비디오 시리즈
Precision labs 시리즈: V³Link™
산업용 이미징 시스템에서 비압축 비디오 데이터, 전력 및 양방향 시스템 제어 신호를 보내도록 설계된 V³Link 시리얼라이저 및 디시리얼라이저를 사용하여 설계하는 방법을 알아보세요.
What are V³Link SerDes?
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리소스
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Hello. I'm Casey McCrea. In this training module, I'll introduce you to V3Link, an exciting new technology, created by TI, to enable uncompressed, low latency video data links in a wide range of industrial, medical, and consumer applications. V3Link is a high-speed bidirectional video SerDes technology that enables uncompressed transmission of video data, control signals, and power using a single wire over several meters.
V3Link SerDes are suitable for many different industries and equipment which require digital, audio, and video transfer over long cable distances. Pictured here are a few examples of V3Link, as used in industrial environments; smart appliances; and medical equipment, such as endoscopes and patient monitors. Through this video, I'll discuss the fundamentals of the V3Link technology, including data interfaces, concepts of serialization and deserialization, the high-speed serial channel itself, and bidirectional communication.
V3Link technology acts as a bridge between protocol-based data interfaces, which require multiple signaling conductors to transfer high-bandwidth data. Some common examples of these interfaces include TMDS, MIPI D-PHY, LVDS, or more. These standards all define different physical layers for transferring audio, video, or other digital data sources. However, generally, these standards are designed for transferring video only over short distances, which may include PCB trace or flex cable.
A wide range of applications, from endoscopes to factory automation, demand that this high-bandwidth data be transferred over longer cable distances using rugged flexible cabling. This presents a challenge, not just because of the signal loss introduced with such a transmission channel, but also due to the introduction of EMI or EMC considerations common to medical or industrial applications, where external noise sources could interfere with the cable as it passes to its destination. Because V3Link can interface to many different parallel data formats, this also means that the technology may be used as a format converter where the source interface may not match the sync interface.
V3Link devices offer engineers a lot of design flexibility by supporting various cable types from a wide range of suppliers. Most applications typically utilize either coaxial or twisted pair cables to carry the information between serializer and deserializer. Coaxial cables tend to have lower insertion loss characteristics when compared with twisted pair cables due to their electromagnetic construction, and they typically come at a lower cost when compared with other cable types. Twisted pair cable types, on the other hand, benefit from the fact that signals are carried differentially across them, meaning that they are typically more immune to the effects of single-ended electromagnetic interference.
Twisted pair cables generally come in three main variants-- STP, or Shielded Twisted Pair; STQ, which means Star Quad; or UTP, Unshielded Twisted Pair. Shielded twisted pair cables provide even stronger immunity against external noise interference by surrounding the twisted pair wires with a thin conductor. Due to the harsh electromagnetic environment of medical or industrial applications, most systems utilize STP or STQ rather than UTP.
Coax cables are often also utilized for transferring DC power to a remote device along the same conductor as the high-speed data since the outer shield can act like a return path for DC current and the AC signal content can be filtered from the DC power, which is also on the line. This scheme is most commonly referred to as Power over Coax, or PoC. Most V3Link devices can support both coax or twisted pair configurations to ensure flexibility in various applications.
The V3Link high-speed forward channel is used to send the serialized video, audio, or other data to an endpoint device with minimal latency. In order to achieve this, the serializer must reformat its incoming data and embed the data clock so that it can be output using less conductors. By utilizing a proprietary echo cancellation technique, V3Link SerDes also allow for full duplex communication over one physical conductor. As high-speed data transfers from the serializer to the deserializer in the forward direction, low-speed data is also transferred back to the serializer simultaneously and without time multiplexing.
The V3Link serializer and deserializer devices automatically establish this bidirectional channel by canceling out their own transmitting signals at each end of the link continuously. The backchannel is typically operated at a significantly lower speed than the forward channel data, in order to facilitate proper separation on both sides, and may include information about the sync device, touch interrupts, control signal, status information, and more.
Using this simultaneous backchannel communication, I2C access and GPIO transfer can be enabled across the length in either forward or reverse directions. In order to compensate for channel insertion loss, which may be significant depending on the speed of operation and type or length of cabling used, V3Link deserializers utilize multiple equalization techniques in order to recover high-frequency signal content and mitigate the effects of intersymbol interference, reflections, or external noise influence.
Consider the eye diagram shown on the left side. The signal eye is sufficiently open when measured at the output of the serializer device. However, once that high-speed data travels through a 10-meter STP cable, the effects of insertion loss and ISI significantly reduce the eye quality to the point where an unequalized receiver could not properly recover the data. After passing through the adaptive equalizer on the deserializer, you can see that the eye has regained both vertical height and horizontal width.
First, the V3Link deserializers utilize Continuous Time Linear Equalizer-- or CTLE-- circuit, which acts is an amplifier to high-frequency signal content that is attenuated more rapidly across the lossy channel. The CTLE circuit has several different gain stages to account for varying channel loss depending on the type or length of cabling used in the system. Next, the V3Link deserializers utilize a Clock Domain Recovery-- or CDR-- circuit, to further improve the signal quality in the time domain by reducing the effects of jitter caused by ISI, reflections, or external noise. For more details on how to interpret eye diagrams, see the Precision Labs video, What is an Eye Diagram?
V3Link transfer works by combining input data into packets or frames in order to be transferred serially at higher speed. Depending on the device family, the forward channel frame size may vary from 28 to 40 bits and uses 8b-/10b-style encoding. For a V3Link forward channel signal, the signaling frequency, in gigahertz, is half of the effective baud rate in gigabits per second. For example, a 4.2 gigabits per second forward channel would utilize a 2.1 gigahertz fundamental carrier frequency.
The forward channel frame can be broken down into four main [AUDIO OUT]. Payload data makes up the majority of the frame. This is the high-bandwidth portion of the data, which could be comprised of video pixel information, audio data, or other types of data including RADAR, LiDAR, and more. There are two embedded clocking bits per frame, which help establish timing for the high-speed signal and establish frame boundaries. DCA and DCB bits are used for both DC balancing the signal and for conveying coding information across multiple consecutive frames in order to establish the link.
DC balancing helps to ensure that the AC coupled channel will not experience the effects of baseband wander caused by the excessive one or zero symbols in the data. Control bits are also included to convey sideband information like I2C, GPIO, SPI, or CRC. V3Link devices utilize a continuous backchannel communication scheme, which allows full duplex communication on the same conductor or conductor pair. The backchannel contains I2C, GPIO, status, or other information to be carried across the link to the remote partner.
One main difference between the forward channel and backchannel communication is that the backchannel is Manchester encoded. This transition-based encoding scheme allows lower data rate symbols to be passed through the AC coupled link without the need for separate clocking channels. From an analog signaling perspective, it is important to note that for Manchester-encoded signals, the symbol rate is equal to the data rate. For example, if the backchannel rate is set for 50 megabits per second, the backchannel frequency will equal 50 megahertz. The frequency spectrums are important to understand for the purpose of EMC optimization as well as power filtering for power over coax schemes.
OK, let's take a quick quiz to test your knowledge. For a V3Link forward channel frequency of 4.1 gigabits per second per lane, what is the fundamental carrier frequency for the signal per lane? Is it a, 4.16 gigahertz; b, 104 megahertz; c, 2.08 gigahertz; or d, 6.72 gigahertz? The answer is c, 2.08 gigahertz. Remember, the V3Link data channel uses an NRZ signaling, which results in 2 bits per clock period.
Number 2-- for a V3Link back-channel rate of 50 megabits per second, what is the fundamental carrier frequency for the signal? Is it a, 10 megahertz; b, 50 megahertz; c, 5 megahertz; or d, 25 megahertz? The answer is b, 50 megahertz. Remember that the V3Link backchannel uses Manchester encoding so the bit symbol rate and megahertz is equivalent to the bit rate. Last question-- true or false? V3Link is a bidirectional half-duplex protocol. False. V3Link is indeed bidirectional but can transfer data in both directions simultaneously, meaning it is actually a full duplex protocol.
To wrap things up, you can access additional technical resources and find V3Link products that fit your application by following the link provided here to ti.com. Thank you very much for taking the time to watch this video and see you again next time.