Counting TI's star products in T-BOX: Automotive Ethernet | Section 1 DP83TC811S-Q1: Automotive Ethernet makes your T-BOX...

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Counting TI's star products in T-BOX: Automotive Ethernet | Section 1 DP83TC811S-Q1: Automotive Ethernet makes your T-BOX... [Copy link]

 

Section 1 DP83TC811S-Q1: In-vehicle Ethernet makes your T-BOX even more powerful

In recent years, the concept of "Internet of Vehicles" has been very popular. Speaking of Internet of Vehicles, of course we have to mention the in-vehicle Ethernet and one of the core components of the Internet of Vehicles - T-BOX!

This article will start with automotive Ethernet 100BASE-T1, deeply analyze the characteristics of automotive Ethernet, and introduce in detail the advantages of TI's star product DP83TC811S-Q1 and its advantages in T-BOX applications.

What is Automotive Ethernet? --- Let’s start with 100BASE-T1

Ethernet has long been widely used in commercial and industrial applications, but it was not until the emergence of 100BASE-T1 that it was widely used in automotive applications. Although some vehicles also use traditional 100BASE-TX for vehicle diagnostics (OBD), it has not been able to develop in the automotive ecosystem because it requires two pairs of twisted pairs and is difficult to meet strict automotive radiation emission standards. IEEE 802.3bw (also known as 100BASE-T1) was developed to meet the needs of automotive systems. Through technical improvements such as adding iterations, encoding, and scrambling mechanisms, its electromagnetic radiation performance has been improved, wiring costs have been reduced, and space occupied has been reduced. Only a single pair of unshielded twisted pairs is required to achieve 100Mbps data transmission. The communication distance can reach at least 15m, and it complies with CISPR 25 Class 5 radiation emission standards, and also meets some other automotive emission standards, such as Open Alliance. In the future, 100BASE-T1 will standardize the in-vehicle ecosystem into a network architecture, thereby simplifying the overall communication system of the ECU and may even eliminate older or less popular protocols such as FlexRay.

Why have the best of both worlds? ---Analysis of 100BASE-T1

Why can 100BASE-T1 meet the many requirements of low electromagnetic interference, low cost, low cable weight, and high bandwidth in automobiles? This is inseparable from the following features of 100BASE-T1:

A special encoding method is used

100BASE-T1 uses a unique 4-bit to 3-bit (4B3B), 3-bit to 2-ternary pair (3B2T) and three-level pulse amplitude modulation (PAM3) encoding scheme to achieve lower electromagnetic interference. Before entering the single pair of unshielded twisted pair, the 100BASE-T1 PHY performs all necessary scrambling and encoding. 100BASE-T1 is transparent to the MAC, and the existing MII is not changed. There are currently four main xMlls for 100BASE-T1: RGMII, MII, RMII, RGMII and SGMII.

As shown in Figure 1 below, after the PHY receives data from the MAC, it encodes, scrambles, and serializes the data. These processes prepare the data for the PHY's analog front end, and the PHY then transmits the data to the corresponding link partner over a single pair of unshielded twisted pair cables.

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Figure-1 xMIIs

The following Figure 2 shows an example of how data is converted from MII to MDI.

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Figure 2 GPHY data conversion from MII to MDI[10]

• 4B Data: The PHY communicating with MAC via RGMII receives 4-bit at a frequency of 25M. (25M*4=100Mbps)
• 3B Data: PHY converts the 4-bit to 3-bit. If it is not divisible by 3, it adds supplementary bits, and the frequency increases to 33.333M. (33.33M*3=100Mbps)
• 2T: Convert 3-bit to 2-ternary pair according to the corresponding relationship in Table 1. For example, 010 is converted to -1 1.

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Table - 1 100BASE-T1 idle symbol mapping

• PAM3: Transmitted on MDI through the three-level pulse amplitude modulation (PAM3) encoding scheme. There are three level states: -1/0/1, and the base frequency becomes 66.66 MHz.

We can compare it. The traditional 100BASE-TX uses multi-level transmission (MLT-3) to achieve 125MHz data transmission. The base frequency it uses is much higher than 100BASE-TX1 (66.66MHz), so a dedicated shielded twisted pair is required for sending and receiving. The bandwidth of 100BASE-T1 is 33.3MHz, which is close to half of the bandwidth of 100BASE-TX. This means that lower-cost cables can be used, but they can provide better radiation emissions and immunity. Both of these are key to automotive applications.

In addition, this data conversion method also improves the spectral efficiency of 100BASE-T1, which can reduce the bandwidth required to transmit the same amount of data.

Data can be sent and received on a single pair of unshielded twisted pair cables

100BASE-T1 is a physical full-duplex interface that supports transmission and reception on the same twisted pair, while 100BASE-T and 100BASE-TX transmit and receive on different dedicated pairs. The shared medium allows 100BASE-T1 to reduce the overall cable weight of the vehicle, which not only reduces material costs but also improves fuel efficiency. Physical full-duplex is achieved through the principle of superposition. The 100BASE-T1 physical system has integrated the hybrid system and uses echo cancellation to remove its own transmitted signal and extract the received information from the link partner. To do this, one PHY is dedicated as the Master and the other PHY as the Slave. When two 100BASE-T1 physicals are connected, they go through a training process so that the device under test (DUT) and the link partner transmit information at the same frequency and the same phase. Figure 3 is a simplified block diagram of the hybrid and echo cancellation in each PHY.

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Figure 3 Simplified block diagram of mixing and echo cancellation in PHY

Adjustable via external components

There are several external components that condition the transmission and reception of PAM3 signals before they leave or enter the board. The goal is to isolate the MDI to prevent ground loops and DC offsets, improve common-mode noise filtering, and reduce radiated emissions while maintaining high immunity.

Minimizes common mode noise

Use a common mode choke (CMC) on the MDI to filter common mode noise. It is extremely important to minimize common mode noise as it can interfere with PHY reception. In addition, since common mode noise is a single-ended energy source, it can result in higher radiated emissions. Table 2 below lists the 100BASE-T1 PHY requirements that the common mode choke (CMC) used in the design must meet.

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Table 2 VCMC parameters for the 100BASE-T1 standard

Using simpler DC isolation

Figure 4 below shows how 100BASE-TX and 100BASE-T1 achieve DC isolation. 100BASE-TX PHY typically uses a transformer with a center tap (on one side of the PHY) connected to the DC voltage at the PHY. The transformer also uses Bob Smith termination (center tap, on the connector side, connected to ground through a resistor) to improve common-mode noise interference. 100BASE-T1 uses a simpler approach, using only two capacitors. Compared to the transformer solution, these two capacitors provide DC isolation and reduce the overall size. Figure 5 shows a typical 100BASE-T1 circuit implementation.

查看详情Figure 4 DC isolation between 100BASE-TX and 100BASE-T1

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Figure 5 100BASE-T1 typical circuit

Thanks to this series of improvements and optimizations, 100BASE-T1 can meet many automotive requirements such as low electromagnetic interference, low cost, low cable weight, and high bandwidth, making it very suitable for use in automotive systems. One of the most typical applications is T-BOX.

DP83TC811S-Q1 for T-BOX

Behind every Internet of Vehicles system is a T-BOX that works silently. Although it does not frequently show its presence like the large in-vehicle entertainment screen, it silently supports the Internet of Vehicles. T-BOX, also known as the Telematics Control Unit (TCU), controls the tracking, positioning and communication of the vehicle. As shown in Figure 6 below, various ECUs on the car can further communicate with T-BOX (TCU) through the in-vehicle gateway (the bridge between various communication domains), so that various ECUs on the car can access the cloud, thereby realizing the wireless upgrade (OTA) of software/firmware or the interactive transmission of data, which provides great convenience for people.

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Figure 6 Gateway system block diagram

Why is 100BASE-T1 a great help for T-BOX applications? First of all, as one of the core ECUs of the car, the selection of T-BOX devices must also meet the requirements of low electromagnetic interference, low cost, and low cable weight that have been repeatedly emphasized above. Not only that, with the rapid development of the communications industry, T-BOX has gradually moved from 2G and 3G to 4G and even 5G. Some 4G (LTE) modem modules in T-BOX can operate at 300-400Mbps. Under this high-speed transmission rate requirement, the 100BASE-T1 and even 1000BASE-T1 PHYs mentioned above that meet the requirements of the automotive system level are undoubtedly a perfect match for the needs of T-BOX! Not only that, using 100BASE-T1 or even 1000BASE-T1 can greatly improve the update and calibration speed of firmware or software, thereby reducing the downtime caused by the update of the automotive system. This is incomparable to 2Mbps CAN FD or 10Mbps FlexRay.

TI's star product DP83TC811S-Q1 is an automotive PHYTER Ethernet physical layer transceiver that complies with IEEE 802.3bw. It provides all the physical layer functions required to send and receive data over an unshielded single twisted pair cable and provides xMII flexibility. Its features are listed below:

• Compliant with IEEE802.3bw (100BASE-T1) standard, it can achieve a transmission rate of 100Mbps on a 60-meter unshielded twisted pair (UTSP) and is compatible with other 100BASE-T1 PHYs.
• Supports multiple MAC interfaces SGMII/RGMII/RMII/MII. Note that only DP83TC811 S -Q1 supports SGMII.
• Only a few external components are required. DP83TC811S-Q1 integrates PMD filter, power filter, MII terminal, PMD terminal. Only a few external components are required.
• Pin compatible with the subsequent 1000BASE-T1
• Compliant with OPEN ALLIANCE TC8 automotive Ethernet ECU test specification
• Support IEEE 1588 SFD
• Optimized EMI and EMC performance
• Low latency (<150ns--- transmit MII)
• Ultra-low active power (210mW ---RMII)

The device also includes a diagnostic tool suite that provides an extensive list of real-time monitoring tools, debugging tools, and test modes. Within the tool suite is the first integrated electrostatic discharge (ESD) monitoring tool. It is able to calculate ESD events on xMII and MDI and provide real-time monitoring through the use of programmable interrupts. In addition, the DP83TC811S-Q1 includes a pseudo-random binary sequence (PRBS) frame generation tool that is fully compatible with the internal loopback and can send and receive data without the use of a MAC.

In summary, TI's 100BASE-T1 automotive Ethernet PHY meets many requirements of low electromagnetic interference, low cost, low cable weight, etc., and perfectly meets the high bandwidth requirements of automotive T-BOX (TCU) applications, making your T-BOX even more powerful! The corresponding TI device DP83TC811S-Q1 will continue to help car networking applications including T-BOX.