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Optoelectronics/Displays??

Transitioning to advanced displays

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

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

Marketing teams are increasingly asking the engineering department for bigger displays with more impressive graphics on their next generation of products. This is true for many applications, such as coffee machines, ovens, factory automation and household boilers, and is naturally presenting a new and interesting challenge for engineering teams.

So the simple question is: How to add a display into your application?
Let us first have a look at what different options you have in selecting a display. There are lots of Three Letter Acronyms (TLAs) used in the industry which are actually very simple but can lead to a bit of confusion.

Everyone in the engineering community is familiar with the Liquid Crystal Display (LCD) concept; even if they have subsequently moved to the dark side and are now working in "sales" they will likely remember the calculator that they once had to use for real work, that had an LCD screen on it.

The basic concept is very simple. A layer of liquid crystals is arranged between two polarised layers (at 90 degrees to each other) such that, without the liquid crystal, no light would pass through at all.

However, the crystal is arranged in a kind of helix pattern which rotates the polarisation of the light between the two polarised screens, meaning all the light can pass though. When an electric charge is applied to the liquid crystals, they unroll or straighten out, thus no longer rotating the light and will appear black.

This "twisted" orientation is the most common configuration inside and LCD, and this is where the Twisted Nematic Liquid Crystal Display (TN-LCD) comes from. "Nematic" is just a way of describing the physical state of the liquid crystal.

A colour display can be built using three separate LCD cells per pixel and applying a red, green or blue filter to each one. The pixels are then lined up in columns and rows, and by applying a voltage to the column and grounding the row, a specific pixel can be turned on or off.

The problem with this approach is, however, that for larger screens, when several pixels in one column and in another row are switched concurrently, there is a long delay as the charge propagates through the matrix, as well as a poor contrast as the charge is distributed across this matrix.

This problem is somewhat overcome by using the STN-LCD module. The STN in this case stands for Super Twisted Nematic screen. In an STN screen, the 90 degree rotated liquid crystal is replaced by a "super-twisted" C or typically 270 degree twistedliquid crystal.

This is, however, still building up a matrix of charged columns and grounded rows, also known as a passive, which has inherent limits as described before.

Moving to an active control gives a much better contrast and a much faster response time. In an active LCD system, there is a dedicated transistor for every single LCD cell, and as such, the transistor controls the switching of that cell or pixel. This gives rise to the phrase "TFT-LCD" which is then an LCD screen controlled by a Thin Film Transistor, and is the most common screen used in graphical display applications today.

The alternative, though not as popular a display technology in this market, is the OLED display. This is essentially a matrix of LEDs, with one OLED per pixel. They are lower power, lighter and can be put on flexible surfaces when compared to their LCD-based counterparts, however, the expensive manufacturing process and the somewhat limited lifetime of the blue component in the displays has delayed their full-scale introduction into the market.

Of course, there are a few other alternatives that are less popular but should nonetheless be understood, and these are listed below:

EPD: the so-called e-paper used in e-readers and in some watches, but not yet suitable for the embedded, non-consumer space.

LCOS: Liquid Crystal on Silicon, used for "near eye" or projectors but have not taken off in the industrial market as yet.

PDP or plasma: Used in older, larger screens but are quickly being replaced by LCD or LED screens.

Driving the display
Now that we know how the picture is actually displayed on the screen and what the technology looks like, let us have a look at how the screens are actually driven. For smaller screens, often using a passive control system, it is common to see new COG (Chip on Glass) technology coming through, where the driver IC for the LCD is actually integrated onto the glass of the panel.

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