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Problems with LVDS to TTL interface (DS90CF386)

Chris Sherwin
Prodigy 20 points
Other Parts Discussed in Thread: DS90CF386

Greetings,


I am currently working on an embedded computing application that requires a small form factor computer and touchscreen.  The computer has an Intel atom processor with native LVDS output.  I am using a DS90CF386 LVDS Receiver chip to convert to 24-Bit TTL signals for a 5.0" Flat Panel Display.  The problem is the display is not working - the best I can get is random vertical lines on the display.  When I look at the TTL signals to the display it looks as if the VSYNC, HSYNC, and DEN signals are all inverted?  This is with a CLK input signal of about 32MHz.  However, if I slow the CLK signal down to about 24MHz by changing the default screen resolution in BIOS to 640x480 (which is not the correct resolution for the LCD) the VSYNC, HSYNC, and DEN signals now appear correct? (I still get garbage on the LCD though).

Any ideas on why the signals would be inverted at the faster clock speed?   Is this a clue that something else is off?  Any advice on LVDS to TTL interfaces in general? 


Thanks much in advance.

-Chris.

  • 0
    TI__Expert 3050 points

    Hi Chris,

    Can you share the schematic page with the DS90CF386?

    Also, it would be helpful if you could collect the following data:

    • scope shot of the LVDS inputs, measured differentially for all clock and data inputs
    • scope shot of the LVDS data inputs relative to the clock, measured differentially
    • scope shot of the power supply noise
    • scope shot of the LVCMOS outputs measured as close to the inputs of the panel as possible

    The goal of the scope shots is to verify LVDS signal integrity, LVDS clock to skew, power supply design and LVCMOS signal integrity.

    Mike Wolfe

    DPS APPS / SVA

  • 0 in reply to Mike Wolfe
    Prodigy 20 points

    7345.2465 Rev 00 LVDS page 1.pdf

    Mike, thanks for the quick reply.  The schematic page is attached.  NOTE: There are 100 ohm terminating resistors across all four LVDS pairs, but those are not shown on that schematic page.

    I'll gather the scope data and then post that as soon as I can.  It appears there may be some problems with the driver that windows is loading to use with the display, so I am going to chase that down as well and verify that is not the culprit.  More info to come...

    Thanks,

    -Chris. 

  • 0 in reply to Chris Sherwin
    TI__Expert 3050 points

    Hi Chris,

    The schematic looks to be in order. You noted that the LVDS terminations are not included on this page. As long as these LVDS terminations are placed as close to the DS90CF386 as possible then there shouldn't be a problem.

    I'll keep an eye out for your reply with the scope shots and update regarding the potential driver issue.

    Cheers!

    Mike Wolfe

    DPS APPS / SVA