This Edge.Auto series of blog posts is focused on TIER IV’s automotive cameras and the technology behind them. The third installment covered image sensor functions and explained the differences between global and rolling shutters. In the fourth installment, we’ll look at the interfaces used in automotive cameras.

Mobile Industry Processor Interface

Many of the high-speed interfaces used in modern image sensors, ISPs, and other camera-related semiconductors follow the Mobile Industry Processor Interface (MIPI) standard. This applies not only to automotive applications but also to products originally designed for mobile phones, as well as industrial equipment and digital cameras.

Developed by the MIPI Alliance, MIPI was originally established to standardize communication protocols for mobile devices. However, in recent years, the number of non-mobile applications adopting the standard has surged.

The standard has developed in response to the increasing pixel count and the resulting need for broader bandwidth in mobile devices, offering:

• Wide data bandwidth
• Low power consumption
• Simple system architecture

These features align well with the requirements of technology beyond mobile devices, so MIPI has been adopted in a wide range of applications. The majority of image sensors and ISPs in automotive cameras use MIPI as their interface standard.

MIPI has developed various standards, including the Camera Serial Interface (CSI) standard. The version that is widely used in products today is CSI-2, the latest of which is v4.0.

CSI-2 is divided into the following layers:

• Physical Layer
• Lane Management Layer
• Low Level Protocol Layer
• Pixel to Byte Conversion Layer
• Application Layer

CSI-2 layer definitions (Source: NXP Semiconductors)

The Physical Layer consists of the following:

• D-PHY
• C-PHY
• M-PHY
• A-PHY

For automotive applications, D-PHY is the most commonly used. In D-PHY version 1.2, data transfer speeds of 2.5 Gbps per lane are achievable. Therefore, a total data transfer rate of 10 Gbps is possible using 4 lanes. The details of A-PHY will be discussed later.

MIPI and automotive cameras

Here, I’ll explain the key features of MIPI, specifically when using D-PHY in the physical layer, and how it applies to automotive cameras.

The low power consumption of the MIPI standard is achieved by reducing the differential signal amplitude to ±200 mV. Additionally, the simple system architecture is realized by separating the data signals from the clock signals.

With regard to signal amplitude, if the amplitude is reduced, the signal transmission distance will be shorter. Additionally, when the data and clock signals are separate, longer transmission distances can lead to signal timing mismatches between the data and clock lines, increasing the potential for data transmission errors.

As a result of these factors, the maximum transmission distance for D-PHY v1.2 is 15 centimeters. For mobile devices, a transmission distance of up to 15 centimeters is generally not a significant issue. It seems that, in formulating the standard, the advantages were prioritized over potential drawbacks.

However, as explained in the first installment of this blog series, the physical locations of the camera and host processor may differ in automotive applications. Therefore, it is not feasible to directly transfer signals from the camera using MIPI. The signals must be transferred over another interface by converting them to a different format.

SerDes transmission

Serializer/Deserializer (SerDes) is the technology widely used for transmission over longer runs with automotive cameras. SerDes technology serializes parallel signals and transmits them over a single pair of transmission lines. In the case of MIPI 4-lane, there are five lines for four lanes plus clock, with 10 signals in total. The serializer converts the signals to a series, while the deserializer converts the serialized signals back into parallel form.

Relationship between serializers and deserializers

SerDes technology has the following advantages:

Reduction in the number of wires
Signal transmission from the camera to the Electric Control Unit (ECU) can be achieved using a single cable.

Simplified wiring
By reducing the number of wires inside the vehicle, cable routing becomes easier, contributing to the simplification of vehicle manufacturing.

Reduction in cable cost and weight
With fewer cables, there is a reduction in both the cost of the cables and their weight (which lowers the overall vehicle weight).

Reduction in electromagnetic interference (EMI)
Fewer signal rise and fall edges during transmission lead to improved EMI performance.

Longer transmission distances
Signals can be transmitted over greater distances.

Texas Instruments' FPD-LINK™ and Analog Devices' Gigabit Multi-Serial Link (GMSL)™ are widely used in automotive cameras. Image signal transmission from the camera to the ECU, along with control signal transmission from the ECU to the camera and power supply, can all be carried out through a single cable with these SerDes technologies. The signal transmission distance can reach up to 15 meters.

All TIER IV cameras use GMSL2, the second generation of Gigabit Multi-Serial Link.

Comparison of GMSL2 and other technologies

Camera interface options

USB (UVC) is commonly used in devices like web cameras. It can connect various devices without requiring dedicated device drivers. However, it has limitations in transmission distance and is generally considered less reliable compared to other connection methods.

Gigabit Ethernet (GigE) is commonly used for industrial cameras and machine vision applications. Its advantages include a long maximum transmission distance and compatibility with existing IP networks, as it operates on an Internet Protocol (IP)-based system. However, IP-based systems cannot fully utilize the transmission bandwidth, and transferring high-resolution, high-frame-rate image data may require compression, which can lead to latency issues.

GMSL2 offers uncompressed signal transmission, sufficient cable length (up to 15 meters), and ultra-low latency, enabling high-bandwidth and efficient data transfer. It also includes a range of features that enhance system safety and reliability. Due to these technical advantages, TIER IV cameras utilize GMSL2.

Emergence of open standards

GMSL and FPD-LINK are proprietary technologies, but in recent years there has been progress in the standardization of SerDes technology based on open standards. Examples include MIPI A-PHY and the Automotive SerDes Alliance (ASA).

MIPI A-PHY is a standard developed by the MIPI Alliance for in-vehicle applications such as advanced driver-assistance systems (ADAS), autonomous driving, and in-vehicle infotainment (IVI). The standard supports data rates of up to 16 Gbps, with a maximum transmission distance of 15 meters. As it is a standard established through the MIPI Alliance, it is characterized by its compatibility with widely used logical layer protocols such as CSI-2 and Display Serial Interface (DSI).

ASA is a standardization initiative driven by OEMs such as BMW, Ford, and Volvo, as well as Tier-1 suppliers like Continental, and semiconductor suppliers including Broadcom, Marvell, Microchip, and NXP.

Both standardization organizations aim to achieve interoperability between different vendors and reduce procurement costs by enabling purchases from multiple companies. As a result, products featuring these interfaces are expected to become widely available in the market within the next three to four years.

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Over the course of four installments, we have provided an overview of automotive cameras, the structure and basic functions of CMOS image sensors, and here we covered interfaces.

TIER IV is fueled by a commitment to make autonomous driving accessible to all and the company supports efforts to develop the technology at all levels. We hope this series of articles has helped enhance the understanding of cameras that serve as the “eyes” of all kinds of automotive systems.

If you’re interested in learning more about products in the Edge.Auto lineup, more installments are in the works. Sign up to our newsletter to ensure you don’t miss the next batch.

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