On 03/30/2015 05:00 AM, George Grey wrote:
Hi Thank you - that will be very helpful
But, the import fact is that: Recent days i and our SoC guys are analyzing the HDMI signal issue. We have tested many mode timings on TV monitors and PC monitors and we found very a few mode timings can work, including the 720P. SoC guys said that TV monitors are follow the CEA 681 standard and our SoC can't math this standard and during our SoC design phase HDMI is not taken into account. I'm afraid this signal can't be fixed and our SoC can't support HDMI well.
Can you explain the problem and the limitations of the SoC in technical detail? You say that a few timings do work including 720p. Can you explain the technical reason for this? Is it possible to determine which timing/configurations will work and which will not?
It's not 720p that's working, at least not the CEA compliant 720p. It's a timings setting where the active resolution is the same as 720p, but the front porch/back porch/blanking timings as well as the refresh rate are different. The reason why this timing works is because the result of the formula that I explained in the earlier mail ("lengthA * clkB / clkA = lengthB") is integer for these settings. And if the result is integer the video timings are uniform in the DSI/HDMI clock domain and hence the monitor will be able to recognize the timings and display the image.
For the timing we are talking about the LDI clock is 72 MHz, the DSI clock is 297.6 Mhz, the total line length in the LDI clock domain is 1650.
The HDMI clock is DSI clock / 4. If you insert that into the above formula you get 1650 * (297.6 / 4) / 72.0 = 1705.0. That's why it works. But it's not 720p and not what the monitor says it supports, but most modern monitors are quite forgiving when it comes to timing as long as the active area is correct and the refresh rate is within reasonable bounds. But you'll not pass compliance testing with this.