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System designer's view of display interface, power-saving problems

Posted: 23 Dec 2011     Print Version  Bookmark and Share

Keywords:PCB  processor  interface 

Being a system designer is not an easy job. You're often handed a high-level set of specifications and told 'make this work.' For example, when a new tablet or smartphone is architected, high-level specifications such as the processor and display used are often chosen by people who don't necessarily examine how easily these components can be integrated together. Regardless of who has chosen these components, responsibility most often falls to the system engineer to solve these issues within the product's cost, size, and design targets.

Let's address three challenges system designers face today, and the options for solving them.

Display/processor mismatches
We've established that when a display and processor is chosen for a particular product, often those are chosen for their consumer value (speed of processor, size and resolution of display), rather than how easily or efficiently those components can interact with each other.

Frequently, both displays and processors support only a single video interface standard. This is done for cost reasons, as supporting multiple interfaces requires more silicon area and usually a longer design time. Typical video interface standards in the mobile market include MIPI, LVDS and RGB. Each of these standards has its own history and 'reason for being' in the mobile space:

MIPI is a mobile consortium-based standard that acts not only as a display interface, but also as a camera and other peripheral interfaces. MIPI is found on many smartphone-sized displays, as well as many smartphone and tablet processors.

LVDS owes its presence to the fact that the larger displays found in tablets are often based on designs from manufacturers who have long supported the notebook computer space, where LVDS is a dominant interface. However, LVDS is rarely found on mobile-targeted processors.

RGB is a legacy interface that has been used in feature phones and smartphones for many years, on both processors and displays.

The variety of interfaces available gives the processors and display manufacturers multiple options during their product design cycle. They'll typically choose the best single interface for their product. However, this then creates a problem, because the single interface chosen by the manufacturer of the processor may not match the interface of the display manufacturer. Per usual, it's now the system designer's job to make this work.

The system designer is faced with a couple of options to solve this problem. First, change the processor or display to match the interface. This is generally not acceptable, as it could negatively affect customer-level product performance specifications, such as the processor speed becoming slower, or the display being smaller or having a lower resolution. Such changes can often make a product non-competitive in performance and/or cost.

The alternative to swapping components is to add an interface 'bridge' chip. These discrete chips perform the single bridging function, but unfortunately are often large in size (up to 14 x 14mm) and can consume significant power resources in systems that are already power-constrained.

The bridge chip option is normally chosen, as it represents the best way to maintain the high level specifications of the OEM product. However, it does add cost and power consumption to the system, along with consuming available PCB space for a single function chip.

Power consumption
One of the key selling points of mobile devices is battery life. Often, after a product is passed to system designers, they are told that it needs to support a certain amount of battery life. And again, it's the system designer's job to figure out how to make that happen.

The option exists of changing out components as mentioned before, but this is most often a non-starter due to product performance requirements. Generally the system designer is stuck with finding ways to reduce the power consumption of the system as-is without changing major components.

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