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NATIONAL ANNOUNCES FIRST LVDS (LOW-VOLTAGE DIFFERENTIAL SIGNALING) DEVICES FOR HOST-TO-LCD INTERFACING

Nov 6, 1995

SANTA CLARA, CA., -- November 6, 1995 -- National Semiconductor today announced the immediate availability of the DS90CR561/2 and DS90CR581/2 host-to-LCD interface devices. These driver and receiver ICs are the first LCD interfacing devices to comply with the LVDS standard and offer a practical way to upgrade laptop computers to XGA screen resolutions. The interfaces are also fully compatible with VGA and SVGA displays.

The DS90CR561/2 and DS90CR581/2 overcome three barriers that have made it impractical for laptop and notebook computer manufacturers to upgrade to higher screen resolutions. First, increasing resolution pushes up the bandwidth on the interface between the computer and the LCD screen by the square of the number of pixels. Second, the resulting higher clock rates generate more electromagnetic interference (EMI). Finally, the higher addressability requirements lead to wider cable/connector interfaces, which are impractical.

To solve these problems, the DS90CR561/2 and DS90CR581/2 use a combination of LVDS signaling and a clock-splitting technique that allows data serialization. As a result, the interface to a 1024x768 XGA LCD with 8-bit color requires only 20 lines (instead of the customary 52) to achieve the necessary bandwidth, yet keeps EMI to much lower levels than is possible using conventional TTL or low-voltage TTL interfaces. Two pairs of the 40-MHz LVDS chips are used to implement the 65-MHz XGA interface. The DS90CR561/2 and DS90CR581/2 support all types of LCD-based products, including PDAs and telecommunications equipment, as well as PCs.

"LVDS is ideal for LCD interfacing," noted Mitchell Abbey, Interface product marketing manager at National Semiconductor. "Its ability to move data at phenomenal rates without high EMI or high power dissipation gives laptop and notebook computer makers an easy way to give customers what they want -- better screen resolutions. National recently became the firstcompany to offer LVDS drivers and receivers," he added. "Now we're first to bring the technology to LCD interfacing. And this is just the beginning."

"LVDS is a breakthrough for PC and non-PC display applications," said Joel Pollack, Sharp's marketing manager for displays. "LVDS will speed the integration of high resolution displays into notebook computers. Having fewer wires simplifies the connection and reduces cost. And the low drive voltage makes it easy to manage EMI. LVDS will also make new LCD applications possible. The ability to operate even an XGA display several meters away from the controller opens up lots of new possibilities for point of sale displays, transportation applications and information kiosks," he added.

LVDS Technology
The key to LVDS technology is the use of a differential data-transmission scheme. Instead of designating a precise voltage level for a logic one or zero, the LVDS standard specifies a voltage differential. This approach ensures outstanding common-mode-noise immunity. Any noise introduced into the medium is seen as common-mode modulations by receivers and is rejected. The receivers respond only to differential voltages.

To achieve high data rates and keep power requirements low, LVDS uses a differential voltage swing of only 345 mV(typ). Further, the LVDS CMOS current-mode driver design greatly reduces quiescent power supply requirements.

National's LVDS devices implement the IEEE (Institute for Electrical and Electronics Engineering) 1596.3 SCI (Scalable Coherent Interface) LVDS and TIA/EIA-644 (Telecommunication Industry Association/Electronic Industry Association) LVDS standards.

The Transmitter/Receiver Chip Sets
The DS90CR581 transmitter converts 28 bits of CMOS/TTL data into four LVDS data streams for interfacing to 24-bit-color LCDs. It sends a phase-locked-loop transmit clock in parallel with the data streams via a fifth LVDS link.

Every cycle of the transmit clock, the transmitter samples and sends 28 bits of data, achieving bandwidths as high as 140 Mbytes per second. The DS90CR581 receiver converts the LVDS data streams back into 28 bits of CMOS/TTL data plus a receive clock.

Similarly, the DS90CR561 transmitter and DS90CR561 receiver implement 18-bit-color interfaces. These devices achieve bandwidths as high as 105 Mbytes per second, working with 21 bits of CMOS/TTL data and a transmit/receive clock.

The phase-locked loop used in the DS90CR561/2 and DS90CR581/2 requires no external components. At a transmit clock frequency of 40 MHz, 18/24 bits of RGB data and 3/4 bits of LCD timing and control data are transmitted at a rate of 280 Mbps per LVDS data channel, resulting in a 1.12 Gbps aggregate bandwidth on the data lines.

The DS90CR561/2 and DS90CR581/2 interfaces are targeted at designs using rising-edge clocks. Devices compatible with falling-edge clocks will be available by the end of the fourth quarter and will have a "CF," rather than "CR," designator. This clock choice will allow the devices to work with a wide variety of graphics and LCD panel controllers.