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TMDS181/TMDS171 CDR circuits agility

Igor Uspensky
Intellectual 670 points

Other Parts Discussed in Thread: TMDS181, TMDS171

Hi,

The latest data sheet document for TMDS181 (SLACE75C) states (see 8.4.1 Retimer Mode, page 31):

"When the video source changes resolution, the internal retimer starts the acquisition process to determine the input clock frequency and acquire lock to new data bit streams. During the clock frequency detection period and the retimer acquisition period that last approximately 7 ms, the TMDS drivers can be kept active (default) or programmed to be disabled to avoid sending invalid clock or data to the downstream receiver. The TMDS181 can be configured to work as a redriver (full range), crossover (redriver-retimer), and retimer (full range)."

Some questions

  1. Is it really always takes up to 7ms (seems that was discussed here early but I can't find)?
  2. The part can operate in automatic redriver to retimer crossover mode (the crossover point is about 1 Gbps). In that mode, when the input clock frequency has changed, is there difference in acquisition period time when the frequency has crossed that point (in comparison with the case the boundary has not crossed)? The Table 6.9 specifies tD3 parameter (CDR active operation after retimer mode initial). Is it applicable to the crossover moment?
  3. How to disable the TMDS drivers during the acquisition process?
  4. I presume the silicon designer's intention was to let the part to handle the input clock frequency changings automatically so it's not necessary for users to disable the drivers. But this feature is mentioned because it can provide some benefits in some (rare) cases. Moreover, there is the document (SLLA358) that discusses how an "advanced user" can manipulate the Page 1 registers to take full advantage of the part. If TI is going to provide similar public document for TMDS171/181?
  5. According to the Table 6.6 (TMDS Differential Input Electrical Characteristics), the redriver mode is practical in the data rate range from 0.25 to 3.4 Gbps. As the TMDS181 supports HDMI2.0, the "full range" term within the document is defined as 0.25 - 6 Gbps. So the statement "The TMDS181 can be configured to work as a redriver (full range)..." looks misleading. Is it correct?

Thanks in advance for your comments

---

regards,

    Igor

  • 0
    TI__Mastermind 23815 points
    Hi Igor,

    1) Clock frequency detection period and clock acquisition period takes approximately 7ms, however this is not the time it takes to see an image in the display. If you just connected the display, you need to wait for video source to read EDID registers, or wait for the start of the next frame.
    2) When entering retimer mode, you need to add td3, this is the time it takes to activate CDR.
    3) if you want to disable Tx, you can do the same for DP159
    {0xFF, 0x01} , // Select Page 1
    {0x10, 0xF0} , //Disable all Tx Lanes
    {0xFF, 0x00} , // Select Page 0

    follow the same procedure in slla358 section 4.3.2 to enable the desired Tx Lanes
    4)There is no plan to release a similar document
    5)Well if you write register 0x0A[1:0]=00, you will have redriver mode, it may be the full range indeed 0.25 to 6Gbps,, but redriver is not HDMI complaint for data rates higher than 3.4Gbps, that's why the redriver range is reduced to 0.25 to 3.4Gbps.

    Regards
  • 0 in reply to Moises Garcia
    Intellectual 670 points

    Hi Moises,

    Thanks a lot for the clarifications. Timely and professionally.

    For the item 5), could you to confirm that my understanding is correct? The DEV_FUNC_MODE field (0x0A[1:0]) does not affect the snooping so let's consider the following scenario:

    Assume the source device is operating in HDMI1.x mode (1/10) and the part's snooping function is enabled.

    1. The user has changed the DEV_FUNC_MODE field from its default value to 0;
    2. The user has toggled PD_EN field (0x09[3]) twice to activate the new DEV_FUNC_MODE value forcing the part to operate as a redriver;
    3. The source device has declared its intention to switch to HDMI2.0 mode (1/40) by setting the TMDS_Bit Clock_Ratio flag in the respective SCDC register;
    4. The part has detected this transition through snooping so the TMDS_CLOCK_RATIO_STATUS bit (0x0B[1]) has changed from 0 to 1 indicating that the part, in turn, has switched to 1/40 mode.

    In that way it's feasible for the part to operate as a redriver at the data rates above 3.4 Gbps. In that case it's the user's responsibility to activate the retimer function (there is no automation).
    ---
    regards,
    Igor

  • 0 in reply to Igor Uspensky
    TI__Mastermind 23815 points

    Hi Igor,

    If user sets full range redriver mode, then source enters x40 video transmission, TMDS181 will keep working as redriver, but it won't work properly.

    In this case the device would have to move manually to retimer mode.

    If you want redriver for data rates below 3.4Gbps and retimer for data rates above 3.4Gbps, you an write 0x0A[1:0]=10b

    Regards

  • 0 in reply to Moises Garcia
    Intellectual 670 points
    Thank you very much. That's exactly what I'd asked