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DP83867IR: RGMII clock skew - Strap & MDIO configuration

Zhou Fang67
Expert 4250 points
Part Number: DP83867IR

Hi Expert,

DP83867 offers both Strap & MDIO configuration for clock skew.

1. Register 0x0032 BIT0 and BIT1 determine whether clock skew enabled. The default value is "0", which means that clock is aligned with data. As a result, whether strap setup for RGMII_Clock_Skew is still available?

 2. If both strap clock skew and MDIO clock skew are set to different value. e.g. Strap clock skew is 2ns with RX/TX[2:0] = 000; MDIO clock skew is 3ns with 0x0086 = 1011 for both transmit & receive clock skew. Which one will take effect, strap clock skew or MDIO clock skew?

Thanks.

Zhou

  • 0
    TI__Mastermind 18425 points
    Hello Zhou,

    1. RGMII clock skew straps are applicable to 48pin QFN device only. The default value of any register controlled by strap pins is decided by the strap conditions. The datasheet default condition assumes that there is no external strap resistor and only the internal 9kohm pull down resistor is used. In this case, the RGMII TX & RX skew will be strapped to '0' and the clock-data will be aligned. If customer is using external strap resistor to strap it to a different skew value then the PHY will over write the default value of 0 skew with whatever the customer has strapped it to.

    2. The PHY will use the strap value as default skew after power-up and reset. MDIO access can be used later to over write the default skew values. If the PHY is power-cycled or hard reset, the clock skew settings configured via MDIO will be lost and the PHY will fall back to the default skew setting decided by the straps. Please note that clock skew straps are valid for 48pin QFN DP83867IR only.

    -Regards,
    Aniruddha
  • 0 in reply to Aniruddha Khadye
    TI__Expert 4250 points
    Hi Aniruddha,

    Thanks for your reply.
    1. Regarding the 1st answer, "the RGMII TX & RX skew will be strapped to '0' and the clock-data will be aligned. "
    However, the default strap mode(Mode 1) for RGMII Clock Skew TX[2:0] = 000, based on the table 8, the default skew is 2ns. It is true?

    2. If we want to switch skew delay from strap to MDIO, we should firstly change register 0x0032 BIT1 & BIT0 to 1(shifted mode), and then set up a delay for register 0x0086? What is the correct procedure to change from strap skew to MDIO skew?

    Thanks.
    Zhou
  • 0 in reply to Zhou Fang67
    TI__Mastermind 19780 points

    Hi Zhou,

    1. Default RGMII delay is 2ns as you mentioned.  By default, as Aniruddha described it, no external resistors are attached to the pins.  Skew is enabled by default, the datasheet values should indicate they are [stap] by default.

    2. If you want to control skew through MDIO, and your default is mode 1, then register 0x86 is the only changes necessary.  If register 0x32bit[1:0] is not equal to 0x3, then you'll need to set it to 0x3 to enable skews.

    Best Regards,