From MIPI CSI-2 to USB 3.0

• Sensor-to-processor interfaces (board level): They connect the image sensor directly to a processor, ISP, or SoC within a device. A widespread example is MIPI CSI-2 (D-PHY / C-PHY). Other examples are LVDS and parallel interfaces.

• Camera-to-host interfaces (system level): System level interfaces transfer image data between a camera unit and a higher-level host system such as an industrial PC. The most important interfaces include GigE, USB, GMSL, and CoaXPress.

Bandwidth is one of the most decisive factors in embedded vision systems since it is the bottleneck of the system. The required bandwidth depends on the resolution, color depth, and frame rate. The higher the bandwidth, the faster data can be captured, processed, and analyzed. If it is too low, the system sees the world either pixelated, too slowly, or with a time delay.

Latency describes the time between the camera being triggered and the image data arriving for processing. A deterministic latency provides the basis for the real-time capability of embedded vision systems.

The distance that the signal can cover without loss varies depending on the interface. The possible lengths are less than 30 cm for the ribbon cable for MIPI CSI-2 and up to 100 m for Ethernet cables for GigE or 5GigE.

Scalability is the ability of a system to grow with increasing requirements without having to redesign the entire architecture. This can mean that more cameras can be integrated into the system or that the requirements for image quality can increase.

System costs differ because, for example, special cables or licenses are required or development and integration costs vary. The long-term availability of components is also a cost factor.

• Bandwidth calculated too tightly: Often only the nominal resolution is multiplied by the frame rate. Factors such as color depth, trigger modes, or future performance reserves are not taken into account. The result: the interface is constantly working at its limit or becomes a bottleneck early on during product updates.

• EMC environment underestimated: Especially in industrial or mobile applications, electromagnetic interference leads to unstable transmissions if the selected interface is not sufficiently robust. Highly integrated board-level solutions, such as MIPI CSI-2, are designed for short transmission paths – not for harsh environments with long cables.

• Cable lengths not defined early on: It may become clear late in the project that several meters need to be bridged between the sensor and processing unit or several cameras need to be synchronized. In this case, what was once a suitable interface may no longer suffice. Technologies such as GigE or GMSL offer greater degrees of freedom than board-level interfaces.

• Scalability not taken into account: What begins as a single-camera system often grows into a multi-camera architecture. A lack of synchronization mechanisms, limited host resources, or a lack of standardization can then make system adaptations necessary. It pays to consider expandability at an early stage.

• Integration effort underestimated: Not every interface comes with a mature software ecosystem or standardized drivers. While established standards such as USB3 Vision or GigE Vision benefit from broad tool support, proprietary or highly specialized solutions can cause additional development effort – especially when it comes to validation and maintenance.

• Let architecture decide instead of requirements: An interface is often chosen because it is already known in the company. However, every embedded vision application has its own requirements in terms of data rate, latency, robustness, etc. The interface should be the result of a requirements analysis – not the starting point.

Conclusion: Structured system analysis to select the interface for your embedded application

When selecting an interface for embedded vision applications, there is no universal solution. The decisive factor is which interface is both technically suitable and economically viable for the specific application. Factors such as bandwidth, latency, cable length, and integration costs must be evaluated.

A structured decision matrix that takes technical and economic criteria into account leads to a reliable decision. An early system architecture analysis is important for this. Defining where image processing will take place, what scaling is planned, and what environmental conditions apply during the concept phase creates planning security – and avoids cost-intensive adjustments later in the project.