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Original question:

LSF0108: application issue of LSF0108

SN74AXC8T245: Help to confirm use SN74AXC8T245 for RGMII signals for level shift communication in system

Milo Wang
Prodigy 100 points
Part Number: SN74AXC8T245
Other Parts Discussed in Thread: SN74LVC8T245

Dear Sir/Miss,

I gonna adopt SN74AXC8T245 as RGMII level shift between 1.8V and 3.3V.  The topology and routing length are as below.Please help to confirm whether it works or not?

  • 0
    TI__Mastermind 44665 points

    Hey Milo,

    Best way to determine if it will work will be to use the EVM and test using similar loads you expect. I can't confirm whether or not it will work in your system. However, i do know others have used this device for RGMII.

  • 0
    TI__Guru 58370 points

    Milo,

    you can also refer to the following app note which talks about RGMII level translation using AXC8T245.

    The speed and prop delay is determined by the output loading, we have tested the AXC family to support the RGMII std speed ( exceeds 125Mhz)  at the rated 15pF loading. with higher loading, the speed would probably decrease a bit but i believe it will still support 125Mhz. The skew as tested is within 500ps ( RGMII stds) for 1.8 <>3.3V translation.

  • 0 in reply to ShreyasRao
    Prodigy 100 points

    Hi ShreyasRao，

    If testing result shows signals rising and falling timing over spec.Could I add pull up resistor for those signals?

    Thanks

    milo

  • 0 in reply to Milo Wang
    Guru 280610 points

    Pull-up resistors would not improve the falling edges, and would make the signal asymmetrical.

    To drive a higher capacitive load, you would need more drive strength, i.e., the SN74LVC8T245 (if it is fast enough).

  • 0 in reply to Clemens Ladisch
    Prodigy 100 points

    Hi Clemens，

    Thanks for your assistance.I want to replace AXC8T245 with AVC8T245.Please help to confirm whether it works or not? Whether the driving strength of the two sides of the chip are the same?For the longer signal of the trace, which side of the chip(A or B?) is used to connect?

    Thanks

    Milo

  • 0 in reply to Milo Wang
    Guru 280610 points

    AXC and AVC have the same drive strength. You can get higher drive strength only with LVC.

    The A and B outputs have the same drive strength.

  • 0 in reply to Milo Wang
    TI__Guru 58370 points

    Milo,

    as Clemens suggested, it seems that the drive strength of the AXC/ AVC is not sufficient for the signals to overcome the loading through the connectors and cables. You can switch to the LVC device which provides double the drive strength that is provided by the AVC/AXC devices for the same 3.3V level. ( 24mA)