Know-How: what is GMSL2? Capabilities and test challenges

Gigabit Multimedia Serial Link™ (GMSL) serializer and deserializers (SerDes) are high-speed communication ICs developed by Maxim Integrated™ (now Analog Devices – ADI) that provide multistream support over a single cable, which can be up to 15 m with coax or shielded twisted pair. It can aggregate protocols like Ethernet and DisplayPort™ over a single link while meeting automotive electromagnetic-compatibility (EMC) requirements. GMSL was introduced in 2017, with 3K video and Gigabit Ethernet support, and then in 2018 GMSL2 was released to provide 4K video support.

The family of GMSL SERDES chips is tailored to handle multiple data streams. This includes the ability to split video streams to cope with the many displays in a car, such as the Information Cluster (IC) and the Central Information Display (CID). Furthermore, GMSL incorporates bidirectional diagnostics, which makes it possible to test the interfaces and cables. This works during development as well as when deployed in the field, providing superior field diagnostics: as an example, the eye-mapping tool essentially offers embedded sampling scope functionality, while the link margin tool can measure the minimum transmit amplitude required to run an error-free link in both directions.

From the low-power requirements of cameras to the widely varying bandwidth of sensor data aggregation, GMSL™ SerDes ICs address the needs of high bandwidth, complex interconnect, and data integrity requirements needed to support evolving automotive infotainment and Advanced Driver Assistance Systems (ADAS). Advanced link integrity and diagnostic capabilities enable a breakthrough in link performance monitoring – a vital capability to the design of automotive safety systems. Using Shielded Twisted Pair (STP) or coax cables of up to 15 meters, GMSL SerDes ICs meet the most stringent electromagnetic compatibility (EMC) requirements of the automotive industry.

GMSL is proprietary, but its functionality is hard to argue with. Reducing the number of cables in a connected car is important, as well as reliability. To improve the latter, GMSL utilizes spread-spectrum technology. Spread-spectrum capability is built into each SerDes design to improve the electromagnetic interference (EMI) performance of the link, without the need for an external spread-spectrum clock. Interoperation of serializers and deserializers within the family allows the use of different interfaces at each end of the link. In addition to driving high-resolution central/rear-seat displays and instrument clusters, GMSL high-speed SerDes can also be used in megapixel camera systems.

Testing of GMSL2™ devices

GMSL2 devices include automotive displays, multi-camera driver assistance systems and infotainment head units, that are subject to thorough V&V and manufacturing test. In the first case (displays), testing of these DUTs (Devices Under Test) requires the generation and transmission of multimedia digital streams, and the programmable generator must be equipped with the Serializer corresponding to the Deserializer mounted on the DUT. In the second case (cameras) the test requires a video frame grabber and real-time analyzer to acquire and analyze multimedia streams coming from the digital stream sources, equipped with the Deserializer corresponding to the Serializer used on the DUT. In the third case (head units), testing requires both a generator and a frame grabber with the aforementioned capabilities.