• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 34 subscribers
  • Views 614 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.

DS90UB927Q-Q1: FPD Link III Overkill for crossing 1 meter in automotive environment?

Martijn de Milliano
Prodigy 190 points
Part Number: DS90UB927Q-Q1
Other Parts Discussed in Thread: SN65DSI83, DS90UB928Q-Q1, DS25BR100, DS25BR440, SN65LVDS93A

Hi, I would like to connect a small TFT display that uses LVDS, 4 channels for data and 1 channel for clock. Data rate is maximum 200 Mbps per lane. Very strict EMC requirements are applicable, namely automotive. What would be the maximum length of a typical LVDS cable that would still work in this environment? Length could be up to 1 meter. I am wondering from what length onwards I should use something like FPD Link III SerDes (e.g. DS90UB927)? Or would that be overkill? BOM cost is important.

Update: the device driving the LVDS signals is TI SN65DSI83.

  • 0
    TI__Genius 12866 points
    Transferring to another group.
  • 0
    TI__Mastermind 35130 points
    Hi Martjin,

    To clarify, is your application DSI to LVDS, with the LVDS link up to 1m in length? And are you certain about the data rate requirement? 200 Mbps is close to the minimum operating frequency for the LVDS output clock.

    Regards,
    I.K.
  • 0 in reply to Ikechukwu Anyiam
    Prodigy 190 points

    Dear I.K.,

    Yes, I have an application processor that has DSI display output, I use SN65DSI83 to change this to LVDS display output, and then connect that to a TFT panel that uses LVDS.

    With SN65DSI83 and the current panel (1024x600@25 fps, 24-bit), the parameters of the LVDS link are: LVDS clock 51.2 MHz, total link data rate is 517 Mbps over 4 lanes meaning 129 Mbps per lane. This works fine. Now I would like to transport this LVDS signal over a longer distance (max 1 m).

    I see that in the SN65DSI83 the maximum LVDS clock rate range is 25 MHz to 154 MHz and for DS90UB928Q-Q1 5 to 85 MHz, so to me it seems that we are operating well within the range of the devices. Where did you see that 200 Mbps is the *minimum* operating frequency of the LVDS output clock?

    Thanks,

    Martijn

  • 0 in reply to Martijn de Milliano
    TI__Mastermind 35130 points
    Hi Martijn,

    I was just going off of the equations for calculating frequency/throughput (#7 here: www.ti.com/.../slla356.pdf). Does your display datasheet state those requirements? If so it should be fine.

    FPD-Link III is a bit overkill for just 1m, as they are are typically good for 10m+ applications. I believe you should be fine with just using the DSI83 as long as you have a good cable. If there's concern about the distance you can use buffers (e.g. DS25BR440 and DS25BR100) rather than a SerDes.

    Regards,
    I.K.
  • 0 in reply to Ikechukwu Anyiam
    Prodigy 190 points

    Dear I.K.,

    Thank you for your advice regarding LVDS versus FPD Link for this case. This is clear, and I consider that to be the answer to my original question. I would be interested in knowing your reasons for believing that just a normal LVDS cable would be fine? Did you do any calculations for this?

    About the frequency: using the equation in the referred document (with Htotal=1356 and Vtotal=636 and fps = 25) I end up with an LVDS clock of 21.6 MHz, which is outside of the range of the datasheet of the panel. Nevertheless, with my past calculations I was able to bring up the screen, so apparently it is quite forgiving. I would have to check this again, perhaps this is a clue for some weird behaviour that we were seeing sometimes! If this is going to prompt further questions on SN65DSI83 I will open a separate question for this. Thanks for pointing out this possible error!

    Martijn

  • 0 in reply to Martijn de Milliano
    TI__Mastermind 35130 points
    Hi Martijn,

    My reasoning is based on our generation 1 FPD-Link and Channel Link devices (e.g. SN65LVDS93A). These devices typically perform well at 1m distances, and the LVDS interface is very similar to that of the SN65DSI83. STP cables should suffice.

    In regards to your frequency and panel datasheet, the datasheet's probably assuming a frame rate of 60fps. Your 25fps frame rate is rather low and would bring the frequency down significantly. Although it is working now, you are still operating outside of the panel's datasheet specs so it can't be guaranteed to work all of the time.

    If you plan to use the SN65DSI83 with these settings (21.6 MHz) we won't be able to support any issues you may see since the minimum frequency specified by our datasheet is 25 MHz, so please go ahead and double check the requirements.

    Regards,
    I.K.