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TMDS171: TMDS171 TX-TERMINATION schematic and control

Ofek Nir
Prodigy 60 points
Part Number: TMDS171

1. How are the 150-300 ohm TX TERMINATION connected? Two pull up's or single resistor for each lane? is it possible to get simplified termination-resistors connection?

2. The definition of the TX_TERM_CTL pin #36 of the TMDS171 is different in the data sheet and in the evaluation board TMDS171RGZEVM. Which is the right one?

3. If jumper J14 at the evaluation board TMDS171RGZEVM is at the low pins (short 2-3) and the data rate is about 0.5G, what is the schematic of TX TERMINAL RESISTORS?

  • 0
    TI__Mastermind 23815 points
    Hi Ofek,

    That feature is Tx Termination impedance, I'm checking if we can disclose how it is implemented.
    The EVM user's guide is wrong, TX_TERM_CTL=L is reserved.
    You can start your testing with automatic TX termination or without tx termination, the HDMI 1.4 specification doesn't require a termination, it was added in this device to improve its performance.

    Regards
  • 0 in reply to Moises Garcia
    Prodigy 60 points
    If it helps, a simplified schematic with the TX termination resistor values and connections when JP14 in the evaluation board is 'DN' will help a lot.
    we connect only single resolution/refrash rate of 1024x768 @ 60hz
    thanks.
  • 0 in reply to Ofek Nir
    TI__Mastermind 23815 points
    Hi Ofek,

    We can't disclose how the Tx terminations are implemented, although this is a differential impedance.

    If the video format is 1024x768, 60Hz, 24bpp the data rate will be 650MHz per lane, in this case, there is no Tx termination, this is how all HDMI1.4 transmitters work.
    You don't have to worry about it, This is the normal setting for HDMI1.4 devices.

    Regards
  • 0 in reply to Moises Garcia
    Prodigy 60 points

    Hi Moises

    As when the EVAL BRD, when placed between our video source and the display solves the original problem of picture quality, we want to place the IC TMDS171 in our RFI card.

    However, the picture is ok only when J14 in the evaluation board is down (2-3); other settings like swing don't affect the quality.

    Also when tried by s/w, the change in TX TERM to 150-300 ohm  cause picture to be OK. Other s/w parameters like swing, emp, eq did not change the visual display quality. ONLY TX TERM.

    Please: what is the change in the EVL BRD behavour when J14 is down or when s/w set to 150-300 ohm.

    Thanks.

  • 0 in reply to Ofek Nir
    TI__Guru 60870 points
    Hi Ofek,

    So it appears that your system will only work if the source (TMDS171 Transmitter) has terminations enabled. The 150-300 ohm terminations can be enabled by the TMDS171 register or setting J14 low enables the 75-150 ohm terminations (which are only valid for HDMI 2.0). Enabling the terminations will have the effect of lowering the average DC level of single-ended signals. What level is your display expecting? Have you tried increasing the Vsadj resistor?

    Regards,
    JMMN