Hardware Converter HDMI-RGB (18/24-bit)

Good day, Habr!



And again, my article will be devoted to video converter. This time it will be HDMI-RGB. I want to devote the article not specifically to development, but to one problem with colors that arose when I connected an 18-bit matrix to a 24-bit driver.

In one of my previous articles, I wrote about the HDMI-LVDS converter, with intermediate conversion to RGB ( HDMI-LVDS. From layout to release ). For the basis of the new development, I decided to take the same TFP401 (HDMI-RGB) chip from Texas Instruments.

I developed the board in the same dimensions, left the microcontroller with the same functionality (sleep mode control, button processing, matrix backlight control). Since the controller was developed for a specific matrix (and, accordingly, a ready-made cable), we also had to install a specific RGB connector, with a pitch of 1.25mm. There are two such connectors on the board, as well as for connecting backlight drivers, since it is necessary to duplicate the image on two matrices at the same time.

A little distracted. Earlier, I already did duplication of LVDS and HDMI interfaces in my projects. At first I tried to use specialized microcircuits, but then I refused them, and began to connect two connectors in parallel (for HDMI, I2C must be torn off so that there are no conflicts on the bus). This option works great on all panels, including more than 42 inches. There are no complaints about the image.

On this board, too, I simply duplicated the RGB connectors and made control of the lengths of the conductors. All tests were performed on one matrix.


Fig. 1. Image from matrix

The first inclusion puzzled me a little. Distortions are especially noticeable in the bottom photo of Fig. 1. It immediately became clear that the problem was in “bitness” - there are no significant bits in all colors. But how to find a problem, because according to the scheme, all the bits come to the matrix "one to one"? I have already seen a similar problem when connecting 6-bit color matrices to my converter, but I did not study it, since we supply products only for 8-bit matrices. He began to dig the Internet in search of a solution to the problem.


Fig. 2. RGB output circuit with TFP401

For a long time I could not find any information on the use of TFP401 in 18-bit mode. In the datasheet, 24-bit is indicated in black and white. If you think about it, this should not affect the picture itself - just a loss of quality and that’s it. The phrase on the TI forum (the support service answered users' questions) that the TFP401 is not able to automatically switch to 18-bit mode of operation (which is already clear from the documentation) and that it is necessary to “reconnect” the panel helped. This suggested that there is still a way.


Fig. 3. Connecting an NL6448BC33-70F matrix from a datasheet

Again I began to study the datasheet on the panel - nothing has changed, 6-bit per channel. In 2014, when developing the HDMI-LVDS converter, I came across some documents saying that you need to shift bits to use the 18-bit mode, but could not remember how many bits. He began to search for information again, but already deliberately by the shift, and, purely by chance, stumbled upon a document in which the table below appeared!


Fig. 4. 24-bit and 18-bit

Thus, to connect an 18-bit matrix and a 24-bit chip, I need to make the connection according to the table in Fig. 4. That is, I have to connect the R0 of the display to the TFP401 R2 and so on. Since I have a connector on the cable, I just “re-twisted” the cable.


Fig.5. Correct Pinout Image

Here are the same pictures, but after the repinning. Everything fell into place. The quality returned to normal, sharp gradients disappeared.


Fig. 6. Appearance of the converter with the connected driver (LD1263) and matrix


Fig. 7. 640 * 480 .......

In Altium, of course, working is not very convenient, but the picture is excellent. Suitable for low-cost dashboards, visualization of any interfaces, etc., if you add a touch, you can use it, for example, for PLCs.

So far, it has not been possible to connect two matrices at the same time (the second is not available), but I think that there will be no problems. Later I will unsubscribe and add a photo.

Thank you for the attention!