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Posted on 03 July 2019

Ethernet In Fast Forward

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Automotive Using Ethernet as Physical Layer Data Bus

Ethernet has been officially be added to the list of automotive networks, such as CAN, LIN, FlexRay and MOST. But why did Ethernet make the list and for what specific automotive applications? Most important of all, can it really meet the challenges of Automotive?

By Mike Jones, Senior FAE, Micrel Inc.

 

Ethernet currently dominates the world of office networking and is also the choice for both factory and home. Simplicity and field-proven open standardisation have significantly lowered the Cost of Ownership which Automotive manufacturers are keen to exploit. Volumes of scale in the office and consumer market, supported by a magnitude of Ethernet vendors, have driven pricing levels far lower than any ‘custom’ designed protocol.

Initial applications for automotive Ethernet now routinely include On- Board-Diagnostics (OBD); for Diagnostics and Software Download of internal ECU static memory and hard disk(s). The adoption of Ethernet will accelerate with the introduction of standardised IP Diagnostics interface, as specified in ISO 13400.

The choice of a standard Ethernet CAT5 cable to interface to the OBD port will allow service centres to seamlessly interface to either a lap top or the Intranet when performing vehicle management diagnostics. Memory downloads for software updates also benefit from the increased speed of 100Mbps (or 1000Mbps Gigabit Ethernet), full duplex bandwidth available by the network. As the demand for increased processor intensive functionality in a car grows, so does the required memory. If present methods continue to be employed, software download times will significantly increase. Off road time results in direct additional servicing costs, due to the need to supply the owner with a temporary replacement vehicle and the additional time associated with workers on the clock.

Implementing Ethernet at the OBD port now allows the car to interface to the World Wide Web, creating endless possibilities. For example, the port can easily be plugged into a wireless unit for remote diagnostics or downloads for in-car navigation, video or music, all from the comfort of the owner’s home! Moving forward, new ‘real time’ Ethernet AVB (Audio-Video Bridging) can also offer high performance infotainment network solutions, again with nearly endless possibilities.

The challenges for Ethernet to also become the de-facto automotive bus are not necessarily unique. By combining Ethernet’s proven ability to reliably transfer high bandwidths of data (home/office) with realtime performance in extreme environments (industrial control), the basis of many automotive needs is formed.

Thermal and EMC Performance

The Industrial Control market has helped to prove that Ethernet networks are able to deliver robust performance in extreme conditions. Extended temperature ranges, heavy vibrations, high EMC radiation and dusty or wet surroundings are typical in many of these applications. Raising the ambient temperatures ‘under the hood’ over the common +85°C will not cause thermal issues for Ethernet devices, due to low power consumption and package selection. For example, Micrel’s KSZ8041NL AM, AECQ-100 Single-Port Fast Ethernet PHY solution, consumes a mere 175mW inside a thermally enhanced 5mm x5mm MLF® package. The KSZ8041NL family also offers a Military specification variant which supports ambient temperatures of up to 125°C.

Due to the demands of the industrial and automotive markets, many newer Ethernet devices offer significantly improved ESD (Electro-Static Discharge) performance. This is a major shift in emphasis from original office applications where ESD rating was not considered of major concern. For example, Micrel’s KSZ8041 PHY and KSZ8851 Controller families have a HBM (Human Body Model) ESD rating of > 6KV. The product’s evaluation board has also been shown to provide > 9kV contact and >16.5kV air ESD ratings, without the need for any external over voltage protection devices. This surpasses general automotive vendor Electromagnetic Compatibility (EMC) requirements such as those set forth by BMW Group Standard GS 95002.

Cables & Connectors

Currently no standard Automotive Ethernet connector or cable exists. The standard Ethernet RJ45 connector and CAT5 cable has proved very robust and remains extremely popular in other applications, including industrial. However, existing vendor specific connectors and wiring looms are likely to be adopted in automotive applications, at least initially. The Ethernet PHY (transceiver) is flexible enough to utilise such connectors and cabling without any significant degradation to performance. Standard CAN cable exhibits similar characteristics to unshielded, twisted pair CAT5. Thorough testing has proven long term error-free transmission of Ethernet over in excess of 100m CAN cable. The major difference between the two is that CAN cable is only partially specified and does not provide a controlled impedance or twist ratio. As a consequence, EMC behaviour and signal integrity cannot be guaranteed, thus making CAN cable generally unsuitable for high speed data transfer. CAN cable is currently used for Ethernet On Board Diagnostics (OBD) and flash updating. Here, the lines can be disabled during normal driving and only active in the repair shop or production plant.

A cable example for high speed data transfer such as LVDS, USB and Ethernet in automotive applications is the Leoni cable, for example the twisted pair Dacar 503. This cable is shielded with controlled 100ohm impedance and qualified up to 1Gbps, giving a performance similar to CAT6 rather than CAT5. It is not actually twisted pair but a four-wire twist known as ‘Stern-Vierer’ (translated as Star Four Wire).

To enhance reliability Cable diagnostics technology, such as Micrel LinkMD®, goes beyond Ethernet-defined standards to provide a solution to such problems. LinkMD® cable diagnostics utilize time domain Reflectrometry (TDR) to analyze each twisted pair for common cable problems, such as open circuits, short circuits and impedance mismatches.

There is an alternative to copper cabling that comes in the form of Plastic Optical Fibre (POF). Car manufacturers are already very familiar with this physical media as it is deployed in MOST networks. The same 1mm LED POF technology from MOST (including new MOST-150) can also be used for 100Mbps Fast Ethernet transmission with reach of 100metres. POF is extremely robust, lightweight and like all fibre, totally immune to electro-magnetic noise as it emits no radiation.

Power Consumption

Power consumption of automobile electronics is becoming increasingly more critical and significant factor in fuel efficiencies. It is important to understand both how and where the power is dissipated in Ethernet circuits to ensure optimum design. In any Ethernet device, the major power dissipation is from the PHY transceiver. Typically, most PHY designs are current-mode drivers and power is consumed both internally to the PHY and externally in the transformer, as shown in Figure 1.

Ethernet PHY Circuit Depicting Power Dissipation in Current-Mode

Ethernet datasheets commonly publish the device only current consumption, Iphy. In which case, to calculate the total circuit current consumption the designer must add typically around 40mA per 100Base-TX or 70mA per 10Base-T PHY for dissipation in the transformers, Itrans. This power consumed externally in the transformer is significant and typically accounts for around 30 to 50 percent of the total PHY circuit current consumption. Micrel’s new generation KSZ8051 Fast Ethernet PHY families differs by utilizing a voltage-mode driver, along with patented DSPenhanced mixed signal architecture, to offer the lowest power consumption in the industry. Device only power consumption is similar to other leading Ethernet PHYs at sub 50mA. However, no current is consumed externally in the transformer, due to the voltage-mode driver design. Hence, a saving of up to 50 percent total circuit power consumption is achieved over competing solutions.

PCB layout design is also simplified and real estate minimised by the unique integrated line termination offered by the KSZ8051 Ethernet PHY as demonstrated in Figure 2.

Integrated Line Termination of the KSZ8031-51 PHY Family

AECQ-100 grade qualified KSZ8051 parts are expected to be available from Micrel in the first half of 2011. To investigate further how to reduce power consumption, one needs to understand how an Ethernet link operates.

When analyzing a network one realizes that there are long quiet periods followed by relatively short bursts of traffic. During these quiet periods, one may expect the Ethernet power consumption to significantly drop, however, this turns out to not necessarily be accurate. Both 1000Base-TX and 100Base-TX are designed so that the link partners are continually ‘synchronized’ to each other. To enable this, when no traffic is being transmitted, the PHY will automatically send out IDLE symbols (11111 5B code), as shown in Figure 3 below.

100Base-TX Idle Pattern

As a consequence, during any quiet period, the PHY transmitter is still operating in a manner similar to full traffic and will therefore consume a similar amount of power. The IEEE recognises this inefficiency and formed a task force whose mission it is to reduce power consumption during periods of low utilization. This task force IEEE 802.3az is commonly known as Energy Efficient Energy. The technique, known as Low Power Idle (LPI), will disable parts of the PHY transceiver that are not necessary, whilst still maintaining the link integrity.

If the Ethernet PHY is, ‘not in use’ then a software or hardware power down mode is usually available. However, even in this low power state the device will still consume ‘in the order of a mA’, which for automotive applications is unacceptable. Hence, it is advised to fully power down the circuit during such periods. For example the diagnostics and software download circuit only need be powered when being serviced by the garage.

Another area where current consumption can be reduced for automotive applications is found in the transmit current drive strength. The IEEE802.3 specification is designed to always provide the capability of operating up to a minimum 100m of CAT5 grade cable. As a consequence, the PHY output drive strength is fixed at this criterion, consuming maximum power, independent of the actual length of cable connected. Automotive networks will never require the capability of 100m cable reach and can guarantee a much shorter length. Consider that one can reduce the PHY transmitter current drive from the standard +/-1V amplitude of the 100Base-TX signal down by up to 50 percent and still operate error free over a 20m reach. The transmitter current drive on Micrel Ethernet devices can be varied either via internal software registers or by modifying the recommended resistance to ground at pin ‘REXT’ (see datasheet for specific value). The output drive strength will vary inversely proportional to the resistance. This method yields significant reduction of both current consumption and EMI (Electro-Magnetic Interference) Radiated Emissions.

Topology – Ring or Star?

Traditional Ethernet networks usually implement a ‘star’ configuration, where a multi-port switch provides point-to-point links to other nodes. However, industrial networks are usually based on a ‘ring’ configuration, which eases the logistics of cable installation. Reductions in the required cabling of an Ethernet ‘ring’ network provides benefits welcome to the automotive industry. Less cabling means weight reductions which has a direct impact on fuel efficiency and hence, overall costs. The basic building block in a ‘ring’ network is the 3-Port switch, for example the automotive AECQ-100 qualified KSZ8873MLL AM from Micrel.

The introduction of Ethernet into the car will most likely favour a ‘ring’ and ‘star’ hybrid topology approach. Figure 4 illustrates a possible automotive Ethernet network.

Depiction of a Basic Automotive Ethernet Ring Network

Each physical media interface can independently be implemented using copper cabling or POF. Here, a central Gateway will interface to the On-Board Diagnostics (OBD) port and other available networks within the car, such as MOST or FlexRay. The Gateway unit can then act as bridge to the Head Unit, further connecting other systems such as Navigation, Vehicle Computer and Rear Seat Entertainment over a ‘ring’ or ‘star’ topology.

Managing an Ethernet Ring

Unlike a MOST networks, it is in fact usually forbidden to configure Ethernet as a true ‘ring’. Any loops within an Ethernet network will result in the duplication of packets that are forwarded in endless loops, quickly degrading the bandwidth and efficiency of a network. To manage the ring, usually protocols such as Spanning Tree are implemented to ‘break’ one of the links and enable again if a link fault is detected elsewhere in the ring. However Micrel’s Automotive switches; both the 3-Port KSZ8873MLL AM and 5-Port KSZ8895MLU offer a unique feature to allow a true Ethernet ring to be implemented without the need for management; MAC Address Source Filtering.

A hardware mechanism of ‘Learning’ and ‘Forwarding’ is utilized by all common Ethernet switches today. A switch will ‘learn’ and then store the ingress packet MAC Source Address and the associated port in a ‘Forwarding’ Table. A port forwarding decision is then made by looking up the packets MAC Destination Address in the ‘Forwarding’ Table. If a match is found, then the packet is forwarded to the associated port in the table entry. Failure to find a match results in the packet being broadcast to all egress ports except the port it arrived at. With this mechanism, the MAC Source Address is only ever learnt and never used in the decision making when forwarding the packet. MAC Address Source filtering enables the filtering of packets based on the MAC Source Address (instead of the MAC Destination Address). Now the switch can detect and filter (drop) any packet that arrives with a MAC Source Address matched to the local processor MAC Address. As a consequence, packets are always removed from the ring following one complete loop. Figure 5 depicts how this can work.

Illustration of a True Ethernet Ring Network

Switch #1 receives broadcast packet at port 3 (processor) with Source Address 1. Packet is forwarded along the ring until it arrives back at Switch #1 Switch #1 drops packet, using MAC Source Address filtering feature. MAC Source Address Filtering also offers the additional benefit of single fault redundant switchover, by exploiting packet forwarding in both a clockwise and anticlockwise direction around the ring. For more details see: ‘Unmanaged Redundant Ring – A White Paper’.

The Future

Ethernet’s unquestionable success in the Industrial networking sector has proven reliability and quality in an extreme environment. This marriage of industrial strength and consumer technology drive provides the perfect physical layer solution for automotive. Ethernet can successfully bridge the gap between lengthy vehicle design cycles and today’s fast moving IP world.

There is nothing complex about Ethernet technology overall; it is simple, proven and open ? the ver reason for its success. Cost is a crucial factor in any market and Ethernet has consistently demonstrated the lowest cost of ownership of any network.

Micrel Automotive Qualified Ethernet devices

Today, Ethernet has already emerged inside the car to provide an IPbased standard interface for diagnostics and software downloading. The next step is for Ethernet to form the backbone of the next generation automotive multi-media networks, carrying ‘live’ traffic. New standards such as IEEE 802.3AVB (Audi-Video Bridging) initially defined for Digital AV Home networking are being adapted to support the same real-time services in the car. Following this, the ultimate goal would to converge other bus systems inside the car into a single common bus; Ethernet.

 

The following Automotive Qualified Ethernet devices are currently available from Micrel. For further details on Micrel Ethernet Solutions go to: http://www.micrel.com/page.do?page=product-info/ether_over.jsp

Note: MOST is a registered trademark of Standard Microsystems Corporation. LinkMD is a registered trademark of Micrel Inc.

 

 

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