NHD-4.3-480272EF-ASXN# Backlight but no Graphic Data

Hi,

We are using the NHD-4.3-480272EF-ASXN# in a product that we expect to deliver soon.
However, we are currently blocked since we haven't been able to solve an issue with the LCD.
Can you please help us fix this?


The issue is that on a cold start (and sometimes almost cold - 100mV to 300mV), the LCD sometimes enters a state where the backlight can be active, but there is no graphic data.
All of the RGB bits are present, PCLK is active, and so are HSYNC and DEN.
VSYNC is held low.

When the LCD is in either a working or a failed state, quick power cycles do not cause a change.
it is only when we let VDD to drain to 0V (cold start) that there isd a chance of the LCD changing from a healthy state to a faulty state, or vice versa.

We have the NHD-4.3-480272EF-ASXN# connected to a BeagleBone Black Industrial, and have tried different timing parameters.

At first, by accident we had the wrong parameters which carried over from a completely different LCD.
These parameters cause the LCD to enter either of the aforementioned states.

clock-frequency = <9200000>;
hactive =  <480>;
vactive =  <272>;
hfront-porch =  <8>;
hback-porch =  <47>;
hsync-len =  <41>;
vback-porch =  <2>;
vfront-porch =  <3>;
vsync-len =  <10>;
hsync-active =  <0>;
vsync-active =  <0>;
de-active =  <1>;
pixelclk-active = <0>;

After we noticed this, we corrected the timing parameters to match the NHD-4.3-480272EF-ASXN# datasheet.
In this state the LCD seemed to appear as though it was always working.
However, the chip that delivers the RGB data, PCLK and sync signals to the LCD automatically filters any HSYNC or VSYNC pulses that are less than 2 full PCLK cycles.
When I disable this filtering to allow HSYNC through, the graphic data slowly fades away, leaving only the backlight.
This led us to believe that the issue is due to timing, and that we were getting closer.
Have a look at this video:
https://rctglobal-my.sharepoint.com/:v:/g/personal/danr_rct-global_com/EWR9me0_vXZPpMP6sw-vQkEBlXDMFyJYGIxhXaVP2iLAqQ?e=P6O0gj

clock-frequency = <12000000>;
hactive =   <480>;
vactive =   <272>;
hfront-porch =   <2>;
hback-porch =   <43>;
hsync-len =   <1>;
vback-porch =   <12>;
vfront-porch =   <1>;
vsync-len =   <1>;
hsync-active =   <0>;
vsync-active =   <0>;
de-active =   <1>;
pixelclk-active = <0>;

We are worried because we can't see VSYNC being generated by the BeagleBone under these settings and that the screen fades away when we let HSYNC through.

The screen seems to work correctly, until we allow HSYNC through.
Are we worrying about nothing here, the filtering is working in our favour?
Will we face problems without VSYNC and from removing HSYNC?
(DEN is active in all of the above scenarios).


We also noticed that the timing in the NHD-4.3-480272EF-ASXN# is completely different to the timing in the ST7282T2 datasheet.
See attached.
Which one is correct?

The typical settings from the ST7282T2 datasheet cause the screen to immediately turn white, just like the first set of settings that we had. This led us to believe that the settings in the NHD-4.3-480272EF-ASXN# is more correct but we are unsure if there is something else causing this.

Kind regards,
Dan