Display specification – avoiding the common pitfalls
01 August 2016
When should I choose the display for my design, and what factors should I think about?
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Can I visualise what my user interface will actually look like? Can I try my content on different screens before making a choice? How can I get a display that matches the resolution provided by the processor?
The display is the first thing that the system user sees and the last thing that the system designer thinks about. A sweeping generalisation but so often true. The first thing the user looks at when they start up the system is the screen, and their perception of the system will continue to be influenced by the information it provides and how it is presented.
Yet system designers typically choose the display late in the design process. Their focus is often on the choice of processor, operating system and the writing of the software. Thinking about how the system is going to be presented to its user at the start allows much more flexibility. So often we see designs which demand a screen resolution not supported by the chosen processor. We also come across instances where the processor will provide a higher resolution than is available in the chosen screen size.
The resolution and size of the display depends on the character of the information to be presented. Is it purely text, icons or graphical? Will it include video? Is there to be a touch interface, and if so, how many options is the user to be offered at any one time, and how much information needs to be presented to help them make their choice.
It’s extremely hard to visualise screen appearance in the abstract – the best way is to obtain samples of the target displays and load up the interface. Platforms like DX from Anders offer pre-integrated motherboards and displays which are shipped working with popular operating systems and processors. Loading your application onto these platforms is quick and simple and you can easily make changes and even try different screen sizes to make an informed decision.
Deciding the size
The first thing to say about specifying the size of a display is to not be constrained by the relatively limited catalogue options that the major display manufacturers offer. Start from a blank piece of paper and decide what size you’d really like – if you choose the right supplier you will be able to achieve the exact size you require at the right price.
Though at first glance a standard product can look cheap, most of the time a semi-custom display is the best option. For example, in many cases we can cut a display to size. This can avoid the situation where you have to increase the size of the case to accommodate the display, or compromise the way information is presented to fit onto a smaller than ideal screen. A display that’s too big can come with many hidden costs – not only is a bigger case more expensive to buy, store and ship, but the bigger display can also use more power.
It is also possible to remove unwanted elements from a semi-custom display. You can choose how many backlight LEDs you need – saving cost and reducing power. You can specify a simpler and cheaper graphics IC, add or remove performance enhancement films etc. to create a screen that delivers exactly what your application requires – no more and no less.
Manufacturers that have an extensive catalogue of display options usually build their standard display range to order anyway – so there are no actual economies of scale. Some manufacturers will produce a limited range of displays in high volume – but if you specify these then you are placing yourself in your suppliers’ hands. If the display market changes and the demand shifts to a different size, they will discontinue models in a heartbeat leaving industrial customers high and dry.
Refresh rate – a new concern
Until recently, refresh rate has barely been an issue for embedded system developers, but virtual reality is starting to change that.
The majority of display controllers refresh the screen at 50 or 60Hz – roughly once a second, and that’s been plenty fast enough even for most video content. Higher refresh rate 90Hz or 120Hz monitors are available, but typically only gamers need their screens refreshing more frequently to keep up with rapidly changing game scenarios presented in real time at high resolution.
Virtual reality headsets, though, will be a professional as well as a consumer electronics product. Currently, simulators used to train pilots, operators of nuclear plants, surgeons and others typically use very large screens to present content in real time. Giving the user a headset would clearly be a great deal cheaper, but does have implications for the display.
Though simulators present fast-changing scenarios developing in real time, on a screen just a few centimetres from the wearers face, a refresh rate of 60Hz is no longer enough to present the motion crisply without blurring, and we are starting to see interest in 90Hz or 120Hz refresh rates.
Emerging technologies like LTPS and IGZO offer higher electron mobility and potentially support faster refresh, but these technologies aren’t readily available on the industrial market. They are expensive and associated with high MoQs. Fortunately, there is no need to move to these technologies to achieve 120Hz refresh. It is perfectly possible to obtain a fast refresh with high resolution amorphous silicon screens.
Doubling the speed doubles the amount of data that the IC has to deliver to the screen. Most driver ICs are designed to work with up to 60Hz refresh – but faster refresh can be achieved using multiple driver ICs in tandem to deliver the required data rate. A good architecture is to use three LCD display driver ICs – two source drives and one gate drive.
Don’t discount the mono option
Up to now, we have assumed that the reader is going with a colour display, but despite the arrival of new and apparently more attractive display technologies, mono character modules remain popular, and continue to be designed into new projects. An enduring attraction is that design-in is typically less challenging than for a TFT-LCD, especially with the help of a specialist distribution partner to help even the most resource-constrained projects achieve a high-quality end result. Segment display technology continues to evolve and now benefits from exciting new developments including versatile colour modes, embedded touch sensing, and sharper, crisper viewing performance. Today, designers can explore many new avenues to communicate more intuitively and deliver memorable user experiences without moving to a full graphical display. Mono doesn’t need to be boring by a long shot.
Among the new technologies now transforming established views of the segment display, bi-colour and field-sequential displays enable designers to utilise colour more extensively than ever before to make their user interfaces clearer, or easier to understand. Bi-colour displays give the opportunity to combine coloured segments with a coloured backlight, breaking the traditional limitation of one colour versus a black background. The backlight colour can be fixed or, with an RGB backlight, can be selected dynamically to be one of eight colours. This allows the display to be adapted instantaneously to communicate special status, such as red for danger and green for “ok” or ready.
The field-sequential display gives even more flexible options for colour-coding the user-interface modes by enabling dynamically reconfigurable RGB display segments: again, choosing from a palette of eight pre-set colours. It is true that an optical filter is able to produce a similar effect, but the new field-sequential displays require no additional components and give colour-change capability that the optical filter cannot equal.
My advice to the display specifier is to be demanding. Run your user interface on a selection of pre-integrated platforms early in the design, to get an idea of the size, resolution and performance of screen that you’re going to need – and do this before your design parameters are too locked down. Then find a supplier that offers flexibility and will deliver what you need. There is absolutely no need to accept the constraints of a standard product range.
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