External Crystal issue when STM33H7 LTDC interface used with RGB to HDMI convertor TI TFP410

[VShet.2]

I am using STM32H743IIT6 MCU to drive HDMI display using LTDC peripheral and Texas Instrument TFP410 for RGB to HDMI conversion. Issue I am facing is I use external crystal as clock source then I see disturbance on my HDMI monitor and its display blinks randomly, say screen is proper for some 5-10 sec then it will flicker and turn off and on then again be stable for some time and these repeats. My guess is that some HDMI timing related parameters are not matched and that's why this behavior on monitor. Surprisingly, this issue resolves if I use internal CSI RC oscillator as clock source.  I even tried doing the PCB layout again using an external agency, but same issue persists in that layout as well. What could be the issue here that it works with internal CSI RC clock source and not external crystal ? Please find below related parameters configured in my project. I have also attached my projects CubeMX .ioc file for reference.

1. Clock Source: External Crystal 25MHz
2. FMC: External SDRAM ISSI IS42S32200L-6TL which is 32bit SDRAM
3. LTDC Settings: 1280 x 720p HD resolution @60Hz (Below parameters are as specified in standard without which if I connect to Dell monitor then it gives timing error on screen)

1. LTDC Clock 74.250 MHz
2. 16bit RGB565
3. LTDC data, clock, HSync, VSync, DE signal goes to TI TFP410 IC whose output are then connected with HDMI connector.
4. Horizontal Timings
   1. Active Pixels 1280
   2. Front Porch 110
   3. Sync Width 40
   4. Back Porch 220
   5. Blanking Total 370
   6. Total Pixels 1650
   7. Sync Polarity pos
5. Vertical Timings
   1. Active Lines 720
   2. Front Porch 5
   3. Sync Width 5
   4. Back Porch 20
   5. Blanking Total 30
   6. Total Lines 750
   7. Sync Polarity pos

[Tesla DeLorean]

Aren't Sync and Back Porch concurrent?

[VShet.2]

No, they are different. 

[MM..1]

Tesla and your image mean , that sync is part of accu back porch, and your nums little miss.

for example

 hltdc.Init.HorizontalSync = 4;
 hltdc.Init.VerticalSync = 4;
 hltdc.Init.AccumulatedHBP = 12;
 hltdc.Init.AccumulatedVBP = 12;
 hltdc.Init.AccumulatedActiveW = 812;
 hltdc.Init.AccumulatedActiveH = 492;
 hltdc.Init.TotalWidth = 820;
 hltdc.Init.TotalHeigh = 506;

back porch here is 8  = 12-4

your nums miss 

1. Blanking Total 370
2. Total Pixels 1650 ...

[AScha.3]

>What could be the issue here that it works with internal CSI RC clock source and not external crystal ?

 

You adjusted PLL/clk for 4M CSI and 25M HSE absolutely to same speed (for core, LTDC, FMC, etc. ) ?

[VShet.2]

Yes, I have adjusted the PLL values such that the core. LTDC, FMC clock remains same in both case.

[AScha.3]

So check with a scope : LTDC  clock, HSync, VSync -> same timing with HSE or CSI ?

[VShet.2]

This issue got fixed as I have replaced External Crystal with External Oscillator in my PCB.