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hdmi_timings pixel clock constraints

Posted: Mon Jun 08, 2015 10:26 pm
by wpt-nathan
Hi All,

Would someone be able to provide me with the constraints for the values provided to the hdmi_timings configuration option? Specifically the pixel clock. It seems at least that there is a lower limit, or that it must be a multiple of some other reference clock.

Thanks,
Nathan

Re: hdmi_timings pixel clock constraints

Posted: Mon Jun 15, 2015 1:51 am
by wpt-nathan
Through some trial and error experimentation I figured out why I couldn't get the pixel clock I wanted. By reading the DPI clock control registers I observed that for low frequencies, such as the 6Mhzish clock I was aiming for, the OSC is used, which is 19.2 Mhz. However, the boot code seems to only set the integer divider, not the fractional, meaning that only frequencies such as 9.6Mhz, 6.4Mhz, 4.8Mhz, etc work. So if the requested clock is not an even divisor, the DPI does not seem to be configured at all.

Fortunately the TFT display I am connecting to the DPI interface allows me to configure the pixel timings, so I was able to get it to work at 60fps with a DPI acceptable clock. I have full frame rate video playback on a 240x320 ( yes, portrait ) display.

--Nate

Re: hdmi_timings pixel clock constraints

Posted: Fri Aug 26, 2016 8:18 pm
by ironic-FR
Hi.
I know, this post is a fex old but, is it possible, now, to use non integer divider pixel clock ?
If i use a HDMI to VGA converter and a VGA to RGB cable, i can use Rpi ont a CRT TV at 720x240x60Hz with hdmi_cvt.
But, if i use a Gert VGA666 (GPIO to VGA) abd the same VGA 2 RGB cable, it works but i can't translate modeline to works.
I don't know how to have a 720x240x60hz resolution.

Re: hdmi_timings pixel clock constraints

Posted: Wed Apr 15, 2020 5:19 pm
by zontar
Hi,
I am experiencing similar problems (see viewtopic.php?f=44&t=270030 ).
Has anybody found values that work or dividers that work, apart from integer dividers of the Oscillator clock (19.2MHz)?

It's a nightmare to use custom mode in HDMI, everything is scarcely documented and there are many references to "defaults" or "normal" or "standard for" which are not normal for laypeople not accustomed to HDMI standards.
BTW I think a clear guide with clear instructions (i.e. put '1' if clock is active low, or put '0' if HSYNC is active High, without ambiguities, should be a nice thing).

Thanks
Z

Re: hdmi_timings pixel clock constraints

Posted: Wed Apr 15, 2020 5:19 pm
by zontar
Hi,
I am experiencing similar problems (see viewtopic.php?f=44&t=270030 ).
Has anybody found values that work or dividers that work, apart from integer dividers of the Oscillator clock (19.2MHz)?

It's a nightmare to use custom mode in HDMI, everything is scarcely documented and there are many references to "defaults" or "normal" or "standard for" which are not normal for laypeople not accustomed to HDMI standards.
BTW I think a clear guide with clear instructions (i.e. put '1' if clock is active low, or put '0' if HSYNC is active High, without ambiguities, should be a nice thing).

Thanks
Z