Page 4 · Putting it to work
Cameras and Displays
The full path from a photon at the sensor to a frame in SoC memory, the display direction, bandwidth sizing, and how to design and debug the link.
Camera to processor, end to end
- Sensor configured. At boot the SoC writes the deserializer over local I²C, brings up each link, and then writes each sensor's registers through the tunnel: mode, exposure, lane count, data type.
- Exposure. A frame-sync pulse from the ECU starts exposure on every camera (below). The sensor reads out its rows as CSI-2 long packets, framed by FS/FE and LS/LE short packets.
- Serialize. The serializer's CSI-2 receiver takes the 4-lane D-PHY bursts, buffers them, wraps them in GMSL packets with CRC, scrambles and drives 6 Gb/s onto the coax.
- Cross the cable. About 4 ns per meter of flight, 20 dB of loss at Nyquist, plus power going the other way on the same conductor.
- Deserialize. CTLE and DFE flatten the channel; CDR recovers the clock; the CRC is checked; the VC is remapped; lines from up to four cameras are interleaved onto a CSI-2 output.
- Into the SoC. The SoC's CSI-2 receiver demultiplexes by VC and DMAs each camera into its own buffer. The ISP debayers and the frames go to perception. Added link latency is microseconds; the frame time is 16–33 ms.
Bandwidth math
| Camera | Raw pixel rate | With ~10% overhead | Link |
|---|---|---|---|
| 1920×1080 RAW12, 30 fps | 0.75 Gb/s | ~0.8 Gb/s | GMSL2 3 Gb/s |
| 1920×1080 RAW12, 60 fps | 1.49 Gb/s | ~1.6 Gb/s | GMSL2 3 Gb/s |
| 3840×2160 (8 MP) RAW12, 30 fps | 2.99 Gb/s | ~3.3 Gb/s | GMSL2 6 Gb/s |
| 3840×2160 RAW12, 60 fps | 5.97 Gb/s | ~6.6 Gb/s | GMSL3 12 Gb/s |
| Radar raw ADC, CSI-2 4 × 600 Mb/s (AWR2243-class) | 2.4 Gb/s | ~2.6 Gb/s | GMSL2 6 Gb/s (3 Gb/s too tight) |
The last row connects to your radar board. The board kept DSP on the sensor because a 100 Mb/s automotive Ethernet link capped what could leave it. A GMSL-class link is the other architecture: ship raw ADC samples to a central processor and do the FFTs there. The trade is cable and central compute cost against sensor-side power, heat and silicon.
Frame synchronization
Displays: the same link reversed
Display links add things cameras do not need: HDCP encryption for protected content, Display Stream Compression on GMSL2 to fit 4K over one cable, and on GMSL3 daisy-chaining several displays from one serializer.
On a robot
- Cables through joints. Coax flexing in a wrist or neck changes its loss and return loss slightly with each bend. Use flex-rated cable, keep the bend radius, and watch the EQ settings and error counters during motion tests. A slip ring adds an impedance discontinuity; check it against the return-loss template.
- Motor noise. PWM drives at 48 V couple common-mode current onto cable shields. Coax relies on its shield as the return, so bond shields to chassis at the connector, and validate with motors switching. STP is an option where shield currents are a problem.
- Power budget. PoC drop through slender joint cables can be larger than in a car. Budget at the minimum bus voltage and maximum camera current.
Board design checklist
- 50 Ω controlled-impedance trace from pin to connector, short, on one layer, with ground vias along both sides.
- Void the reference plane under the AC capacitor pads and connector pads, where pads are wider than the trace, to keep the impedance near 50 Ω.
- ESD protection under 0.5 pF, placed at the connector; avoid SOT-23/SOT-323 packages whose bond-wire inductance resonates in band.
- PoC network: fastest element (ferrite or small L) closest to the signal node; inductors rated well above peak current; verify the impedance mask with the vendor's tool.
- Quiet, well-decoupled PLL and CDR rails, filtered separately from the digital core.
- Measure the channel with a VNA (insertion and return loss) against the GMSL channel template, including both PCBs and the harness.
Debug order when a camera drops
- Power. Read the PoC voltage at the camera through the serializer ADC, or measure it. Low voltage under load means drop or a saturating inductor.
- Lock. No lock: check line-fault status (open/short), connector seating, rate settings on both ends.
- Margin. Lock but errors: read the eye monitor and EQ settings, run PRBS, then heat and cool the harness to find where margin collapses.
- Correlate. Errors only when motors run point to common-mode noise or shield bonding. Errors only when moving point to a cable or slip ring.
- Above the link. Link clean but no image: CSI-2 settings (lanes, data type, VC mapping) and sensor configuration.
Questions to practice
Why does GMSL3 use PAM4 instead of a faster NRZ?
NRZ at 12 Gb/s puts Nyquist at 6 GHz, where cable loss is roughly 10–15 dB worse than at 3 GHz. PAM4 at 6 GBd keeps Nyquist at 3 GHz and pays a 9.5 dB eye-height penalty instead, plus tighter linearity and more coding. On automotive coax the PAM4 trade wins, and it reuses the GMSL2 channel.
How can both directions run on one conductor at once?
The line is linear, so the two waves superpose. The reverse channel sits mostly far below the forward channel in frequency, and where they overlap each end subtracts a replica of its own transmitted signal. Reflections it cannot model become noise, which is why return loss matters.
What limits the PoC inductor design?
It must be high impedance from the reverse channel's lowest frequency (~2 MHz) to above the forward channel's top (~3.5 GHz). That needs both a large inductance and a high self-resonant frequency, which one part cannot do, so a chain of inductors and a ferrite is used. It must also not saturate at the camera's DC current.
Why is the link AC-coupled?
To block the DC ground offset between camera and ECU, and to let DC power share the wire. AC coupling needs DC-balanced, scrambled data so the baseline does not wander and the CDR sees transitions.
How do you synchronize four cameras?
Lock their frequencies (reference over reverse, sensor clock from the serializer), then align their exposures with a frame-sync GPIO sent from the deserializer to all serializers with fixed latency. Timestamp at the deserializer or SoC.
A camera loses link only when the arm moves. What do you check?
Error counters and EQ settings during motion, cable flex damage and connector retention, slip-ring return loss, shield bonding, and whether the PoC voltage dips when motors draw current.