White Screen

I keep getting a white screen on the LCD and it happens randomly which is
rather annoying.

I have configured the kernel to always use an A70 display and I am using a
shorter cable for the LCD bit I still get this annoying issue where the
display goes full white.

It has to be either a driver or power supply issue and I have not been able
to track is down as yet.

I am running with a Mini210 board and it happens with either of the 2 of
the boards I have and any of the displays.

It either happens when I apply power or when the system starts to boot into
Android. I sometimes see the Linux logo but sometimes the screen goes white
a few seconds are applying power.

I am using bench supply that can provide 3A at 5V and I can see that the
normal boot it draws around 1.2A and settles to about 1.1A when Android
finally runs.

I have a Tiny210 with cap LCD and it never does this.

Anyone else seeing this issue?

Like it flashes white for a fraction of a second then starts working
normally, during the early phases of bootup?

My mini2440s often will flash white or with the residue of the screen that
was visible when you reboot or turn-off and then turn on again.

I just took it as normal.

Have yopu only just noticed it on the mini210s?

I've had the units on the shelf, still boxed, for some time and decided to
try and get it working for a small monitoring project.

It happens at times when booting and only just now while debugging the
application, the screen just went white. I have had to do a reset to get it
to work again.

I need to dig into the drivers for the 7" display as this does not happen
with the Mini210 running the W50 5" LCD.

I've compared the Tiny210 kernel settings and they are the same. There is a
difference when it comes to the S70 but this is the A70 I am using.

It's specific for the A70 LCD, while it is not initialized, it's white.

Yeah, once the backlight is on the LCD is initially white but this one
looks like it resets and goes all white during either boot or during
operation.

I left the app running and when I checked it again a few hours later, the
display was white again.

I've changed out the PCB with the other one and the LCD too and nothing is
stable.

I've noticed looking at the schematic for the LCD that VCLK runs along
HSYNC and as VCLK is a 33MHz signal, this will have an effect on the SYNC.
I've seen the exact same issue on an Olimex A20 board where the CLK runs
next to the sync. 

If you look at the LCD connector, they separate VCLK away from the others
and have a GND on either side.

On the Olimex board we have had to use a short cable of no more than 120mm
to get it stable.

I've just swapped out the 200mm cbale I had on it with the shorter cable
that comes attached to the LCD itself and it still goes white so something
is not right here.

I need to look at timing to see if there is something that can be done to
fix this issue.

It's a power supply issue.

I am using a high quality TTi bench supply and outputting 5.00 volts (as
per the meter on the power supply and confirmed with my Fluke multi-meter)

At the board this is 4.65V and when I check at the USB host port, it is
4.3V. This is 4.15 at the LCD.

The 3.3 at the LCD is also low at around 3.1V.

Increasing the bench supply to 5.2 (at the bench supply connectors) gives
me 4.9V at the power connector (the cable is only 1m long but the current
is 1.2A so likely a some voltage drop as the cable size is approx 0.25mm2

I will replace the cable later tonight with a 1mm2 cable to be sure this is
what I am seeing.

With this increase I now get a perfect boot every time.

If this is the case, I just need to make sure that the final build includes
heavy enough cable and that the power supply is rated for 2amps min output
to ensure little or no voltage drop.