• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 33 subscribers
  • Views 220 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.

SN75LVDS83B: CLKOUT problem

Gianluca Angelini
Prodigy 10 points
Part Number: SN75LVDS83B

Tool/software:

Hi,

   we have built a process control board with an HMI interface and we are now testing it.

For the 800x600 pixel TFT display that we have to manage, we have provided both an RGB and LVDS interface. The latter is obtained with the SN75LVDS83BDGG component connected as reported in the attached electric diagram.
LVDS MP106 Rel 030124.pdf
The RGB display works perfectly (we have been using it for years) while the LVDS one does not (this is the first time we have used it).
By investigating with oscilloscope, datasheets, forums, etc. we immediately saw an high ripple on the LVDSVCC power supply which we eliminated by short-circuiting the two choke filters L15 and L16.
After we saw that of the 5 pairs of differential LVDS signals the CLKOUT one does not move, while the other 4 do. 
Both CLKOUT signals are still whether connected to the LVDS display or to a simple 100 Ohm resistor.
On the CLKIN signal there is the 40 MHz clock signal generated by the graphics controller and the /SHTDN pin is off (3.3 V).

After 2 days of research we have not found any explanations and for this we decided to write this request.
Can you help us? Any suggestions are welcome.

Thanks in advance and best regards.
Gianluca Angelini
Software department

  • 0
    TI__Mastermind 33485 points

    Hi Gianluca,

    Thank you for the background information on your issue. I have some suggestions/questions below.

    Gianluca Angelini said:
    By investigating with oscilloscope, datasheets, forums, etc. we immediately saw an high ripple on the LVDSVCC power supply which we eliminated by short-circuiting the two choke filters L15 and L16.

    How high was this ripple? Are L15 and L16 ferrite beads? What is the MPN for these components?

    When you said these elements are short-circuited, do you mean that they are removed and replaced with a 0Ω resistor or a solder bridge?

    Gianluca Angelini said:
    After we saw that of the 5 pairs of differential LVDS signals the CLKOUT one does not move, while the other 4 do. 
    Both CLKOUT signals are still whether connected to the LVDS display or to a simple 100 Ohm resistor.
    On the CLKIN signal there is the 40 MHz clock signal generated by the graphics controller and the /SHTDN pin is off (3.3 V).

    Above is the lower half of the functional block diagram. The CLKIN signal is used to provide the clock for the RGB data shift registers. If you see output on the other LVDS pins, then we know the device is distributing the clock throughout the device.

    Have you seen this behavior of no CLKOUT on multiple boards? You said this is the first time the LVDS83B has been tested.

    Best,

    Jack

  • 0 in reply to Jack Scherlag
    Prodigy 10 points

    Hi Jack,

           thanks for the quick reply. Below I answer your questions and suggestions.

    The ripple on LVDSVCC signal was about 0.4 V.
    Yes L15 and L16 are ferrite beam. Both components are Bourns SDR1005-681KL.
    Now these components are still mounted but there is a short 0.25 mm wire that short circuit their two pins.

    Thanks for the clarification on the multiple functions of CLKIN, I hadn't noticed them.

    Unfortunately we only have one card with the LVDS interface mounted and obviously I have only seen the problem on this one.
    We also thought about trying other cards and now we are waiting for the LVDS connectors in order to mount the interface on other 4 cards.
    I hope that in a few days we will be able to try them and I will immediately report the results.

    Best Regards
    Gianluca

  • 0 in reply to Gianluca Angelini
    TI__Mastermind 33485 points

    Hi Gianluca,

    Gianluca Angelini said:
    Yes L15 and L16 are ferrite beam. Both components are Bourns SDR1005-681KL.

    The Bourns SDR1005-681KL is not a ferrite bead. This component is an "SMD Power Inductor". I do not recommend using an inductor like this for removing high frequency noise. Ferrite beads are classified by their impedance at a specific frequency and not by inductance.

    Gianluca Angelini said:
    Unfortunately we only have one card with the LVDS interface mounted and obviously I have only seen the problem on this one.
    We also thought about trying other cards and now we are waiting for the LVDS connectors in order to mount the interface on other 4 cards.
    I hope that in a few days we will be able to try them and I will immediately report the results.

    Remove components L15 and L16 when testing the new cards. A voltage swing of 0.4v is 4x the maximum noise listed for any VCC terminal.

    Best,

    Jack