinyRex baseboard for USB. Maybe it can be used for other high speed signals too (or maybe something like this exists specifically for Ethernet). Here is the schematic, page 9: http://www.imx6rex.com/wp-content/up...-Schematic.pdf

            The chip for USB is TS3USB221ERSER: http://www.ti.com/lit/ds/symlink/ts3usb221e.pdf

            The chip for Ethernet maybe could be TS3L500AE? http://www.ti.com/lit/ds/symlink/ts3l500ae.pdf
            It was just a quick search, so you need to double check.

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            • #7
              robertferanec this is what i was looking for.
              Appreciate your input.

              A follow up question.

              I am looking for some specifics on upto what frequency we can use the jumper or DIP switches for transmission.

              I tried searching the following on the web by no fate so far:
              Effects of DIP switches when used in data transmission?
              Effects of jumper pins when used in data tranmission?
              and so on...

              Basically i need to understand what will be the disadvantages of using a jumper over using a switch or mux IC and why jumper or DIP switches are not recommended in High speed signals.

              Thank you.

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              • #8
                Jumpers may work, but that is not the right way to do it. Don't forget, Ethernet protocol can repair itself - so everything may look ok, but it may influence performance (e.g. maximum speed), reliability or EMC/EMI. Here, on this picture, I wired up gigabit Ethernet, and I could connect to internet and download files, so, it somehow worked, but, is it the right way to connect it?

                Click image for larger version

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              • #9
                Thank you robertferanec

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                • #10
                  robertferanec
                  In my application I am using TS3L500AE for which I am planning to use a jumper setting to set it High or Low. (Thanks to your suggestion above).
                  The point here which I am concerned about is using a current limiting resistor at the "select" pin input when it is pulled High, However the datasheet has given no such information of what value pull up/Pull down to be used.

                  I understand this is CMOS based structure so the "select" pin for TS3L500 is of high input impedance (Need your citation here, if this is right?) but does this means I can connect +3.3V and Ground directly via jumper for High and Low state respectively of the IC.

                  Thank you.

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                  • #11
                    Standard solution is to use pull up (e.g. 10k) and fit a JUMPER to short to GND. When Jumper is not fitted, 10k pulls the pin to VCC, when the Jumper is fitted, the pin is shorted to GND.

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                    • #12
                      Thank you robertferanec ,
                      I have used a similar solution where I have used a 3 pin header connecting as below:
                      Pin 1 - 3.3V with 10K
                      Pin 2 - Connected to SEL pin of the MUX/DEMUX
                      Pin 3 - Connected to ground via 1K

                      Essential placing the jumper between Pin1 -2 will pull the SEL pin high
                      and between the Pin 2-3 will pull it to ground via 1K.

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                      • #13
                        I do not use that solution as one of the possibilities is - no jumper fitted. This means, there is a way to set your board into state when status of the input pin is not defined (the good practice is, that all your input pins should have ALWAYS defined status).

                        Also, I often do not use resistors to GND (sometimes I do, but it depends on situation), as Low logic level is sometimes defined only between 0-0.8V (for TS3L500AE it is 0-0.8V) and any small voltage drop on that resistor can put the input level into undefined state. With logic level High there is no this kind of problem, because the input voltage range is much bigger (for TS3L500AE it is 2-5.5V).

                        For CMOS it is sometimes easier 0V to 1/3VDD (Logic Low) vs 2/3VDD to VDD (Logic High), but often I do not want to cross 0.6V for low level anyway - just to be sure it always works ok.

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                        • #14
                          Appreciate for the insight robertferanec .
                          Thank you.

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