Why the PIXCLK frequency is not consistant ?

leejey — Wed, 21 Mar 2012 10:43:08 GMT

Hi,

I am using iMX51 connected a WVGA LCD.

I measured the PIXCLK frequency and it is a variant waveform. It means that the high time on the clock pulse varies in a scope. As we know that Pixel clock frequency is directly related with the frame refresh rate. But I don't know why?

Re: Why the PIXCLK frequency is not consistant ?

FlorentAuger — Wed, 21 Mar 2012 15:59:17 GMT

Hello,

The typical structure for the IPU driver of Linux is the following.

struct fb_videomode {
const char *name;   /* optional */
u32 refresh;       /* this parameter is ignored */
u32 xres;       /* pixel */
u32 yres;       /* pixel */
u32 pixclock;       /* pico second */
u32 left_margin;   /* pix_clk */
u32 right_margin;   /* pix_clk */
u32 upper_margin;   /* line */
u32 lower_margin;   /* line */
u32 hsync_len;       /* pix_clk */
u32 vsync_len;       /* line */
u32 sync;
u32 vmode;
u32 flag;
};

The timing diagrams are the following:

___ <- hsync_len ->______________________________________
      |____________|          <--- xres --->       |________
                             <- left_margin -> _________ <- right_margin ->
______________________________|             |_______________________

___ <- vsync_len -> ____________________________________________
   |_____________|                  <-------- yres ------>                  |____
                   <- upper_margin ->___   _..._    ___ <- lower_margin ->
________________________________| |_|   |_| |____________________

The period of the pixel clock is given in pico seconds. The dividers inside the IPU are not integer, and depending on the value of pixclock, the clock might have a duty cycle that is different from 50%.

The calculation for the dividers being made automatically by the driver, it is sometimes necessary to slightly adjust pixclock to a higher or lower value to get a nice clock.

Re: Why the PIXCLK frequency is not consistant ?

leejey — Thu, 22 Mar 2012 07:19:25 GMT

Hi Florent Auger,

You supply me a great answer. Thanks a lot.

I will adjust the pixclk frequency for a better duty cycle.

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Why the PIXCLK frequency is not consistant ?

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