Cable length of MIPI camera is always an issue in real application. Herewith, we want to share some experience with customer. First of all, we need to understand a bit of MIPI standard, we attach at the end of this essay some info which is abstracted from some technical articles, for information.

From our expereice, impedance of cable is important, shielding of twist pair cable is also important. I do believe people knows PCB layout for these signal path is critical but people may not know the shorter the openings of twisted for soldering is also critical, it is a small trick. Adding a small capacitor between DP/DN might help the stability of data transmission but it depends on the actual setup. Sometimes, it may hurt the data on contrary. In our solution of mCam701 with C8207, cable length in between the module can be 4Meter. It is a good example to illustrate this, sometimes, we got complains that system doesn’t work after customer resoldering the connector.

Techincal Information about MIPI

The Camera Serial Interface (CSI), a division from the MIPI Alliance, was originally designed for the mobile industry, it’s a universal camera interface solution with higher bandwidth, power efficiency, and improved scalability, overcoming the disadvantages of common parallel interfaces. 

It provides a high-speed sensor interface that links a camera to a host processor, and along with the explosion of smartphones and surging demands of embedded camera systems, MIPI CSI has become “currently the most widely adopted camera interface in mobile devices.” 

As a processor-to-camera interface, the MIPI CSI-2 protocol has a 3-layer structure, with a Physical Layer (C-PHY/D-PHY) for signaling, a Transport Layer for data transmission (Lane Management, Low-Level Protocol and Pixel-to-Byte Conversion), and an Application Layer for high-level encoding and data interpretation.

For a stable connection, the maximum cable length you get with CSI-2 sits around 30 centimeters/11.8 inches, this is way too short for when the camera needs to be placed far away from the host. How to extend the cable length of mipi signal? The solution is to use Shielded Twisted Pair Cable.

A shielded twisted pair is a type of twisted pair cable that contains an extra wrapping foil or copper braid jacket to protect the cable from defects like cuts, losing bandwidth, noise, and signal to the interference. It is a cable that is usually used underground, and therefore it is costly than UTP. It supports the higher data transmission rates across the long distance. We can also say it is a cable with metal sheath or coating that surround each pair of the insulated conductor to protect the wire from external users and prevent electromagnetic noise from penetrating.