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TMDS181: DDC issues with Level Shifter

Martin Seeger
Expert 5411 points
Part Number: TMDS181
Other Parts Discussed in Thread: TMDS361B, , TCA9802, TMDS361

Dear Team,

our customer moved from HDMI1.x to HDMI2.x and changed the TMDS361B to the TMDS181.

The TMDS181 does not do the zero level regeneration anymore, so they chose the TCA9802 as level shifter (+3V3 to +1V8) and also to get zero level regeneration:

The TCA9802 works well with copper cables but when testing fiber optic cables the DDC line is failing for them. Attached you see the behavior off the DDC lines: Blue: SDA@Source, Green: SDA@TCA9802Aside, Yellow: SDA@TCA9802Bside, Red: SCL@ TCA9802Bside.

Looking forward to your feedback.

Best Regards
Martin

  • 0
    TI__Guru 60870 points
    HI Martin,

    Is the TMDS181 configured in DDC snoop mode? What pullups are installed in the system? Can you share the DDC schematic?

    Regards,
    JMMN
  • 0 in reply to JMMN
    TI__Expert 5411 points
    Hi JMMN,

    thanks for the answer.
    Let me see if I can get the schematics.

    In general the TMDS is setup as mentioned as sink side device as in figure 36 of the datasheet.
    Between the LTC4307 and TCA9802 there is a 4.7K pull-up.

    Best Regards
    Martin
  • 0 in reply to Martin Seeger
    TI__Expert 5411 points

    Julie,

    still working on the schematics - keeping you posted.

    Going one step back, we try to understand the changes between the TMDS361 and TMDS181:

    The TMDS361 has detailed information about the DDC block, see datas heet figure 42:

    The TMDS181 data sheet just shows a 'ACTIVE DDC BLOCK', see the figure in section 8.2:

    Thanks and best regards

    Martin

  • 0 in reply to Martin Seeger
    TI__Guru 60870 points

    Hi Martin,

    The TMDS361 is a switch that repeats DDC traffic only.  The TMDS181 is an active DDC repeater, so it acts as a slave to the source DDC master and a master to the sink DDC slave if DDC is routed through the TMDS181.  Due to some HDMI devices not implementing clock stretching we have been recommending that customers implement the DDC in snoop mode for TMDS181 instead of routing the DDC through the device.

    Regards,

    JMMN