Flexible architecture drives innovative LCD applications

Liquid crystal displays (LCDs) have become very common in today’s consumer and automotive electronics. LCD technology has produced a large number of products that meet a wide range of size, power, and image quality requirements.



The popularity of standard LCDs has led to a number of fixed-function semiconductor solutions to drive them ranging from ASSPs to full-custom ICs. However, these solutions often restrict the options that product developers need for a flexible solution to support specific applications or a scalable solution to support multiple, similar applications in a specific product line.



Enter two-way viewing LCDs. Today there is no standard IC driver solution for this new breed of LCD. Support can only be garnered with a custom solution. This article examines an innovative FPGA-based architecture and soft-core embedded processor that supports two-way and standard viewing.



THE FPGA SOLUTION

Limitations of fixed-function devices for driving LCD panels can be overcome by replacing them with a combination of an FPGA and a soft-core embedded processor. The inheritant design flexibility of FPGAs supports a variety of LCD panel requirements. FPGA reconfigurability also allows system designers to change the LCD driver functionality after the product has been manufactured, enabling easy in-field product upgrades and enhancements.

By utilizing a soft-core processor development environment, designers can quickly and efficiently assemble the elements needed to drive a wide range of LCD panel configurations for implementation within an FPGA.



Providing an LCD driver architecture that addresses the design challenges of two-way and standard viewing enables product developers and manufacturers to choose from several devices, each based on their needs and without concern about obsolescence. An architecture like Altera’s Cyclone FPGA is based on five modules: two BT-656 video inputs, an LCD controller, an SDRAM controller, and an I2C controller. The processor performs the overall control and arbitration of the system. When employed in an FPGA and bus connected, these modules provide all of the necessary throughput and video bandwidth to drive a two-way viewing LCD panel with 800 x 480 resolution (see Figure 1).



Describing the elements of this FPGA-based LCD driver architecture, is useful in understanding how the two-way viewing LCD panel operates in comparison to a standard LCD panel. This technology allows simultaneous display of two independent images on the same physical glass surface of an LCD. The images are not shown side-by-side or picture-in-picture, but concurrently, separated by two optical viewing cones: one for viewing from the right and one for viewing from the left (see Figure 2). In an automotive application, the driver views the display from the left (dashboard instruments, navigation information, etc.), while the passenger views the display from the right (movie viewing). Both images would appear across the full surface of the individual displays.



Electrically, a two-way viewing LCD panel is identical to a standard LCD panel. As with a standard LCD panel, data is written to the display in a row-column pixel matrix that traverses the display from left to right and top to bottom. Additionally, both timing and synchronization signals remain identical to standard LCDs. The dual image effect is derived by physical means. Above the liquid crystal material is an optical director layer that routes light from the subpixels to each independent field of view.



Since the optical director is static, it relies on the organization of the subpixel data beneath it for proper image creation and alignment. Therefore, reorganization and mixing of the subpixel data must occur prior to the data being written to the display. In a two-way viewing LCD panel, the subpixels of one image alternate with subpixels from the other. Since two images are “woven” together, they divide the effective display resolution in the horizontal direction.



VIDEO INPUT MODULE

The BT-656 video input module is designed to be compatible with the ITRU-BT656 digital video standard. This module is responsible for color space conversion (CSC), clipping, de-interlacing, scaling, and an RGB 565 pack. Each operation is performed sequentially and is parameterized by registers controlled by a Nios II processor.



The LCD controller module is a multilayer LCD controller containing three 16-bit color image layers and, optionally, two 8-bit palette layers. Each layer is formed and controlled by a separate DMA master, thus creating efficient partitioning of the external DRAM-based frame buffer.



As each layer master reads from memory, the LCD controller module combines (or flattens) the layers into one combined frame. This combined frame is then sent to the LCD and the LCD controller module combines the layers, it can apply an alpha value or transparency factor to each.



The finite layer control provides one method of two-way viewing LCD panel control. Each video source is routed to separate and equally dimensioned frame buffers. To construct the two-way viewing image, the DMA controllers of the two video layers grab alternating pixels from their frame buffers. The layers are offset from each other by one pixel and then flattened into one layer for the output image frame. This process preserves the overall display timing parameters and enables the FPGA to weave the pixel information together.



RESOURCE UTILIZATION AND IMPLEMENTATION

An FPGA-based LCD driver architecture easily scales to support LCD panels of various sizes, both two-way viewing and standard. Additional capabilities, such as customized on-screen displays, splash screens, graphics acceleration, 3D rendering, or support for any number of LCD panels can be included by adding functionality to the FPGA design and selecting the appropriately sized FPGA for implementation. Product developers thus not only gain design flexibility when using an FPGA-based architecture, but also cost effectiveness without sacrificing performance.



About the authors

Martin Won has more than 16 years of experience with programmable logic and programmable logic applications. He joined Altera Corporation in August 1990 as an applications engineer and has held various positions in his career, during which he founded Altera’s customer training program and managed several other technical and marketing programs. He holds a BSEE degree from the University of California at Santa Barbara.



Patrick Hourigan joined Jabil Circuit Inc. in 2005. Before joining he held a number of technical and management positions with software and hardware firms, including lengthy assignments in Japan. He holds a BSEE degree from The Ohio State University and an MBA from Florida Atlantic University.



David Anderson is a product design innovator specializing in electrical design. He joined Trilogy in 2005 and over his career has developed various products and technologies in areas including satellite radio, vehicle navigation, digital audio broadcasting, Bluetooth audio and hands-free interfaces, and vision based inspection. His design experience includes developing module-based SDARS system for Sirius Satellite Radio and FPGA-based controllers for LCD displays like Sharp’s two-way viewing panel. He holds a BSEE degree from Purdue University.



Click here for Illustrations:



Figure 1



Figure 2