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