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DRA80M: What is RGMII series resistor value and absolute maximum ratings

Jason Hale
Mastermind 34310 points

Part Number: DRA80M

Hello E2E community,

My customer is asking generic questions about the DRA80xM device:

 

What is the value of the series resistor to avoid overshoots and undershoots within the specification?

We want to make sure that the slow, typical and fast models are working with our layout and the series resistor. For this we need the max. and min. allowed voltage for over/undershoots at the RGMII interface with 1V8 and 3V3 IO Supply. I think all have the LVCMOS buffer. The min. high level threshold and the max. low level threshold is available for the LVCMOS buffer.

Checking the Data Manual, Technical Reference Manual and the AM65 product page and "Technical Documents" on www.ti.com, I believe the following is true:

  1. RGMII is using LVCMOS buffer (Source: SPRSP31A, Data Manual vA, page 22/277 : RGMII balls all show LVCMOS).
  2. Series resistor value is 22-ohm (Source: SPRACI9.pdf, AM65x / DRA80x Schematic Checklist, Section "2.14 CPSW Ethernet"
  3. RGMII balls are in the VDDSHV2_WKUP domain (Source: SPRSP31A, Data Manual vA, page 22/277 : RGMII balls all show LVCMOS)
  4. VDDSHV2_WKUP (therefore RGMII) has min/nom/max range of 1.71V/1.8V/1.89V for 1.8V IO and 3.14V/3.3V/3.46V for 3.3V IO.
  5. But the Schematic Checklist shows that RGMII is only supported with 1.8V IO (and 3.3V IO is not supported).

Are all these statements correct?

Have I interpreted the data correctly?

Thank you!

  • 0
    TI__Mastermind 39520 points

    The value of the series termination resistor is dependent on the specific design.  The PCB's target impedance could drive different resistor values.  It is possible no resistor is required.  Recommend using the IBIS model customer's PCB simulation to determine if resistor is required and if so, the specific resistor value.

    I believe the five statements are correct.  RGMII is supported at both 1.8V and 3.3V IO.  However, there is an errata that states 1Gb speeds for RGMII are only supported for 1.8V IO.

  • 0 in reply to Robert Eschler
    TI__Mastermind 39520 points

    Have the questions been answered, and this thread closed?

    Thanks

    Robert

  • 0 in reply to Robert Eschler
    TI__Mastermind 39520 points
    I've done some research into the 1.8v vs 3.3v RGMII IO at 1Gbps issue that I mentioned. It appears that is not an issue. I was involved in early development, and was made aware of this concern. However - its since been resolved, and there is no issue. RGMII interface can operate at 1Gbps for both 1.8V and 3.3V IO. I apologize for any confusion.