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Choosing the best display for your application

Posted: 11 Jun 2015     Print Version  Bookmark and Share

Keywords:LED  LCD  display  OLED 

Now we have RAM filled with 2.6MB of picture data for our VGA screen with 24bit colour depth. There are, of course, ways to both scale this up and scale this down.

It is possible to take a smaller screen and this will reduce your memory requirement. For every less pixel wide your screen is, you save the 480 other pixels in the column, but the same is true in the other direction too. The XGA screen, which is 720px x 1024px, needs 2.1MB of RAM for the background image using 24bpp colour depth, rather than the 900KB of the VGA screen.

The other thing that can been changed is how many bits need to be used for the colour of each pixel. We have used 24bits per pixel in this example, as it is the most popular choice at the moment. It gives the most flexibility and ensures that the display does actually show what the graphical team would like to be on there.

It is, of course, possible to use an 8bit colour set up, however this brings with it some other issues. With only 255 colours, there are many issues that will be encountered. For example, something as simple as displaying text is difficult.

Modern fonts are not simply made up of white background and black letters with a width of an exact number of pixels. Even simple letters are made up of an array of blacks and greys so they are easily recognisable to the human eye.

It is then possible to use a small screen and an 8bit colour to display some data, but it is questionable as to whether Steve Jobs would have put the Apple name on your end product!

With this in mind, there are a number of options in terms of processor or controller to drive the screen. The general rule of thumb is that a microcontroller is a good low cost option to drive smaller screens with a lower colour depth; this makes sense up to about QVGA (320 x 240) size, which needs only about 150KB (using 16bpp) of RAM for the background image.

However, above this, very often the bandwidth of the MCU and the performance of the core in the MCU is insufficient to create a truly rich user experience. Thus, above the QVGA size, the standard choice tends to be either an MPU, or a new embedded-MPU, now available from a number of suppliers and is essentially an MPU with the memory embedded in the device already.

In conclusion

So in summary, the embedded electronics world is being swept along by the consumer trend of adding a screen to many systems. This simple step allows OEMs to differentiate and add value to their end products, but creates a new challenge for design engineers taking their first step into this world.

There are many different options of screen in terms of technology, colour and size. It is also an area where there are many new changes happening on a near daily basis, as the mobile and consumer world continues to drive the display technology in new and interesting directions.

For the engineer newly moving into the arena, the key design criteria to be fixed early in the process is what the screen size will be (measured in pixels), and from there most of the other decisions are relatively simple.

If the screen is going to be large, then you will need a lot of RAM and you will need a controller/processor that supports that much RAM, supports that interface and also has sufficient performance to drive it.

- David Parsons
  EE Times Europe/consultant
  Renesas Electronics Europe

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