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Change ISS clock can influence video data frame rate

Albert Ke
Prodigy 160 points

I got strange relationship of ISS clock and video data frame rate in DM8148.

My configuration is connect camera sensor OV10630 with discrete sync in VIN1 PortA of HDVPSS.

The VIN format is BT.656 8bit.

When I set ISS clock as 400Mhz, the video frame rate is about 14fps.

But, when I reduce ISS clock to 200Mhz, the video frame rate raise to about 28fps.

According to data sheet, the clock source of ISS and Media Controller (VPSS-M3 and Video-M3) is PLL_MEDIACTL.

When I increase ISS clock to 400Mhz, the M3 clock should also become 400Mhz, it should increase M3 processing frequency and the video frame rate should become higher theoretically.

Does anyone can help to explain this testing result???

Best regards,

Albert Ke

  • 0
    TI__Expert 8780 points

    Albert Ke,

    The M3 & ISS actually works at PLL_MEDIACTL clock /2 frequency i.e. if you set the frequency of MEDIA_CTL as 400 Mhz - internally ISP and M3 will work at 200Mhz.

    More details on clocking and MEDIA controller architecture are available in the TRM.

    Regarding your result, what is the incoming frame rate of BT.656 data?Also, how are you calculating the frame-rate?

    Is it possible for you to count the Hsync / Vsync interrupts received and print the frame rate that is being captured? I suspect the frames are getting dropped somewhere.

    Regards

    Rajat