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Transitioning to advanced displays

Posted: 24 Jun 2015 ?? ?Print Version ?Bookmark and Share

Keywords:LCD? Nematic? LVDS? RAM? MCU?

This can provide a good cost reduction for smaller screens but is yet to make the breakthrough into larger colour screens, the likes of which the aforementioned Steve Jobs would have put onto his smartphones or other gadgets. These screens for the most part include an additional PCB, which contains the control mechanism for the screen and can additionally contain a controller for the touch interface.

There are then two further options available in terms of displays. There are so-called display modules, and there are standard displays. A module is just as you'd expect C a complete module with all the memory required to save the picture data which is being displayed on the screen and normally a simple SPI interface.

We will not go into these in much more detail, but suffice to say that they are slightly more expensive than a standard display but offer a much simpler solution for the novice user.

There are two standard interfaces to a standard display.

RGB signals or LVDS signals. It tends to be that displays over a certain size (about WVGA) will offer an LVDS interface and smaller ones will offer an RGB interface. There is of course no hard and fast rule here but the bigger the screen the more likely it will be to have only an LVDS interface.

RGB is essentially a parallel interface whereby each colour (Red, Green and Blue) is represented by a parallel bus. Thus for a 24-bit colour display there will be 24 "data" bits.

This is the most simple of interfaces as there is a standard one to one transfer of every pixel data on the bus to the way it is stored in RAM. There are several different notations of RGB standards, such as RGB565 of RGB666, which simply denotes the number of bits taken for each colour. In RGB666, there are 6bits reserved for each colour, and it is therefore an 18bpp colour. In addition to these data signals there are also the clock signals to synchronise the panel.

A panel clock (or pixel clock) sets the pace for the whole interface and subsequently the data transfer occurs. There is then an Hsync clock, or horizontal synchronisation clock, which indicates after a number of pixel clocks when to jump to the next line.

Then at an even slower frequency, there is the Vsync signal (vertical synchronisation) which in turn indicates when all the rows have been written to, and it is possible to then start the next picture or frame.

Of course, the bigger the display, the higher the speed required for the pixel clock in order to meet the refresh rate of the screen. As external signal frequencies get higher, the risk of signal corruption also increases.

For this reason, larger screens now tend to use an LVDS interface instead of the standard parallel RGB. LVDS stands for Low Voltage Differential Signal. LVDS technology is used in many applications where signal integrity is very important, especially at high frequencies.

The LVDS signal uses a two-wire interface (per channel) and has a common voltage (normally 1.2 V). Then to create a "high" signal, the voltage on one line is raised by 100 mV and the signal on the second line is lowered by 100 mV. This allows for low power, high frequency, high-reliability signals to be transmitted.

In a display, there are typically four LVDS channels. These channels are used for the red, green, blue and clock signals in turn, and then the data is transferred serially rather than in parallel.

Managing content
Now we know how the screens are set up, let us move to the other side of the application and see how the graphical content is created. Let's look first at how the images are stored in memory. We are all now familiar with our holiday photos being stored as a JPEG on our PCs at home.

Sadly, this is not the format that is used; the image is saved as a raster image or a bitmap. This is, of course, significantly larger than the JPEG that you use for your holiday photos, so let's have a look at the way that these images are actually saved in RAM and how much of it you actually need.

As we said, the basic picture is stored as a bitmap, whereby every pixel in the picture is stored and represented by unique data. There is no data compression, like in other formats. It could be a 16-bit or 24-bit colour depth; for a 24-bit colour, that each pixel would be represented by 3 bytes.

You can then immediately see that this means a lot of memory will be used. For example, a VGA screen of 640 by 480 pixels would have 307K pixels, and as such need about 900 KB of data per image on the screen. Sadly, however, the RAM usage story is not over yet.

A typical GUI application will be made up of several picture layers. These layers would be then displayed on top of each other. For example, one layer could be the corporate background image, and the next layer might be a frame around the outside of the picture with some data displayed on it, such as the temperature and the time.

A third layer could then be a graph showing real-time measured data in your application. The reason that you store these pictures in different layers, and therefore in different areas of the RAM, is so that you only need to change one small picture rather than re-calculate the whole image. If you needed to re-work the entire GUI every time that the graph was updated or the temperature changed, it would just take too much CPU power.

These layers are then combined together, either by hardware acceleration or by software, using a number of different mechanisms.

The two key concepts here are alpha blending and chroma keying. Alpha blending defines what is known as an alpha channel, which is an additional 8-bit value added to the 24-bit colour signal for every pixel.

This alpha value defines the transparency of each pixel such that the layers can be placed as semi-transparent on top of the background layers. Chroma keying is slightly less memory intensive, and is again very useful for combining pictures. Chroma keying is the special effect which we are all familiar with from the movies, which is also known as "green-screen" whereby the actor stands in front of a green curtain and the green is replaced by a film showing the angry dinosaurs that are chasing him.

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