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C667x: decoupling on the SERDES power supply

Sara Venhuizen
Prodigy 80 points

We are basing a new HW design on the C6678 EVM kit.  We are not planning to use the HyperLink SERDES function, but are planning to use one pair of the SGMII SERDES to/from an Ethernet PHY.  We are tight on board space.  For the SERDES supply, do we need to add all decoupling components (around 50) that are on the EVM kit reference design, keeping in mind we are using SGMII only?

  • 0
    TI__Mastermind 42545 points

    There are a number of power supply pins for serdes regulator and termination that are specific to a particular serdes interface.  The correct supply voltage must be connected to these pins even if the serdes is not used, however, the filter components and additional bypass capacitors shown in the EVM schematic are not required for the unused serdes.

    Based on your question the SGMII interface is the only serdes interface you will be using.  The SGMII serdes uses the VDDT2 termination voltage and the VDDR3 regulator voltage.  The filters and additional bypass capacitors for these pins are required for proper operation of the SGMII interface.  VDDT1 and VDDR1 are specific to the hyperlink serdes.  VDDR2 is the regulator voltage for the PCIE serdes and VDDR4 is the regulator voltage for the SRIO serdes.  If these serdes interfaces are not used then the filter components and the additional bypass capacitors for VDDT1, VDDR1, VDDR2 and VDDR4 are not needed.  The VDDT1 pins may be connected directly to CVDD1.  The pins for VDDR1, VDDR2 and VDDR4 may be connected directly to DVDD15.  In the C6678 EVM design this would eliminate four filter components and seventeen bypass capacitors.  Note that this is well below your estimate of fifty capacitors.

  • 0 in reply to Bill Taboada
    Prodigy 80 points

    Thanks for the answer.  That's what we were looking for.  To clarify, I was asking if we could decrease the number of caps from around 50 total on the SERDES supplies to some lower number.  Based on your response we can at least get rid of a few - we weren't expecting to get rid of all 50.