Advanced driver assistance systems (ADAS) are improving driver and pedestrian safety, providing vehicle capabilities such as pedestrian detection, lane departure warnings, collision avoidance, and much more. The increasing use of cameras throughout vehicles is enabling many ADAS capabilities. For ADAS applications involving cameras, one critical design challenge is to move image data from the camera to the processing unit and from the processing unit to each display as quickly and efficiently as possible.
When designing ADAS camera systems, there are some key factors to be aware of:
Bandwidth — Depending on its purpose, performance demands are different for each camera. For example, a back-up assistance camera with wide-angle lens may require 1.3 megapixels with 18-bit color per pixel at 30fps. Factoring in the control bits and encoding for balance, this single camera would generate >1Gbps of data.
Latency — A vehicle travels 91.13 ft (27.8m) every second when moving at a speed of 62.5mph (100km/hr). Obviously, when it comes to passenger and traffic safety, every second counts.
Reliability — To ensure that the vehicle continues running smoothly, it's essential to be able to adapt to wear and tear over the vehicle's lifetime and to detect when service is required.
Power consumption — As the number of electronic systems in vehicles continues to grow, staying within battery capacity and distribution constraints becomes increasingly difficult.
Cost — To keep system costs low and the technology competitive, it's important to find ways to reduce the number of components and cables in vehicles.
Image quality — High-quality images are critical for ADAS applications that are based on vision-based object detection.
Serializer-deserializer (SerDes) technologies can enable high-performing camera systems with robust, compact, and flexible communication links. This article takes a look at how these technologies can help automotive engineers design safer, smarter cars.
Gigabit Multimedia Serial Link (GMSL) SerDes Technology
The right SerDes products can provide high reliability and flexibility for uncompressed camera-feed transmission systems. SerDes chipsets can take uncompressed parallel video output from an image sensor and combine it with control inputs to serialize it into a single high-speed output. The chipsets then transmit this data across a cable and convert the received signals back into the original parallel video output on the deserializer side. Many systems are designed to provide both power and high-speed bidirectional data through the same cable. The Maxim Integrated MAX967xx family is an example of a set of products that offers safety and reliability features specifically for ADAS applications:
Control channel error-detection and retransmission, which ensures 100% accuracy when configuring a link;
Crossbar switch supporting any parallel input to any parallel output;
Enhanced cable drive with 50 coax or 100 shielded twisted pair;
Eye-width monitor and adaptive equalization;
Flexible data input up to 1.74Gbps;
Dedicated frame sync GPO.
Crossbar Switch Eases Layout Constraints, Supports Design Reuse
By including a crossbar switch in an ADAS design, any data input can be configured to route to any data output. This eases layout constraints and enables design reuse, which could significantly cut development costs. If image sensors with different output buses are supported for a given application, all sensors can interface to the same serializer board. In each scenario, the crossbar switch can be configured to ensure that the signals applied to the serializer are routed to the appropriate deserializer output. Design time is significantly reduced by designing a single serializer board that interfaces with different camera modules. The deserializer side can enjoy the same benefit. For the combination of a single camera module and serializer, a number of different deserializer boards and graphics processor combinations can be used to interpret the incoming camera data. The increased compatibility is simply enabled via an internal crossbar switch.
Detecting Line Faults
Some parts in the MAX967xx product line provide built-in line-fault detection. By attaching an external resistor network from the serial link to the LMN0/LMN1 pins and including a reference voltage between 1.5V and 1.7V, the system can automatically detect the physical state of the serial link. An optional hardware pin, LFLTB/GPIO1, can be used to provide an alert upon detection of an open cable, short to battery, or short to ground. Two line-fault monitor pins, LMN0 and LMN1, are included for use with single-conductor coax cables and shielded twisted pair (STP) cables.
The normal operating threshold for the LMN0/LMN1 pins is 0.57V to 1.07V. If the cable is shorted to ground, the line voltage is pulled below this threshold. If the cable is open, the line voltage is pulled up to the reference voltage between 1.5V and 1.7V. If the cable is shorted to the battery, the line voltage is pulled higher than 2.5V.
Overcoming Voltage Issues in Power-Over-Coax Circuits
In many systems, one STP cable actually contains two pairs, one for power and one for data. There are some advantages to using coax cables instead of STP cables for SerDes links, however. Coax cables are cheaper, lighter, more flexible, and less lossy at high frequencies. To be competitive, low-cost coax cables must provide both power and data through a single cable. To achieve this, the available frequency spectrum on the inner conductor is divided into power, reverse-channel data, and forward-channel data bands. Filtering passes the appropriate frequency band to its corresponding circuit. A series capacitor to the transceiver inputs AC-couples the data channels.
The DC power typically uses the low-pass quality of series inductors to construct filters whose impedance rises above 1k in the reverse-channel and forward-channel frequency bands. Since the data channels operate with 50 termination, the 20x increase in impedance is sufficient to couple the DC voltage and filter the high-frequency content. Every inductor has parasitic capacitance that causes self-resonance and a corresponding drop in impedance at high frequencies. Inductors of different sizes are therefore chosen to filter out all the bands of interest.
The current delivered across the cable must pass through each inductor in the power filter, constraining various aspects of inductor parameter selection, including saturation current (ISAT), DC winding resistance (DCR), and package size. If a current greater than ISAT flows through an inductor, the inductor saturates, and the inductance drops steeply. There is a power loss proportional to the square of the current multiplied by the inductor's DCR, which causes self-heating to occur. If the power delivery rail does not include a built-in voltage margin, then the voltage drop across the power filter may lead to insufficient voltage levels at the load.
You can avoid each of these three potential problems by applying a higher voltage to the cable, which lowers the cable current. You can also choose inductors with sufficient size and saturation current rating to manage the required cable current.