How Ethernet AVB is playing a central role in automotive streaming applications

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Ethernet is emerging as the network of choice for infotainment and advanced driver assistance systems, Atmel’s Tim Grai explains.

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Imagine you’re driving down the highway with the music blaring, enjoying the open road. Now imagine that the sound from your rear speaker system is delayed by a split second from the front; your enjoyment of the fancy in-car infotainment system comes to a screeching halt.

Ethernet is emerging as the network of choice for infotainment and advanced driver assistance systems that include cameras, telematics, rear-seat entertainment systems and mobile phones. But standard Ethernet protocols can’t assure timely and continuous audio/video (A/V) content delivery for bandwidth intensive and latency sensitive applications without buffering, jitter, lags or other performance hits.

Audio-Video Bridging (AVB) over Ethernet is a collection of extensions to the IEEE802.1 specifications that enables local Ethernet networks to stream time synchronised, loss sensitive A/V data. Within an Ethernet network, the AVB extensions help differentiate AVB traffic from the non-AVB traffic that can also flow through the network. This is done using an industry standard approach that allows for plug-and-play communication between systems from multiple vendors.

The extensions that define the AVB standard achieve this by:

• reserving bandwidth for AVB data transfers to avoid packet loss due to network congestion from ‘talker’ to ‘listener(s)’
• establishing queuing and forwarding rules for AVB packets that keep packets from bunching and guarantee delivery of packets with a bounded latency from talker to listener(s) via intermediate switches, if needed
• synchronizing time to a global clock so the time bases of all network nodes are aligned precisely to a common network master clock, and
• creating time aware packets which include a ‘presentation time’ that specifies when A/V data inside a packet has to be played.

Designers of automotive A/V systems need to understand the AVB extensions and requirements, as well as how their chosen microcontroller will support that functionality.

AVB: A basket of standards

AVB requires that three extensions be met in order to comply with IEEE802.1:

• IEEE802.1AS – timing and synchronisation for time-sensitive applications (gPTP)
• IEEE802.1Qat – stream reservation protocol (SRP)
• IEEE802.1Qav – forwarding and queuing for time-sensitive streams (FQTSS).

In order to play music or video from one source, such as a car’s head unit, to multiple destinations, like backseat monitors, amplifiers and speakers, the system needs a common understanding of time in order to avoid lags or mismatch in sound or video. IEEE802.1AS-2011 specifies how to establish and maintain a single time reference – a synchronised ‘wall clock’ – for all nodes in a local network. The generalized precision time protocol (gPTP), based on IEEE1588, is used to synchronize and syntonize all network nodes to sub-microsecond accuracy. Nodes are synchronized if their clocks show the same time and are syntonised if their clocks increase at the same rate.

This protocol selects a Grand Master Clock from which the current time is propagated to all network end-stations. In addition, the protocol specifies how to correct for clock offset and clock drifts by measuring path delays and frequency offsets. New MCUs, such as the Atmel | SMART SAMV7x (shown above), detect and capture time stamps automatically when gPTP event messages cross MII layers. They can also transport gPTP messages over raw Ethernet, IPv4 or IPv6. This hardware recognition feature helps to calculate clock offset and link delay with greater accuracy and minimal software load.

Meanwhile, SRP guarantees end-to-end bandwidth reservation for all streams to ensure packets aren’t delayed or dropped at any switch due to network congestion, which can occur with standard Ethernet. For the in-vehicle environment, SRP is typically configured in advance by the car maker, who defines data streams and bandwidth allocations.

Talkers (the source of A/V data) ‘advertise’ data streams and their characteristics. Switches process these announcements from talker and listeners to:

• register and prune streams’ path through the network
• reserve bandwidth and prevent over subscription of available bandwidth
• establish forwarding rules for incoming packets
• establish the SRP domain, and
• merge multiple listener declarations for the same stream

The standard stipulates that AVB data can reserve only 75% of total available bandwidth, so for a 100Mbit/s link, the maximum AVB data is 75Mbit/s. The remaining bandwidth can be used for all other Ethernet protocols.

In automotive systems, the streams may be preconfigured and bandwidth can be reserved statically at system startup to reduce the time needed to bring the network into a fully operational state. This supports safety functions, such as driver alerts and the reversing camera, that must be displayed within seconds.

SRP uses other signalling protocols, such as Multiple MAC Registration Protocol, Multiple VLAN Registration Protocol and Multiple Stream Registration Protocol to establish bandwidth reservations for A/V streams dynamically.

The third extension is FQTSS, which guarantees that time sensitive A/V streams arrive at their listeners within a bounded latency. It also defines procedures for priority regenerations and credit based traffic shaper algorithms to meet stream reservations for all available devices.

The AVB standard can support up to eight traffic classes, which are used to determine quality of service. Typically, nodes support at least two traffic classes – Class A, the highest priority, and Class B. Microcontroller features help manage receive and transmit data with multiple priority queues to support AVB and ‘best effort class’ non AVB data.

Automotive tailored requirements

Automotive use cases typically fix many parameters at the system definition phase, which means that AVB implementation can be optimised and simplified to some extent.

• Best Master Clock algorithm (BMCA): the best clock master is fixed at the network definition phase so dynamic selection using BCMA isn’t needed.
• SRP: all streams, their contents and their characteristics are known at system definition and no new streams are dynamically created or destroyed; the proper reservation of data is known at the system definition phase; switches, talkers and listeners can have their configurations loaded at system startup from pre-configured tables, rather than from dynamic negotiations
• Latency; while this is not critical, delivery is. Automotive networks are very small with only a few nodes between a talker and listener. It is more important not to drop packets due to congestion.

Conclusion

The requirement to transfer high volumes of time sensitive audio and video content inside vehicles necessitates developers to understand and apply the Ethernet AVB extensions. AVB standardization results in interoperable end-devices from multiple vendors that can deliver audio and video streams to distributed equipment on the network with micro-second accuracy or better. While the standard brings complexities, new MCUs with advanced features are simplifying automotive A/V design.

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This article was originally published on New Electronics on October 13, 2015 and authored by Tim Grai, Atmel’s Director of Automotive MCU Application Engineering. 

4 designs tips for AVB in-car infotainment

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AVB is clearly the choice of several automotive OEMs, says Gordon Bechtel, CTO, Media Systems, Harman Connected Services.

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Audio Video Bridging (AVB) is a well-established standard for in-car infotainment, and there is a significant amount of activity for specifying and developing AVB solutions in automobiles. The primary use case for AVB is interconnecting all devices in a vehicle’s infotainment system. That includes the head unit, rear-seat entertainment systems, telematics unit, amplifier, central audio processor, as well as rear-, side- and front-view cameras.

The fact that these units are all interconnected with a common, standards-based technology that is certified by an independent market group — AVnu — is a brand new step for the automotive OEMs. The AVnu Alliance facilitates a certified networking ecosystem for AVB products built into the Ethernet networking standard.

According to Gordon Bechtel, CTO, Media Systems, Harman Connected Services, AVB is clearly the choice of several automotive OEMs. His group at Harman develops core AVB stacks that can be ported into car infotainment products. Bechtel says that AVB is a big area of focus for Harman.

AVB Design Considerations

Harman Connected Services uses Atmel’s SAM V71 microcontrollers as communications co-processors to work on the same circuit board with larger Linux-based application processors. The software firm writes codes for customized reference platforms that automotive OEMs need to go beyond the common reference platforms.

Based on his experience of automotive infotainment systems, Bechtel has outlined the following AVB design dos and don’ts for the automotive products:

1. Sub-microsecond accuracy: Every AVB element on the network is hooked to the same accurate clock. The Ethernet hardware should feature a time stand to ensure packet arrival in the right order. Here, Bechtel mentioned the Atmel | SMART SAM V71 MCU that boasts screen registers to ensure advanced hardware filtering of inbound packets for routing to correct receive-end queues.

2. Low latency: There is a lot of data involved in AVB, both in terms of bit rate and packet rate. AVB allows low latency through reservations for traffic, which in turn, facilitate faster packet transfer for higher priority data. Design engineers should carefully shape the data to avoid packet bottlenecks as well as data overflow.

Bechtel once more pointed to Atmel’s SAM V71 microcontrollers that provide two priority queues with credit-based shaper (CBS) support that allows the hardware-based traffic shaping compliant with 802.1Qav (FQTSS) specifications for AVB.

3. 1588 Timestamp unit: It’s a protocol for correct and accurate 802.1 AS (gPTP) support as required by AVB for precision clock synchronization. The IEEE 802.1 AS carries out time synchronization and is synonymous with generalized Precision Time Protocol or gPTP.

Timestamp compare unit and a large number of precision timer counters are key for the synchronization needed in AVB for listener presentations times and talker transmissions rates as well as for media clock recovery.

4) Tightly coupled memory (TCM): It’s a configurable high-performance memory access system to allow zero-wait CPU access to data and instruction memory blocks. A careful use of TCM enables much more efficient data transfer, which is especially important for AVB class A streams.

It’s worth noting that MCUs based on ARM Cortex-M7 architecture have added the TCM capability for fast and deterministic code execution. TCM is a key enabler in running audio and video streams in a controlled and timely manner.

AVB and Cortex-M7 MCUs

The Cortex-M7 is a high-performance core with almost double the power efficiency of the older Cortex-M4. It features a six-stage superscalar pipeline with branch prediction — while the M4 has a three-stage pipeline.  Bechtel of Harman acknowledged that M7 features equate to more highly optimized code execution, which is important for Class A audio implementations with lower power consumption.

Again, Bechtel referred to the SAM V71 MCUs — which are based on the Cortex-M7 architecture — as particularly well suited for the smaller ECUs. “Rear-view cameras and power amplifiers are good examples where the V71 microcontroller would be a good fit,” he said. “Moreover, the V71 MCUs can meet the quick startup requirements needed by automotive OEMs.”

The infotainment connectivity is based on Ethernet, and most of the time, the main processor does not integrate Ethernet AVB. So the M7 microcontrollers, like the V71, bring this feature to the main processor. For the head unit, it drives the face plate, and for the telematics control, it contains the modem to make calls so echo cancellation is a must, for which DSP capability is required.

Take the audio amplifier, for instance, which receives a specific audio format that has to be converted, filtered and modulated to match the requirement for each specific speaker in the car. This means infotainment system designers will need both Ethernet and DSP capability at the same time, which Cortex-M7 based chips like V71 provide at low power and low cost.

Atmel tightens automotive focus with new Cortex-M7 MCUs

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Large SoCs without an Ethernet interface typically have slow start-up times and high-power requirements — until now. 

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Atmel, a lead partner for the ARM Cortex-M7 processor launch in October 2014, has unveiled three new M7-based microcontrollers with a unique memory architecture and advanced connectivity features for the connected car market.

According to a company spokesman, E70, V71 and V70 chips are the industry’s highest performing Cortex-M microcontrollers with six-stage dual-issue pipeline delivering 1500 CoreMarks at 300MHz. Moreover, V70 and V71 microcontrollers are the only automotive-qualified ARM Cortex-M7 MCUs with Audio Video Bridging (AVB) over Ethernet and Media LB peripheral support.

Atmel is among the first suppliers to introduce the ARM Cortex-M7-based MCUs, whose core combines performance and simplicity and further pushes the performance envelope for embedded devices. The new MCU devices are aimed to take the connected car design to the next performance level with high-speed connectivity, high-density on-chip memory, and a solid ecosystem of design engineering tools.

Atmel’s Memory Play

Atmel has memory technology in its DNA, and that seems apparent in the design footprint of E70, V70 and V71 MCUs. The San Jose-based chipmaker is offering a flexible memory system that is optimized for performance, determinism and low latency.

Jacko Wilbrink, Senior Marketing Director at Atmel, said that the company’s Cortex-M7-based MCUs leverage Atmel’s advanced peripherals and flexible SRAM architecture for higher performance applications while keeping the Cortex-M class ease-of-use. He added that the large on-chip SRAM on SAM E70/V70/V71 chips is critical for connected car and IoT product designers since it allows them to run the multiple communication stacks and applications on the same MCU without adding external memory.

On-chip DMA and low-latency access SRAM architecture

Avoiding the external memories reduces the PCB footprint, lowers the BOM cost and eliminates the complexity of high-speed PCB design when pushing the performance to a maximum. Next, Tim Grai, another senior manager at Atmel, pointed out another critical take from Cortex-M7 designs: The tightly coupled memory (TCM) interface. It provides the low-latency memory that the processor can use without the unpredictability that is a feature of cache memories.

Grai says that the most vital memory feature is not the memory itself but how the TCM interface to the M7 is utilized. “The available RAM is configurable to be used as system RAM or tightly-coupled instruction and data memory to the core, where it provides deterministic zero-wait state access,” Grai added. “The arrangement of SRAM allows for multiple concurrent accesses.”

Cortex-M7 a DSP Winner

According to Will Strauss, President & Principal Analyst at Forward Concepts, ARM has had considerable success with its Cortex-M4 power-efficient 32-bit processor chip family. “However, realizing that it lacked the math ability to do more sophisticated DSP functions, ARM has introduced the Cortex-M7, its newest and most powerful member of the Cortex-M family.”

Strauss adds that the M7 provides 32-bit floating point DSP capability as well as faster execution times. With the greater clock speed, floating point and twice the DSP power of the M4, the M7 is even more attractive for applications requiring high-performance audio and even video accompanying traditional automotive and control applications.

Atmel’s Grai added an interesting dimension to the DSP story in Cortex-M7 processor fabric. He pointed out that true DSPs don’t do control and logical functions well and generally lack the breadth of peripherals available on MCUs. “The attraction of the M7 is that it does both—DSP functions and control functions—hence it can be classified as a digital signal controller (DSC).”

Grai quoted the example of Atmel V70 and V71 microcontrollers used to connect end-nodes like infotainment audio amplifiers to the emerging Ethernet AVB network. In an audio amplifier, you receive a specific audio format that has to be converted, filtered, modulated to match the requirement for each specific speaker in the car. So you need Ethernet and DSP capabilities at the same time.

Grai says that the audio amplifier in infotainment applications is a good example of DSC: a mix of MCU capabilities and peripherals plus DSP capability for audio processing. Atmel is targeting the V70 and V71 chips as a bridge between large application processors and Ethernet.

Most of the time, the main processor does not integrate Ethernet AVB, as the infotainment connectivity is based on Ethernet standard. Here, the V71 microcontroller brings this feature to the main processor. “Large SoCs, which usually don’t have Ethernet interface, have slow start-up time and high power requirements,” Grai said. “Atmel’s V7x MCUs allow fast network start-up and facilitate power moding.”

The SAM E70, V70 and V71

Atmel’s three new MCU devices are aimed at multiple aspects of in-vehicle infotainment connectivity and telematics control.

SAM E70: The microcontroller series features Dual CAN-FD, 10/100 Ethernet MAC with IEEE1588 real-time stamping, and AVB support. It’s aimed at automotive industry’s movement toward controller area network (CAN) message-based protocols holistically across the cabin, eliminating isolation and wire redundancy, and have them all bridged centrally with the CAN interface.

SAM V70: It’s designed for MediaLB connectivity and leverages advanced audio processing, multi-port memory architecture and Cortex-M7 DSP capabilities. For the media-oriented systems transport (MOST) architecture, old modules are not redesigned. So Atmel offers a MOST solution that is done over Media Local Bus (MediaLB) and is supported by the V70 series.

SAM V71: The MCU series ports a complete automotive Ethernet AVB stack for in-vehicle infotainment connectivity, audio amplifiers, telematics and head control units. It mirrors the SAM V70 series features as well as combines Ethernet-AVB and MediaLB connectivity stacks.

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Majeed Ahmad is the author of books Smartphone: Mobile Revolution at the Crossroads of Communications, Computing and Consumer Electronics and The Next Web of 50 Billion Devices: Mobile Internet’s Past, Present and Future.