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low-voltage

Low-voltage differential signaling

Low-voltage differential signaling, or LVDS, is an electrical signaling system that can run at very high speeds over inexpensive twisted-pair copper cables. It was introduced in 1994, and has since become very popular in computers, where it forms part of very high-speed networks and computer buses.

Differential vs single-ended signaling

LVDS is a differential signaling system, which means that it transmits two different voltages which are compared at the receiver. LVDS uses this difference in voltage between the two wires to encode the information. The transmitter injects a small current, nominally 3.5 mA, into one wire or the other, depending on the logic level to be sent. The current passes through a resistor of about 100 to 120 Ω (matched to the characteristic impedance of the cable) at the receiving end, then returns in the opposite direction along the other wire. From Ohm's law, the voltage difference across the resistor is therefore about 350 mV. The receiver senses the polarity of this voltage to determine the logic level. This type of signalling is called a current loop.

The small amplitude of the signal and the tight electric- and magnetic-field coupling between the two wires reduces the amount of radiated electromagnetic noise.

The low common-mode voltage (the average of the voltages on the two wires) of about 1.25 V allows LVDS to be used with a wide range of integrated circuits with power supply voltages down to 2.5 V or lower. The low differential voltage, about 350 mV as stated above, causes LVDS to consume very little power compared to other systems. For example, the static power dissipation in the LVDS load resistor is 1.2 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal. Without a load resistor the whole wire has to be loaded and unloaded for every bit of data. Using high frequencies and a load resistor so that a single bit only covers a part of the wire (while traveling near light speed) is more power efficient.

Logic Levels:

Vee VOL VOH Vcc VCMO
GND 1.0 V 1.4 V 2.5–3.3 V 1.2 V

LVDS is not the only differential signaling system in use, but is currently the only scheme that combines low power dissipation with high speed.

Applications

LVDS became popular in the latter half of the 1990s. Before that, computers were too slow to make use of such fast data rates, and the need to run twice as many wires for the same amount of data outweighed the speed benefits. Yet multimedia and supercomputer users, both of whom needed to move large amounts of data over links several meters long (from a disk drive to a workstation, for instance) maintained a widespread interest in LVDS.

Two examples of LVDS use in computer buses come from HyperTransport and FireWire, both of which trace their ancestry back to the post-Futurebus work which also led to SCI. LVDS is supported in SCSI standards (Ultra-2 SCSI and later) to allow higher data rates and longer cable lengths. Serial ATA, RapidIO, and SpaceWire utilize LVDS to allow high speed data transfer.

LVDS can also transport video data from graphics adapters to computer monitors, particularly flat panels, using the Flat Panel Display Link (FPD-Link), LVDS Display Interface (LDI), or OpenLDI standards. These standards allow a maximum pixel clock of 112 MHz, which suffices for a display resolution of 1400 x 1050 (SXGA+) at 60 Hz refresh. A dual link can boost the maximum display resolution to 2048 x 1536 (QXGA) at 60 Hz. FPD-Link works with cable lengths up to about 5 m, and LDI extends this to about 10 m.

Comparison with parallel transmission

LVDS is often used for serial data transmission, which involves sending data bit-by-bit down a single pair of wires. This is in contrast to conventional parallel transmission, in which several wires are used with a common ground to carry several signals at once. The high speed of LVDS, and its use of in-channel synchronisation, allows more data to be sent using fewer wires than can be done with a normal parallel bus. The device for converting between serial and parallel data is called a serializer/deserializer, abbreviated to SerDes.

Multipoint LVDS

When serial data transmission is not fast enough, data can be transmitted in parallel form using an LVDS pair for each bit or even byte (as in PCI Express or HyperTransport). This system is called bus LVDS, or BLVDS. Standard LVDS transmitters are designed for point-to-point links, but multipoint bus systems can be made using modified LVDS transmitters with high-current outputs that can drive multiple termination resistors. Bus LVDS and LVDM (by TI) are de facto multipoint LVDS standards. Multipoint LVDS (MLVDS) is the TIA standard (TIA-899) that has evolved and is used in AdvancedTCA for some clock distribution.

MLVDS has two types of receivers. Type-1 are nearly compatible with LVDS and use a 0 Volt threshold. Type-2 use a 100 mV threshold to handle in a consistent way various errors such as open and short circuits. For MLVDS:

Input Min/Max Output Common Mode Min/Max Output Amplitude Min/Max
-1.4 / 3.8 V 0.3 / 2.1 V 0.480 / 0.650 V

SCI-LVD

The present form of LVDS was preceded by an earlier attempt, SCI-LVD, which was a subset of the Scalable Coherent Interconnect (SCI) specified in the IEEE 1596.3 standard. It was designed for interconnecting multiprocessing systems.

Standards

The ANSI/TIA/EIA-644-A (published in 2001) standard defines LVDS. This recommends a maximum data rate of 655 Mbit/s over twisted-pair copper wire, but predicts a possible speed of over 1.9 Gbit/s for an ideal transmission medium.

See also

External links

References

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