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Speaking of success: Epson Europe Electronics

Jiro Morinaga explains how Epson's recently introduced lcd controller can help save energy in a range of systems.

The USB interface continues to find more applications, even in the embedded world. Apart from audio equipment and home gateways, USB ports are becoming more popular in cars, where they allow music files and navigation data to be transferred from iPods and memory sticks.

PC vendors such as Toshiba discovered the advantage of USB power some years ago and started to supply power to connected USB devices, even when the pc was still in power saving mode. Today, the implementation of sophisticated power saving states is ubiquitous for modern PCs and the Equipment under Power (EuP) Eco Design Guideline aims to reduce standby power consumption for all equipment to 1W, or 2W for products with a status display.

USB deploys two differential data lines – DP and DM – the V-bus line for 5V power supply and the ground wire GND common to the other three lines.

Data lines usually use 28AWG wire, whereas the V-Bus and GND lines are usually 20AWG. As the power loss of copper wires is inversely proportional to cross section, these wire dimensions improve power loss by a factor of two or three compared to LVDS/APIX cable, with its symmetrical structures and smaller cross section wires.

USB is built on a tiered star topology, enabling hubs to be deployed. By choosing proper cabling approach, wire lengths and thus power loss can be reduced substantially. Such measures are not possible when using point to point connections like LVDS or APIX, unless you allow separation of data and power cables.

The USB standard defines power modes and power supply characteristics exactly, something which is not done in other display interface technologies. Interfaces such as LVDS APIX do not mention cable length, nor wire dimensions, which substantially affects the planning security.
A special feature of USB is its suspend and resume capability, enabling interrupt messaging from the peripheral to the main system, even in low power mode.

Years of development have meant that USB high speed circuits are smaller and less power hungry than other interfaces. Estimates show that USB designs can consumer 50% less power than APIX or LVDS.
Because APIX or LVDS are targeted at systems with higher refresh rates than USB, means more switching activity is needed in the whole system, resulting in higher power consumption.

Epson's S1D13U11 LCD controller features a USB interface to the main CPU and contributes to power reduction through:
• display refresh
• power management mode, USB Suspend/Resume
• Epson's low power design
• Simple graphic control, plus second layer, bit block transfer

Systems using LVDS require the main cpu (or SoC) to transfer images to the display continuously in refresh mode, regardless of whether image content changes. This requires continuous switching activities in the main cpu and the circuits within the image processing chain, including LVDS transmission, resulting in more power consumption.

But the S1D13U11 only handles image refresh after the transmission path. For most of the time, display content consists of still images and only one picture needs to be transferred from the cpu to S1D13U11. This means the transmission path consumes a fraction of the power needed by systems built using, for example, LVDS.

Apart from handling display refresh, the device can also place the main system into a deactivated status while maintaining the display of still images, something which is not possible with conventional LVDS or DVI systems.

The S1D13U11 also supports such power management functions, such as PLL switching, sdram refresh during sleep and USB suspend and resume, in which wake up by interrupt – a touch panel control or key control connected to SPI or GPIO, for example – can be fed to the main cpu. It is also possible for the display to be maintained while the system is in standby, for touch or key press functionality to be maintained or even the display's backlight to be switched off, reducing power consumption even further.

However energy saving efforts should not be restricted to standby or reducing the power consumption of components; application and use should also be considered, as should the system architecture.

Many home gateways can, to a large extent, work independently of a PC, allowing VoIP calls or music and video to be distributed within the home. Even email accounts can be installed on the gateways and read via DECT clients, but on a small display.

A system featuring the S1D13U11 can connect to the USB port of these gateways, but use a larger display. While a power efficient PC and a 17in monitor consumes 130W, a USB based display controlled by the gateway would consume just few Watts. For example, an S1D13U11 evaluation board featuring a 3.5in QVGA touch panel draws just 170mW on the 5V V-Bus.

A media centre – either PC, set top box with Ethernet or WLAN, or Network Attached Storage with its own media management – must always be switched on, or at least be in standby, with a TV used for the display, with 32in models consuming more than 100W. A USB based display would allow large power savings. Many games PCs, multimedia PCs or notebooks integrate media centre capabilities and are used as such. If a sub display based on a USB connection with the above mentioned characteristics is included, this would not only contribute to energy saving efforts, but would also speed start up times.

Many industrial displays do not require high refresh rates or resolution, as they are primarily used for status display or GUI. Instead of connecting displays via LVDS or DVI, a USB display would bring a substantial reduction in power consumption. The same applies to the area of network management, as the main display content would be network connection status and traffic.

Using USB for display interfaces has better power efficiency than other interface solutions. As a leading eco-friendly enterprise, Epson offers multiple chip solutions for USB based display connection, which contribute to the reduction of CO2 emissions.

Jiro Morinaga, Epson Europe Electronics

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