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Flickering -rippling- problem on LCD, using DM6437

Elric
Expert 1250 points

Hi,

We are using dm6437 on our custom board to interface to LCD. The LCD model is TX07D09VM1CBB-2 Hitachi 240*320 LCD.

We configured the OSD and VENC module (with the aid of the Digital Video Using DaVinci SoC document), so that we can take the digital output on LCD currently. All the signals (vsync, hsync, vclk...) are met with the LCD's requirements. So everything seems good so far, but we have a flickering (or the precise word is "rippling") problem.

We are using an external clock of 6.4MHz, which is a standard for the LCD we use. The vsync value is about 67-68Hz.

The problem is, it is like we see the vertical lines constantly processing from up to down (especially on the black areas it is much more visible), an effect like our vsync is very very low. I know this is a very general question, but do you have any ideas what should cause this problem? Can we have a wrong configuration on the DSP side or may it be related to LCD capabilities?

Also, when we use the external clock higher than 6.4MHz, for example 11MHz, this flickering problem is gone. We are definitely out of the LCD requirements (Max DCLK for LCD is 6.5MHz, which we have 11MHz,and max vsync value is 68Hz while we have 110Hz), but the output we take is certain and looking very beatiful, though we could't understand how it works fine with such higly values.

If we use the LCD with these values, what kind of consequences may happen?

Thanks in advance,

Erman

 

 

  • 0
    TI__Mastermind 41665 points

    Elric said:
    The problem is, it is like we see the vertical lines constantly processing from up to down (especially on the black areas it is much more visible), an effect like our vsync is very very low. I know this is a very general question, but do you have any ideas what should cause this problem? Can we have a wrong configuration on the DSP side or may it be related to LCD capabilities?

    The first thing that comes to mind would be noise or crosstalk, though I do not see that happening with the low frequency, and I do not see how that would correlate with it going away at a higher frequency. It could be some timing issue with the LCD, though it sounds like you have already verified the timings. To narrow this down further you could take a look at the data coming out of the VENC with a logic analyzer to see if you can see the rippling in the digital data coming out of the VENC, this should tell you definitively if the problem is coming from the VENC or the LCD itself though it may not be practical to actually do depending on your hardware.

    Elric said:
    Also, when we use the external clock higher than 6.4MHz, for example 11MHz, this flickering problem is gone. We are definitely out of the LCD requirements (Max DCLK for LCD is 6.5MHz, which we have 11MHz,and max vsync value is 68Hz while we have 110Hz), but the output we take is certain and looking very beatiful, though we could't understand how it works fine with such higly values.

    If we use the LCD with these values, what kind of consequences may happen?

    This is something you will want to check with your LCD manufacturer, they may have some explanation for the clock and why it is specified how it is. It seems strange to me that you can overclock the LCD to almost double the max spec and still have it function at all, let alone work better than when it was at its correct specification. Perhaps there was some confusion within the LCD specifications to begin with?

  • 0
    Prodigy 90 points

    Dear Elric,

    Do you use MXI for clock source?

    We have the same problem with our DM355 MCU , we are using MXI2 for clock source with 6.4 Mhz crystal.

    How did you arrang on the "structure vpbe_videomode_prgb" values?

    Thank you.