When it comes to the notion of "embedded," the microprocessor may have to renounce its sovereignty and share its scepter with the flat-panel display. The reason is simply that embedded may soon connote displays as well as microprocessors. When a product contains a display, which is the case in more applications, designers will be pushing to embed as many functions as possible onto the display substrate itself, assuming that the volumes warrant nonrecurring engineering costs.
So, which functions will migrate to the display next? Some display drivers, phase-locked loops (PLLs), and analog-to-digital converters (ADCs) have already been integrated. It's likely that many functions of a product may eventually wind up on the display—be it a cell phone, a PDA, or an entire computer.
Putting a computer on a display, however, probably is an application for the distant future. But it does seem like a natural progression in an integration sequence, following closely in the footsteps of the CPU, which migrated all of the attendant functions onto a single chip and took the name "microprocessor."
Whatever migrates, though, low-temperature polysilicon (LTPS) will undoubtedly be the enabling technology to move functions onto the display. Today, this manufacturing technology is being applied to active-matrix, thin-film-transistor (TFT) LCDs, as well as organic-light-emitting-diode (OLED) displays just now starting to trickle into products delivered to the market place.
"We see the growing adoption of LTPS as a natural evolution of the amorphous-silicon version of the active-matrix TFT LCD," says Vincent F. Sollitto Jr., Photon Dynamics' CEO and president. The company provides yield-management solutions for flat-panel displays, pc-board assembly, and semiconductor packaging. Sollitto points out that virtually every major display manufacturer has some research going on in LTPS.
The distinction in quality between LTPS and amorphous silicon is like night and day with respect to mobility and the ability to fabricate good circuits, with LTPS winning hands down. The most appealing benefit of LTPS display technology is its ability to integrate row and column driver circuitry right on the glass substrate, eliminating the need for separate driver devices. Its high carrier mobility, which ranges somewhere between 100 and 300 cm2/Vs, makes this possible.
By comparison, the carrier mobility of amorphous silicon, today's predominant display fabrication technology, is a paltry two orders of magnitude lower—from 0.5 to 1 cm2/Vs. Because of their far higher carrier mobility, LTPS circuits are much faster. So unlike amorphous-silicon circuits, they can handle high-definition images.
With LTPS, while you're fabricating the transistors for the display, you might as well print the circuitry for the row and column drivers. But why stop there? Adding the multiplexers, the shift registers, and more is a simple, logical step. "Now you're moving toward the computer-on-glass, or system-on-glass," notes Sollitto.
Systems-on-glass could be mobile, like an electronic book, viewer, DVD, car navigation, wireless telecommunications, or fully integrated LCDs on an entire "sheet" computer with a 32-bit CPU and 64-Mbyte memory. But fabricating an entire computer on glass means overcoming the major hurdle of mixing different geometries on a glass plate.
Processor circuitry and display circuitry have quite different resolution requirements. Today's processor geometries are down to nearly 0.1 µm, whereas design rules for displays are approximately 1 µm. This large discrepancy is important because maintaining uniformity and particulation across a display panel is extremely difficult due to a panel's huge size relative to that of a chip.
Integrating just the display's peripheral circuits offers great benefits. Consider the interconnection burden for a super video graphics array (SVGA) of 800 by 600 pixels made with TFT LCD technology. With drivers off the glass, amorphous silicon requires some 4000 connections to the row and column matrix on the display. With drivers on the glass, the interconnection count plummets to just 200—and that's for a slightly higher-resolution display, such as an extended graphics array (XGA) of 1024 by 768 pixels. What's more, no tape-automated-bonding/chip-on-glass (TAB/COG) ICs are required. Yields rise too because of fewer rejects due to fewer interconnections.
"Make no mistake about it, everything we think is coming in the brave, new world will become display-centric," says Sollitto. "And five years from now, if you're not a player in the display business, you won't be a player in the electronic equipment business either because there will be fewer and fewer products that don't employ a display."
Unburdening The Microprocessor
SmartPanel is another way of describing the integration—and here we're talking about moving entire chips, rather than circuits (Fig. 1). "A distinct benefit is lifting the burden on the system processor for refresh and control and putting the display to sleep," says Chuck McLaughlin, president of the McLaughlin Consulting Group, a market-research organization specializing in displays. "Instead, these smart functions are moving into the controller that's already on every LCD, but had been basically a dumb controller in the past," he adds. Cell phones already have a smart controller, thereby off-loading the microprocessor and the DSP chips.
A lot of the inexpensive, passive-matrix LCDs that wind up in cell phones today have all kinds of smarts built into their control chip to unburden the microprocessor and reduce the power drain. Also, techniques to reduce power in the standby mode are being incorporated into the display module itself. This task was formerly assigned to the microprocessor.
According to McLaughlin, leading display companies using LTPS to integrate row and column drivers right on the glass include Toshiba, Sanyo, and Sony. There are reports that LTPS fab lines are in some state of preproduction development or full production. Toshiba is making LTPS LCDs in the 8- to 10-in. range, and Sanyo is manufacturing small video panels for camcorders and TV sets, as well as the 2- to 3-in. National Television Standards Committee (NTSC) monitors found on camcorders and digital still cameras.
As for high resolution, whereas amorphous silicon circuitry limits TAB package pitch to 130 pixels/in. (PPI), moving the drivers onto glass enables pixel pitches up to 300 PPI. That makes possible high resolution, sometimes defined as PPIs of 113 and above.
Higher Resolution Is On the Way
It's well known that the highest-bandwidth-delivery system to the human brain is through the eye. So if you want the benefits of broad bandwidth—whether it's for the Internet, television, computer output, and so on—the medium must ultimately be display-centric. Clearly, displays will be at the heart of many computing, communications, and entertainment products during the next 10 years.
"The area of greatest dynamic change will be in the market sectors comprising PDAs, cell phones, and Internet appliances," says McLaughlin. "And it's the place where all of the tough and nasty design issues converge—power reduction, pixel counts, color, panel weight, and daylight viewability."
But as he points out, in going to 150 or 200 PPI on a flat panel, you're moving to resolutions that are difficult, if not impossible, for CRTs to achieve cost-effectively. Only with the yet-to-be-achieved active-matrix LTPS LCD will costs fall enough to avoid a huge increase in display price.
As Sollitto of Photon Dynamics sees it, the higher resolutions will start in Asia where there are vertically integrated manufacturers building both displays and systems. "It won't begin with the major domestic computer OEMs because they don't make displays. What's more, they have difficulties securing committed supplies for such displays," he notes.
To assist designers developing products at higher resolutions, Analog Devices has introduced a device that's said to be the world's first analog LCD interface to break the 200-MHz barrier. It's aimed at providing superior image quality in UXGA (1600- by 1200-pixel) LCD monitors and projectors.
The AD9888 is a complete monolithic analog interface for capturing RGB graphics signals from PCs and workstations. Its 205-MHz encode rate, combined with a full-power bandwidth of 500 MHz, will support resolutions of up to UXGA, as well as high-definition television (HDTV) and digital-TV formats. This device fully integrates three 8-bit, 205-Msample/s ADCs, an on-chip PLL to generate a pixel clock from HSYNC and COAST inputs, mid-scale clamping for HDTV and digital-TV (YUV) signals, plus programmable gain, offset, and clamp controls.
A second entry from Analog Devices, the AD8380 LCD driver chip, is aimed at reducing the power and cost of driving small LCDs in projector systems (Fig. 2). It provides high accuracy and low settling time, and accommodates full, dynamic-range imaging signals with no need for external amplifiers or calibration cycles. The chip also features laser-trimmed output accuracy. This means that if additional AD8380s are added to accommodate still higher resolutions, all of the devices will maintain color-matched, high-quality display signals without calibration.
A Plug-And-Play Display
On the road toward migrating components onto the display is an integrated TFT display panel introduced by Sharp Microelectronics last summer. The company calls it the "All-in-One," because it provides a single-component solution that has been optimized by incorporating interface circuitry. It's intended to reduce total cost and development time in PCs and industrial monitors.
Packed into the All-in-One are the TFT-LCD panel, video decoder, graphic engine, controller, and an inverter to power the backlight. A designer only has to hook the display up to a power supply, devise an enclosure, and provide an input signal.
Both the 15-in. XGA version (LQ150X1MG51) and the 16-in. SXGA version (LQ160E1MG11) of the All-in-One display include the same high-density analog connectors (15-pin sub-D type) used on most monitors. Also provided is a standard 24-pin, digital video input (DVI-D) connector.
The built-in controller has provisions for adjusting display brightness and contrast, as well as for selecting image format and language shown. Graphics and text can be displayed on a 1024- by 3- by 768-dot panel for the 15-in. version, and on a 1280- by 3- by 1024-dot panel for the 16-in. version.
Sharp has also introduced a higher-performance technology version of the All-in-One that includes the company's ASV technology. ASV provides a wide viewing angle of up to 170° (horizontal) by 170° (vertical) and 80- to 150-ms response times. Future improvements should include faster response speed and improved color accuracy.
Besides newer displays, display controllers continue to evolve. Genesis Microchip has introduced the fourth in its series of dual-interface LCD controllers, targeting the new "SmartPanel" market sector. The gm5115 system-on-a-chip (SoC) contains most of the control functions required by an LCD monitor, including a programmable timing controller. As a result, the chip eliminates an entire pc board of circuitry from the LCD monitor, reducing costs and increasing the end product's reliability.
The on-chip panel-timing controller connects directly to the display's row- and column-driver chips. This fully programmable device is compatible with a wide range of LCD panels from many different manufacturers. The gm5115 also includes a triple-ADC and PLL.
A number of other controller chips have hit the market as well (see the table). For example, Texas Instruments (TI) has introduced a high-speed LCD chip set, the TFP74x3 LCD timing controller and the TMS57538 LCD source driver. The chips offer the advantages of high resolution and savings in both board space and cost for flat-panel display resolutions ranging from XGA (1024 by 768 pixels) to QXGA (2048 by 1536 pixels).
TI uses a new process to manufacture the chips for lower power consumption compared to its prior-generation mini-LVDS devices, the TFP7401 LCD timing controller and the TMS57534 LCD source driver. Developed by TI in conjunction with IBM, the mini-LVDS cuts the number of signal traces by 60%.
Is There Any Hope For Standardization?
A successful effort to establish standards for display panels could help to drive up volumes and drive down costs. But of course, it requires both mechanical and electrical standards. Yet is it realistic to expect that a flat-panel display will one day plug into a standard socket just as a light bulb has for over 70 years?
The answer depends on whether or not display technology ever reaches a point where the technology and size become sufficiently stable to warrant the herculean efforts to standardize. Unfortunately, display technology is still relatively young and is still developing rapidly. This is why bringing OEMs and display manufacturers together with the hope of agreement on standards appears to be nowhere near at hand.
To view the table, go to www.elecdesign.com and click on "Examples Of New Controller Chips For High-Resolution Displays."
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