Interface primer
GMSL Primer
GMSL is Analog Devices' way of moving camera and display video over a single coax: a serializer at one end, a deserializer at the other, with control and power on the same wire. This primer builds it up from the physics of the cable.
What it is
GMSL (Gigabit Multimedia Serial Link) is a serializer/deserializer (SerDes) pair that stretches a short board-level video interface across a long cable. On a camera, the serializer sits next to the image sensor and takes its CSI-2 output. On the compute board, the deserializer turns the cable back into CSI-2 for the SoC. For displays the direction reverses: the SoC's DSI or DisplayPort goes into a serializer, and a deserializer in the display panel drives the timing controller.
Maxim introduced it in 2008 for infotainment displays and backup cameras. GMSL2 (2018) brought 6 Gb/s and a fast full-duplex control channel; GMSL3 (2021) doubled the payload rate to 12 Gb/s by switching from two-level to four-level signaling while keeping the same 3 GHz channel bandwidth. ADI reported more than 500 million links in cars from over 25 automakers by 2022.
The physics problem it solves
CSI-2 over D-PHY was designed for a few centimeters of PCB. It falls apart on a harness for four physical reasons, and each one maps to a GMSL feature.
| Problem on a 15 m harness | Physics | What GMSL does |
|---|---|---|
| Ground is not the same at both ends | Return current through harness resistance makes a DC offset (I·R) of volts between camera and ECU. D-PHY is DC-coupled with a 200 mV common mode. | AC-coupled link with DC-balanced coding: the receiver never sees the offset. |
| High frequencies die in the cable | Skin effect and dielectric loss: −20 dB or more at 3 GHz over 15 m. Bits smear into their neighbors (ISI). | Adaptive equalization (CTLE + DFE) rebuilds the eye. GMSL3 uses PAM4 to keep the Nyquist frequency at 3 GHz. |
| Skew between pairs | Four lanes plus a forwarded clock drift apart by tens of picoseconds per meter, more than a bit period at Gb/s. | One serial lane with an embedded clock. The receiver's CDR extracts timing from the data itself. |
| Wires cost weight, connectors and money | Ten conductors for D-PHY, plus power, plus I²C. | Video, control, GPIO and power all on one coax center conductor and its shield. |
Generations
| GMSL1 | GMSL2 | GMSL3 | |
|---|---|---|---|
| Year | 2008 | 2018 | 2021 |
| Forward rate | up to 3.125 Gb/s | 3 or 6 Gb/s | 12 Gb/s |
| Line code | NRZ | NRZ | PAM4 (6 GBd) |
| Nyquist frequency | ~1.6 GHz | 1.5 / 3 GHz | 3 GHz |
| Reverse channel | low-rate control | 187.5 Mb/s | 187.5 Mb/s |
| Typical use | VGA–1 MP cameras, displays | up to 8 MP cameras, FHD displays, Ethernet tunneling | multi-camera aggregation, multiple 4K streams, daisy-chained displays |
| Compatibility | — | GMSL1 mode on some parts | GMSL3 parts also run GMSL2 rates |
What it is not
- Not a standard. Both ends must be ADI parts. The open alternatives are MIPI A-PHY and ASA Motion Link; TI's equivalent is FPD-Link.
- Not a network. It is a point-to-point link (with some daisy-chain and aggregation features), not a switched fabric like automotive Ethernet.
- Not compression. Camera video is raw (RAW10/12, YUV). GMSL2 display links can add DSC, but the camera path is uncompressed so the ISP gets every bit with only microseconds of added latency.
The pages
The one-minute answer
It's a SerDes that carries CSI-2 or display video over one 50 Ω coax. The serializer packetizes the CSI-2 stream, scrambles it for DC balance and transition density, and drives it NRZ at 6 Gb/s (or PAM4 at 12 Gb/s for GMSL3, which keeps the Nyquist frequency at 3 GHz). The cable loses around 20 dB at 3 GHz, so the deserializer runs adaptive CTLE and DFE, recovers the clock from the data, and rebuilds CSI-2 for the SoC. At the same time a 187.5 Mb/s reverse channel runs the other way on the same wire, separated by frequency and echo cancellation, and carries I²C to the sensor plus GPIO such as frame sync. Camera power rides on the center conductor through inductor networks that look like open circuits from a few MHz to several GHz.