Tips for Smart Integration in Flat Panel Displays

Flat panel displays are taking the computer market by storm; rising sales rates for LCD monitors, graphic projectors, and plasma displays are reminiscent of the early days of personal computers. To remain competitive in a rapidly growing arena, manufacturers cannot afford to stand still. They must continually improve their products, lowering costs, improving image quality, increasing reliability, and adding new user features. Most importantly, manufacturers must improve the manufacturability of their products, making them easier and less expensive to produce. One key to achieving all of these design goals is to employ "intelligent integration" in the display interface.

Integrating discrete functions into a single chip can reduce the number of parts and production costs, and PC manufacturers have adopted this strategy for a while in the past decade. However, because of the huge technical challenges to overcome when combining display interface functions, displays need to be intelligently integrated. Integrating high-performance analog and high-speed digital circuits into a single design requires extensive experience in specialized mixed-signal design techniques. Products with integrated interfaces need to meet cost and size reduction goals without sacrificing image quality or manufacturability.

A typical LCD display offers many opportunities for integration (Figure 1). These include:

1. Bringing together analog and digital interfaces to provide legacy compatibility with over a billion PCs in use worldwide today, while still remaining “digital ready” for the future.

2. Combines the interface with the image controller function, which is the main image-related circuit in the flat-panel display.

3. Reduce interconnections and simplify board layout to reduce distortion that affects image quality.

4. Add new features such as USB hub and USB client functions as low additional cost for flat panel displays.

Figure 1: Flat panel display module diagram.

Integrated analog interface

Despite the continued growth in interest in new digital flat panel interfaces, analog interfaces remain the primary choice for LCD displays currently on the market. Integrated analog interfaces such as the AD9884 from Analog Devices and the TDA8752H from Philips have combined all the traditional analog functions required for flat panel interfaces into a single chip (Figure 2). This first step of integration has reduced costs, improved image quality, and simplified board layout.

Figure 2: Analog interface block diagram.

Integrated analog interface chips are being mass-produced using mixed-signal CMOS or BiCMOS wafer fabrication processes. These high-performance processes are required if the chips are to meet the high-speed (>100 MHz) A/D converters with typical linearity specifications (INL/DNL) of ±0.5 LSB and >200 MHz input bandwidth required for flat panel interface applications. However, standard CMOS wafer fabrication processes do not provide the mixed-signal characteristics required to achieve high-performance analog signal processing and A/D converters.

Increase digital connectivity

The next logical step in integrating flat panel interfaces is to combine the analog and digital interfaces in a single chip. This makes it easier to provide legacy compatible displays that are also "digital ready" for the future.

Having a digital interface on the same monitor may encourage consumers to choose a digital connection when they upgrade their graphics card or computer system. It is generally believed that the vast majority of PC users use their monitors for five to seven years and typically upgrade their PCs several times to adopt high-performance technology before purchasing a new monitor. Dual-interface monitors allow users to continue to get long-term use out of their monitors before discarding them.

The new Digital Display Working Group (DDWG) Digital Video Interface (DVI) v1.0 and the Video Electronics Standards Association (VESA) Plug and Display standards allow for this dual-interface display. All of these direct addressing display standards specify connectors, cables, and electrical characteristics to support dual-interface displays. All standards also specify transition minimized differential signaling (TMDS) as the digital interface.

AD9887 Integrated Interface

The AD9887 is the world's first integrated dual interface for flat-panel displays (Figure 3). Its analog interface is based on the AD9884 analog interface and operates at a maximum frequency of up to 140 MHz for SXGA displays. Its digital interface uses TMDS and can also provide image resolutions up to SXGA (112 Msps). These interfaces share the same output, saving 48 pins compared to a two-chip analog/digital interface design. The chip also provides automatic detection of the interface connected to the display, as well as an option that allows the user to select the interface on a serial data connection.

The chip is available in two grades: the AD9887 for XGA displays and the AD9887-140 for SXGA displays. This choice allows designers to select the most cost-effective solution for their displays. The AD9887 is currently ready for product sampling in November.

Figure 3: AD9887 dual-interface module diagram.

Reduce interconnection

The image quality achievable with flat-panel displays is greatly dependent on the design and layout of the display interface circuit board. One of the benefits of higher integration in displays is fewer interconnections between devices, resulting in higher image quality. It is notoriously challenging for display manufacturers to build interface circuit boards next to “noisy” digital circuits, such as noise-sensitive PLLs and A/D converters, and still achieve acceptable image quality. David Mentley, vice president of Stanford Resource, said display companies “spend a lot of money, time, real estate and effort to make analog interfaces work without artifacts, but not all of them succeed.” Higher levels of interface integration allow for simplified board designs and produce higher image quality compared to older discrete device board designs.

Integrated control functions

Early graphics controllers, also called scalers or zoom scalers, such as the Genesis gmZ1, generally included only image magnification functions and required that other flat panel display electronic functions be handled by discrete peripheral chips. Frame rate conversion, frame buffers, external PLLs, microcontrollers, OSD controllers, and EEPROM chips were required to complete the display circuit board.

Second-generation graphics controllers such as the Macronix 88282 and Sage Cheetah integrated the scaler and frame rate conversion functions into a single chip, resulting in higher image quality, lower cost and simpler circuit board layout.

wiring

LCD monitors (LCMs) offer smaller form factors, which has led designers to consider integrating more user features into these displays. As USB quickly becomes the best communication link with PC peripherals, it is natural to place USB hubs and client functions in desktop LCD monitors. The 12 Mb/s data rate of USB 1.1 is sufficient for a wide range of PC peripherals, such as phones, digital cameras, keyboards, mice, digital joysticks, graphics tablets, wireless base stations, cassettes, tapes and disks, digital speakers, scanners and printers. The higher bandwidth USB 2.0 products will appear in the second half of 2000, allowing higher-function PC peripherals, including higher-resolution video conferencing cameras, new generation scanners and printers, and high-speed storage units. The higher data rate of USB 2.0 (>120 Mb/s) will also open up the possibilities of existing new PC peripherals.

Almost all LCD displays with USB hubs on the market today have the connector and associated circuitry in the display substrate. However, the VESA Flat Panel Physical Mounting Interface Standard (FPPMIS) describes standard mounting diagrams for removing the display substrate, mounting the LCD flat panel directly to the wall, and how to mount it to maximize desktop space. In order to comply with FPPMIS, the USB hub circuitry and connector must be migrated to the display circuit board on the back of the LCD flat panel. In line with the smart integrated design strategy, flat panel interface products will quickly integrate on-chip USB hub functions to comply with FPPMIS, save circuit board space, and further reduce the cost of the entire display.

Complexity issues

Some graphics controller manufacturers are designing and producing products that integrate analog interface/graphics controllers into a single chip. These integrated products are mainly focused on XGA (100 MHz or lower) screen resolutions and are expected to achieve lower costs and simplify circuit board layout. Although they emphasize "single chip implementation", they still provide products that require one or more external PLLs, microcontrollers, OSD (on screen display) controllers, EEPROM memory, and other external circuits. These integrated analog interface/graphics controller products are usually limited to scaling of low-resolution images. Frame rate conversion does not have or require an external frame rate controller and external frame buffer memory.

Since most of the die area consists of digital gates, this integration strategy requires the use of 0.25 ? or 0.35 ? digital CMOS production processes to be cost-effective. Embedded DRAM processes are used in some cases to integrate large frame buffer memories with graphics controllers on-chip, but are not well suited for implementing high-performance analog functions.

To optimize high-performance analog circuits, mixed-signal CMOS production processes were required. Because mixed-signal processes were not commonly used and designers with high-performance mixed-signal experience were still rare, the analog performance of these early integrated products was compromised, as evidenced by the significantly reduced display image quality compared to stand-alone analog interface products.

Successful testing

Products with integrated interface/graphics controllers are true mixed-signal devices that require 100% high-speed mixed-signal electrical testing to ensure the highest quality performance. Mixed-signal test solutions for A/D converters are now almost as complex as the chip design itself.

Digital designers are extremely proficient in the development of digital test programs, where test vectors are typically input into automatic test pattern generator (ATPG) software to develop digital test patterns. Joint Test Action Group (JTAG) boundary scan and internal scan techniques are also used to verify 98% or more fault coverage within digital circuits. Existing digital logic testers are frequently used to test these digital circuits.

However, high-performance mixed-signal circuits require "fast" testing to verify electrical performance. Testing high-speed analog circuits requires test R&D engineers to develop code and unique test hardware to achieve high-precision measurements of A/D converter jitter, signal-to-noise ratio (SNR), and linearity. High-performance mixed-signal test platforms such as Teradyne Catalyst and LTX Fusion combine the ability to test high-performance analog circuits with the ability to test high-speed, high-pin-count digital circuits contained in one integrated device. Integrated interface products that are not rigorously tested using high-performance mixed-signal test platforms can have severe deviations in their performance due to normal distribution on the wafer production process.

Out-of-spec components can cause serious problems for LCM manufacturers who have demanding product schedules and limited engineering resources. For example, consider an interface product that has lot-to-lot variations in a critical parameter that only affects 10% of the batch tested. If the supplier cannot screen out all of these bad parts, the display manufacturer will be forced to add additional testing to 100% of its displays, increasing the cost of the final product. Today, cautious designers are asking more questions about how to test integrated interface products to ensure they receive only the highest quality products.

Leap to the next level

Smart integration is the key to increasing the success of LCM and other direct-addressed display products. Reducing the number of components can improve image quality, reduce cost, reduce size, and make the product easier to produce. However, the resulting device will be very complex. Unless these devices are properly designed and adequately tested, attempts to achieve a higher degree of integration will produce disappointing results. Engineers need to foresee the product's specifications in order to ensure that they have components they can rely on for their display design.