Low voltage differential signaling
Encyclopedia
Low-voltage differential signaling, or LVDS, is an electrical digital 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 computer
s, where it forms part of very high-speed networks
and computer bus
es.
system, meaning that it transmits information as the difference between the voltages on a pair of wires; the two wire voltages are compared at the receiver. In a typical implementation, the transmitter injects a small current, typically 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 ohms (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.
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. At 2.5 V supply voltage the power to drive 3.5 mA becomes 8.75 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal.
Logic Levels:
LVDS is not the only differential signaling
system in use, but is currently the only scheme that combines low power dissipation with high speed.
and supercomputer
users, both of which 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.
The first widespread application for LVDS was to transport video data from graphics adapters to computer monitors, particularly flat panels in notebook computers, using the Flat Panel Display Link (FPD-Link
) by National Semiconductor (NSC
). The LCD display module vendors commonly use the term LVDS instead of FPD-Link
when referring to the input signal to their modules. The higher performance follow-ons were LVDS Display Interface (LDI) and 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. Mobile devices have begun to use Display Serial Interface
which utilizes LVDS signaling.
Two examples of LVDS use in computer buses come from HyperTransport
and FireWire, both of which trace their development 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
, PCI Express
, RapidIO
, and SpaceWire
utilize LVDS to allow high speed data transfer.
Intel and AMD expect that the LVDS LCD-panel interface would no longer be supported in their product lines by 2013, with Embedded DisplayPort
and Internal DisplayPort as preferred solutions.
, which involves sending data bit-by-bit down a single pair of wires. This is in contrast to 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 synchronization, allows more data to be sent using fewer wires than can be accomplished with a parallel bus. The device for converting between serial and parallel data is called a serializer/deserializer, abbreviated to SerDes
.
scheme is typically utilized when DC balance is desired, a necessity for AC-coupled transmission paths (such as capacitive or transformer-coupled paths).
or HyperTransport
). This system is called parallel LVDS.
LVDS, or BLVDS (by NSC
) was the first variation of LVDS designed to drive multiple LVDS receivers, and uses 100-ohm terminations at each end of the differential transmission line to maintain the signal integrity. 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 circuit
s. For MLVDS:
(SCI) specified in the IEEE
1596.3 1995 standard. It was designed for interconnecting multiprocessing
systems.
/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.
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s, where it forms part of very high-speed networks
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
and computer bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
es.
Differential vs. single-ended signaling
LVDS is a differential signalingDifferential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
system, meaning that it transmits information as the difference between the voltages on a pair of wires; the two wire voltages are compared at the receiver. In a typical implementation, the transmitter injects a small current, typically 3.5 mA
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, 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 ohms (matched to the characteristic impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...
of the cable) at the receiving end, then returns in the opposite direction along the other wire. From Ohm's law
Ohm's law
Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points...
, the voltage difference across the resistor is therefore about 350 mV
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
. The receiver senses the polarity of this voltage to determine the logic level.
The small amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
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. At 2.5 V supply voltage the power to drive 3.5 mA becomes 8.75 mW, compared to the 90 mW dissipated by the load resistor for an RS-422 signal.
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
Differential signaling
Differential signaling is a method of transmitting information electrically by means of two complementary signals sent on two separate wires. The technique can be used for both analog signaling, as in some audio systems, and digital signaling, as in RS-422, RS-485, Ethernet , PCI Express and USB...
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 cost of twice as many wires for the same amount of data outweighed the speed benefits. Yet multimediaMultimedia
Multimedia is media and content that uses a combination of different content forms. The term can be used as a noun or as an adjective describing a medium as having multiple content forms. The term is used in contrast to media which use only rudimentary computer display such as text-only, or...
and supercomputer
Supercomputer
A supercomputer is a computer at the frontline of current processing capacity, particularly speed of calculation.Supercomputers are used for highly calculation-intensive tasks such as problems including quantum physics, weather forecasting, climate research, molecular modeling A supercomputer is a...
users, both of which needed to move large amounts of data over links several meters long (from a disk drive to a workstation
Workstation
A workstation is a high-end microcomputer designed for technical or scientific applications. Intended primarily to be used by one person at a time, they are commonly connected to a local area network and run multi-user operating systems...
, for instance) maintained a widespread interest in LVDS.
The first widespread application for LVDS was to transport video data from graphics adapters to computer monitors, particularly flat panels in notebook computers, using the Flat Panel Display Link (FPD-Link
FPD-Link
Flat Panel Display Link by National Semiconductor is a high-speed interface connecting the output of a video controller in a laptop computer, computer monitor or LCD television set to the display panel. Most laptops, LCD computer monitors and LCD TVs use this interface internally...
) by National Semiconductor (NSC
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...
). The LCD display module vendors commonly use the term LVDS instead of FPD-Link
FPD-Link
Flat Panel Display Link by National Semiconductor is a high-speed interface connecting the output of a video controller in a laptop computer, computer monitor or LCD television set to the display panel. Most laptops, LCD computer monitors and LCD TVs use this interface internally...
when referring to the input signal to their modules. The higher performance follow-ons were LVDS Display Interface (LDI) and OpenLDI
OpenLDI
OpenLDI is a high-bandwidth digital interface standard for connecting flat panel LCD monitors to computers. While it is virtually unknown in standalone displays, it has a strong basis in the interconnect for internal displays, as in laptops. OpenLDI is based on the de-facto FPD-Link specification...
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
LDI (disambiguation)
LDI may refer to:* Lady Davis Institute for Medical Research in Montreal, Canada* Liability-driven investment strategy* LVDS Display Interface, a low-voltage differential signaling interface for flat-panel displays...
extends this to about 10 m. Mobile devices have begun to use Display Serial Interface
Display Serial Interface
The Display Serial Interface is a specification by the Mobile Industry Processor Interface Alliance aimed at reducing the cost of display sub-systems in a mobile device. This is commonly targeted at LCD and similar display technologies...
which utilizes LVDS signaling.
Two examples of LVDS use in computer buses come from HyperTransport
HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...
and FireWire, both of which trace their development back to the post-Futurebus
Futurebus
Futurebus is a computer bus standard, intended to replace all local bus connections in a computer, including the CPU, memory, plug-in cards and even, to some extent, LAN links between machines. The effort started in 1979 and didn't complete until 1987, and then immediately went into a redesign...
work which also led to SCI
Scalable Coherent Interconnect
SCI, for Scalable Coherent Interface, is a high-speed interconnect standard for shared memory multiprocessing and message passing. The IEEE Std 1596-1992, IEEE Standard for Scalable Coherent Interface was approved by the IEEE standards board on March 19, 1992...
. LVDS is supported in SCSI
SCSI
Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...
standards (Ultra-2 SCSI and later) to allow higher data rates and longer cable lengths. Serial ATA
Serial ATA
Serial ATA is a computer bus interface for connecting host bus adapters to mass storage devices such as hard disk drives and optical drives...
, PCI Express
PCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
, RapidIO
RapidIO
The RapidIO architecture is a high-performance packet-switched, interconnect technology for interconnecting chips on a circuit board, and also circuit boards to each other using a backplane...
, and SpaceWire
SpaceWire
SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency in collaboration with international space agencies including NASA, JAXA and RKA...
utilize LVDS to allow high speed data transfer.
Intel and AMD expect that the LVDS LCD-panel interface would no longer be supported in their product lines by 2013, with Embedded DisplayPort
DisplayPort
DisplayPort is a digital display interface standard produced by the Video Electronics Standards Association . The specification defines a royalty-free digital interconnect for audio and video. The interface is primarily used to connect a video source to a display device such as a computer monitor...
and Internal DisplayPort as preferred solutions.
Comparison with parallel transmission
LVDS is often used for serial data transmissionSerial communications
In telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
, which involves sending data bit-by-bit down a single pair of wires. This is in contrast to parallel transmission
Parallel communications
In telecommunication and computer science, parallel communication is a method of sending several data signals simultaneously over several parallel channels...
, 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 synchronization, allows more data to be sent using fewer wires than can be accomplished with a parallel bus. The device for converting between serial and parallel data is called a serializer/deserializer, abbreviated to SerDes
SerDes
A Serializer/Deserializer is a pair of functional blocks commonly used in high speed communications to compensate for limited input/output. These blocks convert data between serial data and parallel interfaces in each direction...
.
Comparison with 8b/10b transmission
LVDS does not specify a bit encoding scheme (i.e. start bit and stop bit). LVDS is the physical mechanism to transport bits across wires. Any user-specified encoding scheme can be sent/received across an LVDS link, including 8b/10b data. An 8b/10b encoding8B/10B encoding
In telecommunications, 8b/10b is a line code that maps 8-bit symbols to 10-bit symbols to achieve DC-balance and bounded disparity, and yet provide enough state changes to allow reasonable clock recovery. This means that the difference between the count of 1s and 0s in a string of at least 20 bits...
scheme is typically utilized when DC balance is desired, a necessity for AC-coupled transmission paths (such as capacitive or transformer-coupled paths).
Parallel 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 ExpressPCI Express
PCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
or HyperTransport
HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...
). This system is called parallel LVDS.
Multipoint LVDS
BusComputer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...
LVDS, or BLVDS (by NSC
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...
) was the first variation of LVDS designed to drive multiple LVDS receivers, and uses 100-ohm terminations at each end of the differential transmission line to maintain the signal integrity. 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
Texas Instruments
Texas Instruments Inc. , widely known as TI, is an American company based in Dallas, Texas, United States, which develops and commercializes semiconductor and computer technology...
) are de facto
De facto
De facto is a Latin expression that means "concerning fact." In law, it often means "in practice but not necessarily ordained by law" or "in practice or actuality, but not officially established." It is commonly used in contrast to de jure when referring to matters of law, governance, or...
multipoint LVDS standards. Multipoint LVDS (MLVDS) is the TIA
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...
standard (TIA-899) that has evolved and is used in AdvancedTCA
Advanced Telecommunications Computing Architecture
Advanced Telecommunications Computing Architecture is the largest specification effort in the history of the PCI Industrial Computer Manufacturers Group , with more than 100 companies participating. Known as AdvancedTCA, the official specification designation PICMG 3.x was ratified by the PICMG...
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 circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
s. For MLVDS:
| Output | Input | ||
| Common mode | Amplitude | ||
| Min | 0.3 V | 0.480 V | −1.4 V |
| Max | 2.1 V | 0.650 V | 3.8 V |
SCI-LVDS
The present form of LVDS was preceded by an earlier attempt, SCI-LVDS, which was a subset of the Scalable Coherent InterconnectScalable Coherent Interconnect
SCI, for Scalable Coherent Interface, is a high-speed interconnect standard for shared memory multiprocessing and message passing. The IEEE Std 1596-1992, IEEE Standard for Scalable Coherent Interface was approved by the IEEE standards board on March 19, 1992...
(SCI) specified in the IEEE
Institute of Electrical and Electronics Engineers
The Institute of Electrical and Electronics Engineers is a non-profit professional association headquartered in New York City that is dedicated to advancing technological innovation and excellence...
1596.3 1995 standard. It was designed for interconnecting multiprocessing
Multiprocessing
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...
systems.
Standards
The ANSIAmerican National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...
/TIA
Telecommunications Industry Association
The Telecommunications Industry Association is accredited by the American National Standards Institute to develop voluntary, consensus-based industry standards for a wide variety of ICT products, and currently represents nearly 400 companies...
/EIA
Electronic Industries Alliance
The Electronic Industries Alliance was a standards and trade organization composed as an alliance of trade associations for electronics manufacturers in the United States. They developed standards to ensure the equipment of different manufacturers was compatible and interchangeable...
-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.