TFP401A: No video output on display Riverdi RVT70HSTNWN00

Hey Damien,

Sorry for the delayed response, this thread was assigned to the wrong person originally. 

Can you confirm which version of HDMI you are using? The raspberry pi compute 4 which outputs HDMI 2.0, but it looks like you are using HDMI 1.0 in the E2E.

From the datasheet:

H_total = 1344               Refresh Rate = 60Hz

V_total  = 635

Pixel clock = H_total * V_total + RR = 1344 * 635 * 60 = 51,206,400

The pixel clock expected to be seen is 51.2 MHz. Looks like the pixel clock is halved for some reason.

TMDS Clock = (Pixel_clock * 24bpp * (10/8) encoding) / (3 lanes * 40 hdmi 2.0 clock )

TMDS Clock = (51.2 MHz * 24 * 10/8) / (3*40) = 12801600

The TMDS clock should be 12.8MHz

It seems that the timings are off on the HDMI side.

Hello,

I’ve very probably found the issue, the blue channel was connected to Odd-pixel output, whereas the green and red channels are connected to the Even-pixel output of the TFP401.

The Pixel clock was running at 24.5 MHz instead of 49MHz due to the pin PIXS HIGH. The HDMI clock was measured at 49MHz.

I will soonly do a PCB modification (with bulge wires) for re-routing the blue channel correctly and I will inform you of the results.

Best regards

Ok sounds good

BTW, does my decoupling is correct? (ferrite bead + 2x4.7uF + 100nF cap at each VDD pin), separated for OVDD, AVDD, PVDD, DVDD

I use 100nF caps at proximity of the chip VDD pins, but the datasheet sometimes specify 10nF, sometimes 100nF. What's better?

The implementation is good. when deciding what caps to place the system power supply noise must be taken into account. The 100nF is a general purpose value that will account for a majority of noise frequencies on the supply line. You will need to take noise measurements on the power plane and isolate the frequencies at which the noise is the highest and filter those specific frequencies out.

Using the 100nF you have implemented should be fine. 

Thanks, last question : for confuring the pins (ST, STAG, PIXS) etc, a direct pull up to GND or VDD is acceptable or a resistor is recommanded?

I recommend adding a pull-up and pull-down resistor, or a switch on the control pins as it will provide greater flexibility on the PCB as you will be able to change the settings easily. If you directly connect a pin to the ground or power plane you cannot easily change the board configuration. 

So I've done my bulge wire modification, re-routing the blue channel to the EVEN output correctly.

I finally got an image, colors are fine but the picture is shifted.

The HDMI output seems good and works well with a very similar display (a Waveshare 7inch 1024x600 display also using a TFP401 as an interface IC).

Try changing PIXS to low

So it was the timings of the HDMI output that were off.

After a lot of test and errors with the EDID file (especially the H and V front porch timings), I finally got the timings right, the picture is now perfect and centered.

Thanks for you help