18-bit LVDS panel on 24-bit framebuffer

Hi,

To use 18-bit LCD on AM335x 24-bit LCD interface, you have to configure your software for RGB888 interface, but in hardware to connect only the upper 6 bit of every color to your panel. 

The color assignment of LCD pins is described in Silicon Errata (sprz360f.pdf) section 3.1.1.

You may take a look on this forum thread:

http://e2e.ti.com/support/arm/sitara_arm/f/791/t/303632.aspx

Hope this helps,

BR,

Vidin

Vidin,

That apply to TTL parallel RGB interface only. I am asking the LVDS interface.

-Eric

Biser,

Our customized board is using AM335X. Our LVDS interface is designed 24-bit and works fine for 24-bit LVDS panel. The LVDS in implemented via a LVDS transmitter. Now, our clients would like to use a 18-bit LVDS panel for 24-bit frame buffer.  The work around now is shift two bits of RGB in application level. We would like to know if it is possible to work this around in driver level.

-Eric

The lowest two bit will be encoded by LVDS transmitter. The way to make it work is try to the 3rd LSB bit become LSB bit. And the fourth LSB bit become the 2nd LSB bit, and so on...

Is there any way to implement it in drivers?

Is there any way to work this around instead of from user space?

Biser,

I am using 3.2 from SDK6.0.0. I think the link that you pointed is not related.

-Eric

Biser,

Please do that. And hopefully I can get some feedback early next week. have a nice holiday!

-Eric

Biser,

Any news?

-Eric

 Eric Lee4, have you find solution for your 24bit into 18 bit conversion? Did you change the software or drivers?

I have the same problem described...

Thank you in advance!

Andrea S.

Andrea,

I do not think it is possible to do this in driver level. However, you can shift two bits in your application like this:

ifdef RGB24

                                *(mem) = c.b & 0xff;

                                *(mem+1) = c.g & 0xff;

                                *(mem+2) = c.r & 0xff;

                                *(mem+3) = c.a & 0xff;

#else // RGB666

                                *(mem) = (c.b >> 2) & 0x3f;

                                *(mem+1) = (c.g >> 2) & 0x3f;

                                *(mem+2) = (c.r >> 2) & 0x3f;

                                *(mem+3) = c.a & 0xff;

#endif

I am dealing with the same issue because SGX driver works only with 16 or 32 bit framebuffer. Anyone knows what is the best way to shift the colors in case of Qt/QtQuick (QPA) with eglfs platform?

There in an extra LVDS channel for 24 bit (vs. 18 bit).  The majority of displays run the extra LSBs over the extra channel, but there are a small number of displays that run the MSBs over the extra channel.  This means in most cases you leave the software in 24bit mode and an 18 bit panel just does not have a receiver for the extra LVDS channel.  No hardware changes required.

Your real issue is likely because the AM335x is screwed up in that the bits are swapped between 18bit mode and 24bit mode, this is a silicone error.  So when you set the software to 18 bit mode with a 24bit hardware layout, colour bytes are inverted.  Try keeping the software in 24bit mode and connect the 18bit panel, it should just work.  There is zero chance of damage (unless the LCD connector pinout is different, or power supply spec is different, etc.).