[Resolved] LVDS Display Troubleshooting

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[Resolved] LVDS Display Troubleshooting

Postby SITAComnet » Mon Jan 11, 2016 5:22 pm

New to Wandboard (and Linux-ish), and we have a Kyocera TCG070WVLR 7" LVDS touch panel connected.

The screen is not working and I have some questions.
I have a Wandboard Quad, with ubuntu 14. I can RDP into the device. On power up, I get nothing but backlight from the touchscreen.

1) Are there settings in ubuntu I need to set to get LVDS display to show anything?
2) Are there any tools that allow me to check the status of the LVDS display (like, a simple "are you there" kind of tool, similar to lsusb)?

Last edited by SITAComnet on Tue Jan 19, 2016 12:55 pm, edited 1 time in total.
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Re: LVDS Display Troubleshooting

Postby Tapani » Tue Jan 12, 2016 6:28 am

Hello Brian,

adding a 'randomly chosen' LVDS panel is not as easy as plugging in a monitor to a Windows PC.

Yes, you need to tell the kernel where you want your display output(s), and what kind of display you have - the default is likely HDMI.
Generally you can do that in u-boot by somehow passing suitable video arguments to the kernel (video=mxcfb0 .. ).
Which these are depends on your panel and the setup you want (single channel, dual channel, multiple displays).
If your panel supports CVT timings it becomes easier, since you only need to specify one resolution -- otherwise we have to pass all the display timings
(which might not even be fully documented!).

To change u-boot settings you need a debug console.

Also, LVDS has no one standard on which order the different RGB bits are sent over the LVDS (diff pair) pins.
(There PSWG, VESA and JEIDA for instance)
If you are unlucky you need to change the driver to force your bit order. (This is the last of your problems,
if your output picture appears as "colouful noise" but with shapes vaguely recognisable - then this might be needed).

LVDS panels cannot be probed usually. Some do have an I2C EDID which you might be able to see with an
Code: Select all
i2cdetect -y 0

(replace 0 with the I2C channel you wired up to your panel, 0=I2C1, 1=I2C2, 2=I2C3)

Note that the touchscreen and display are two completely separate devices. There is no connection in software,
they need to be enabled spearately. I have assumed you are starting with the LVDS display.

For touchscreen - you can try probing it by 'lsusb' or 'i2cdetect' depending on how you connect the TS to
the board. After that you probably need to add your TS to the device tree and

Sorry for this not being easier for you.
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Re: LVDS Display Troubleshooting

Postby SITAComnet » Tue Jan 12, 2016 6:58 pm

Thanks for the help Tapani! Your suggestions at least got me to some place a little farther. I've got serial debug attached, and can see and modify UBoot. Now the real problems seems to be how to correctly modify UBoot to get things to work. I've messed around with it most of the day with no luck. I've also tried the Android image to see if it made a difference, but still nothing. All of these magic values in UBoot are killing me.

I was given this device by engineering, so I have no idea if it was proven out to ever work. They have assured me that the LVDS connection is correct.

Anyway, thanks for your help. If anyone has any UBoot suggestions, tips or trick, please send them my way.

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Re: LVDS Display Troubleshooting

Postby SITAComnet » Fri Jan 15, 2016 6:33 pm

Finally got it to work.

Here is what I did:
1) Connect to the Wandboard using a serial port and null modem cable
2) Use Putty to connect to the serial port (8,n,1)
3) Ubuntu14 DOES NOT WORK (yet, more correctly... I've not figured it out) with this device, use Ubuntu12 image
4) Boot the Wandboard, at the uboot countdown, press any key to stop booting
5) At the uboot prompt, enter "printenv" to see what the environment variables are set to
6) Enter this line at the uboot prompt (without the double quotes): "setenv bootargs 'console=ttymxc0,115200 root=/dev/mmcblk0p1 rootwait rw video=mxcfb0:dev=ldb,LDB-WVGA,if=RGB24' "
7) Enter "saveenv" at the uboot prompt (this will save the previous line)
8) Enter "boot" at the uboot prompt (this will continue the linux boot process)
9) You should see penguins as linux boots on your touchscreen
10) You should see the system boot on the screen. If things are all wonky, you need to play around with LDB/LCD WVGA/XVGA RGBXXX values

WVGA = anything that is X by 480 (800x480 for instance.. wide vga), there are other values that this can be, and I found not definitive area where the possible values are enumerated at.
LDB vs LCD, not sure what the difference is, but LCD would not work for me, while LDB would
RGB24, RGB666, RGB565 are all modes that you have to know if your screen is set up to support

Thanks for the support in figuring this stuff out.
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