Embracing the First Automotive SerDes Interfaces

Article By : James Goel

The MIPI Alliance recently released MIPI A-PHY v1.0, the first automotive SerDes physical layer interface specification. Let's explore what Automotive SerDes interfaces are for and why they are important...

The MIPI Alliance recently released MIPI A-PHY v1.0, the first automotive long-reach serializer-deserializer (SerDes) physical layer interface specification. Let’s explore what automotive SerDes interfaces are for and why they are important.

New transformative automotive trends are accelerating the evolution of vehicle electronics. Advanced driver assistance systems (ADAS), in-vehicle infotainment (IVI) and autonomous driving systems (ADS) are increasing the need for more sophisticated cameras, sensors, displays and onboard processing power.

The benefits are clear: More advanced sensors and electronics improve ADAS, higher-resolution and larger in-vehicle displays increase driver awareness, and ADS help eliminate human error. However, these trends require new innovations in safety and driver applications, making vehicles more technically complex. Automotive OEMs and suppliers can support these advancements by taking advantage of existing technologies while simplifying development through standardization.

A sample use case: Better backup cameras
Interfaces for cameras, sensors, displays and other components play an important role in new automotive capabilities. Take the example of a backup camera: Video from a rear-mounted camera must reach a display on the dashboard in real time, and any interruption, delay or glitch could be a matter of life and death.

Today, the physical layer interfaces that link these components across a vehicle are not standardized. An industry-backed technical specification for this critical interface will increase innovation, reduce cost and improve functional safety, security and performance.

A standardized interface will also bring new technologies to market faster, offer OEMs greater choice, foster interoperability, and provide enhanced support services such as test and software resources from a growing ecosystem.

Introducing MIPI A-PHY
The MIPI Alliance recently released MIPI A-PHY v1.0, the first automotive long-reach serializer-deserializer (SerDes) physical layer interface specification. It provides an asymmetric data link in a point-to-point or daisy-chain topology, providing high-speed unidirectional data, embedded bidirectional control data and optional power delivery over a single cable. The Alliance also introduced MIPI Automotive SerDes Solutions (MASS), a series of full-stack connectivity solutions built on A-PHY, which will enable end-to-end security and functional safety for applications involving ADAS, IVI and ADS.

To support rapidly advancing in-car electronics, A-PHY defines a reliable, resilient, low latency, high-bandwidth interface with a path to future performance increases. Key features include:

  • High reliability: Ultra-low packet error rate of 10-19 or less than one error in the lifetime of the car
  • High resiliency: Ultra-high immunity to EMI effects
  • High performance: Data rates as high as 16 Gbps, with a roadmap to 48 Gbps and beyond; v1.1, already in development, will provide a doubling of the high-speed data rate to 32 Gbps and increase the uplink data rate to 200 Mbps
  • Low latency: Fixed latency as low as ~6µs, depending on speed gear
  • Long reach: Up to 15 meters
  • Supports power over cable
Click the above to enlarge. (Source: MIPI)

MIPI A-PHY implementations: Automakers are expected to integrate A-PHY into their systems in two phases. First, A-PHY will offer a standard long-reach interface between bridge chips. As more components support A-PHY, cameras and displays will be able to use CSI-2 and DSI-2 natively over A-PHY (without bridge chips) to reduce cost, complexity, wiring, weight and use of space.

As shown in the diagram above, the primary purpose of MIPI A-PHY is to transfer high-speed data between cameras and displays and their associated electronic control units (ECUs). It does this by allowing higher-layer protocols—including the well-established MIPI CSI-2 and MIPI DSI-2 specifications, as well as other approved protocols such as VESA’s DisplayPort and Embedded DisplayPort (DP/eDP)—to operate directly over physical links throughout a vehicle.

Eventually A-PHY integration directly into the ECU will free OEMs from having to support external PHY bridge ICs.  Key advantages include the reduction of cost, cable and development time.

Glass-to-glass image transmission
In the backup camera example, MASS provides a full-stack solution that uses the same physical interface and the same safety and security features, creating a “glass-to-glass” solution from camera to display. Here’s how it works:

Click the image above to enlarge. (Source: MIPI)

The rear camera pixel stream is transmitted directly to an ECU using CSI-2 over A-PHY. The ECU performs image signal processing with graphics overlay and sends the video stream to one or more displays. On this leg, the data can be transmitted using either MIPI DSI-2 or VESA DP/eDP over A-PHY.

The MASS protocol stack contains end-to-end functional safety and security enhancements for CSI-2 and DSI-2 to prevent failures and malicious or non-malicious tampering. Additionally, high-bandwidth digital content protection (HDCP) can be applied to prevent copying. The result is the ability to transport higher resolutions than HD video from the backup camera to the driver’s display.

Other MASS applications include lane-keeping and sign-detection sensors; 360-degree camera, lidar and radar systems; and multiple high-resolution instrumentation, control and entertainment displays, including advanced head-up displays and side mirror displays.

The MASS protocol stack
The MASS stack incorporates elements required for safety, reliability and security and can accommodate multiple protocols from MIPI and other approved protocols.

Beginning at the physical layer, A-PHY includes features such as a PHY-level retransmission scheme to recover damaged packets and ensure steady connections. A-PHY’s protocol-agnostic data link layer allows multiple protocols to be multiplexed over one A-PHY link.

Lower-layer protocols such as MIPI CSI-2 and DSI-2 are integrated with A-PHY through a set of safety and security extensions and protocol adaptation layers (PALs) that map these protocols to A-PHY’s A-Packet format. PALs are in development for MIPI CSI-2, MIPI DSI-2, VESA’s DP/eDP, I2C, GPIO, I2S and Ethernet. A PAL for the MIPI I3C bus interface is also forthcoming.

Upper-layer functional safety and security capabilities are added using MIPI Camera Service Extensions (CSE) and Display Service Extensions (DSE) on top of CSI-2, DSI-2 and other native protocols. These extensions, along with a MIPI A-PHY Reference Compliance Test Suite, are expected to be available in early 2021.

Together, these elements will help system-level engineers adhere to the ISO 26262 safety standard and build systems that meet Automotive Safety Integrity Level (ASIL) requirements at any level, from ASIL B to ASIL D.

While new vehicle development takes years, a standardized end-to-end solution for in-vehicle connectivity can give OEMs and their suppliers more confidence to integrate future technologies that enhance safety, security and the in-car experience.

— James Goel is chair of the Technical Steering Group at MIPI Alliance.

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