• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 98 subscribers
  • Views 1198 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Should we add the serial termination resistors for GPMC and MMC0 bus?

Cosmo Yu
Intellectual 300 points
Other Parts Discussed in Thread: AM3352

Hi Sir,

Should we add the serial termination resistors for GPMC and MMC0 bus? If yes, which resistor value is suitable, 22 Ohm or 33 Ohm?

Cosmo

  • 0
    TI__Guru**** 393215 points
    Hi Cosmo,
     
    This is entirely up to you. The serial resistors help reduce board EMI. They are helpful if your board traces are long. If you are in doubt, I would suggest that you place 0Ohm resistors, which can be later changed when you see how the board behaves.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 300 points

    Hi Biser,

    Thanks a lot for your quick reply! Are these serial resistors only for board EMI control? These traces are not long on our board. We place the NAND flash and Micro SD closed to AM3352. If we place these 0 Ohm resistors, it will be a challenge to layout due to the limit room between AM3352 and devices. Now I only place the serial resistors (22 Ohm) on GPMC_CLK and MMC0_CLK. Is that OK?

    Best regards,

    Cosmo

  • 0 in reply to Cosmo Yu
    TI__Guru**** 393215 points
    Hi Cosmo,
     
    I think this should be OK. My personal opinion has always been that it's 50-50 between the usefulness of these serial resistors and having to break traces, place vias etc. in order to place them. Especially if the bus runs along internal board layers I see no point in exposing traces to an external layer just to place a resistor.
  • 0 in reply to Biser Gatchev-XID
    Intellectual 300 points

    Hi Biser,

    Yes, that's our situation (the bus runs along inner layers). Thanks again for your great support!

    Best regards,

    Cosmo

  • 0 in reply to Cosmo Yu
    TI__Guru**** 393215 points
    Hi Cosmo,
     
    A clarification from the AM335X team:
     
    There are cases were TI devices will source a clock through the output buffer of a bi-directional IO to the terminal which is connected to the input buffer of the same bi-directional IO and external device.  This is done to help with timing closure by minimizing clock skew between the internal clock and external clock.  However, there is a nasty side-effect caused by using this approach.  The side-effect can be improved by inserting a series resistor as close as possible to the source terminal.
     
    I have attached part of an old application note from several years ago that includes a section that describes this issue. 5342.CLK_issue.pdf 
    There are cases where the series resistor is required to prevent the internal clock from getting glitches.  In this case, the resistor is not an option.
     
    AM3xxx devices have several clock outputs that are looped back at the terminal and require a series resistor.  This is normally mentioned in the respective schematic checklist.