Set the camera timestamp over USB3 Vision, not UART lines

You have a USB3 Vision camera, you found the UART option in the I/O chapter of its manual, and the obvious idea follows: feed time-of-day in over those two serial pins and every image comes out stamped with real time. It does not work that way. The UART lines on a USB3 Vision camera are a transparent serial pipe between your host software and an external device; the camera forwards bytes and understands none of them. The camera clock is an internal free-running counter, and the only route to it is the GenICam feature set over the USB3 Vision control channel. What follows is what the UART lines carry, where the timestamp comes from, and the arithmetic that anchors it to your host clock with an accuracy you can budget.

What the UART lines actually carry

The I/O connector of a USB3 Vision camera mixes opto-isolated inputs, GPIO lines and two UART pins. The UART function exists so the camera's cable harness also reaches a serial peripheral sitting next to the camera: a lighting or lens controller, a stage, a small PLC. Your software writes bytes to a serial path the camera exposes over USB; the camera pushes those bytes out of the UART pins unchanged and returns whatever the peripheral sends back. The camera is a wire with two connectors. It never parses the stream, and no camera feature is reachable through it - device control lives on the USB3 Vision protocol by design, on every conforming camera.

So the answer to "can I set the time over the I/O UART" is no, structurally. A time string sent down those pins reaches the peripheral wired to them; the camera's timestamp counter keeps counting as if nothing happened, because for the camera nothing did.

Where the camera timestamp comes from

Every frame the sensor delivers, the camera latches the current value of a free-running hardware counter and attaches it to the image as chunk data. Enable the timestamp chunk and each buffer arrives carrying that value, taken at exposure end on cameras that follow that convention - the chunk data timestamp at exposure end and how to latch it covers the details. Frame-to-frame intervals are hardware-latched, so they are good to well under a microsecond. What the counter cannot tell you on its own is where it sits in your lab's time; that anchor is yours to build.

Two device properties decide the arithmetic:

• The tick frequency. Current machine-vision cameras commonly run the counter at 1 GHz - one tick per nanosecond - but the value is a device property. Read the tick frequency from the camera's feature list at start-up and convert with what you read.
• The counter width. A 64-bit counter at 1 GHz wraps after 2^64 / 1e9 ≈ 1.84 × 10^10 s, about 585 years. Narrower latched values wrap sooner; camera timestamp clock rollover and monotonic time mapping shows how to live with the wrap.

Under auto exposure the exposure value rides the same metadata route, and reading exposure time from frame metadata under auto exposure shows how to pull feature values back per frame.

Setting and anchoring the clock over USB

Across USB3 Vision cameras, the GenICam feature list exposes timestamp control as a small set of commands: a latch that freezes the current counter into a readable value, a reset that zeroes it, and on some models a write that loads a value you supply. Writing your host's time-of-day straight into the counter is tempting, but the write lands somewhere inside the USB round trip, so the value is already old when it takes effect. The latch is the better anchor on every camera, because it freezes the counter at a moment the camera itself defines and lets you measure the rest.

Writing a feature through a GenICam API is the same call you already use for exposure - setting camera exposure time through a GenICam API in Python shows the pattern - only the feature differs. The anchor routine is three steps:

1. Read the host clock, send the latch command, read the latched value back, read the host clock again, and pair the latched ticks with the midpoint of the two host samples. The midpoint centres the round-trip error.
2. Convert the latched ticks to seconds and subtract from that midpoint to get the offset.
3. Convert every frame's chunk timestamp the same way and add the offset.

Worked example: ticks to wall time

Assume the camera reports f_tick = 1,000,000,000 ticks per second, read from the feature list at start-up.

1. The latch returns ticks_latched = 4,200,000,000,000 ticks, with host clock samples straddling it at a midpoint of t_host_mid = 452.123456 s.
2. t_latched = 4,200,000,000,000 / 1e9 = 4200.000000 s.
3. offset = 452.123456 - 4200.000000 = -3747.876544 s.
4. A frame arrives with a chunk timestamp of 4,213,762,455,100 ticks = 4213.7624551 s.
5. t_frame_host = 4213.7624551 - 3747.876544 = 465.8859111 s, so about 465.886 s on the host's monotonic clock.
6. If that host's monotonic zero was 12:00:00.000, exposure ended at 12:07:45.886 - 465.886 s is 7 min 45.886 s.

Drift sets your re-latch interval

The counter runs on the camera's oscillator, typically within ±50 ppm of nominal. At that tolerance the offset error grows 50 microseconds every second:

After 60 s that is 60 × 50e-6 = 3 ms; after 600 s it is 30 ms. To hold 1 ms, re-latch every 0.001 / 50e-6 = 20 s.

If you need absolute time tighter than about a millisecond, stop pushing the USB path: route one electrical trigger to both the camera and a host input that timestamps it, and anchor camera and host to that shared event. Sub-microsecond alignment across several cameras is the territory of GigE Vision with IEEE 1588, not USB3.

Four routes to real time, compared

What the UART is for, then

Point the UART at the peripheral it was designed for. A lighting controller bolted beside the camera can take its level commands from the host through the camera's own power and I/O cables, with replies riding back the same way, and you save a separate serial run out to the line. Aligning that serial traffic with images is a host-side job - the camera forwards the bytes but never timestamps them - and synchronize serial data with triggered image capture works the pattern at 60 fps.

Meanwhile every latch command and every chunk value rides the USB3 data cable together with the image stream, so that cable is part of the timing chain:

The same control architecture holds across the area scan cameras category - filter by interface in the camera finder, for example:

Checks before you build the anchor

• Read the tick frequency from the feature list at start-up and convert with it everywhere. Never hard-code 1 GHz.
• Enable the timestamp chunk and confirm the values are present and non-zero - empty chunk data from a GigE camera: isolate camera or host walks the split if they are not.
• Sample the host clock twice around the latch and use the midpoint, on the same thread that reads the value.
• Re-latch every 20 s at ±50 ppm; halve the interval to target 0.5 ms.
• Watch the host's USB stack: the control channel and the image stream share it, and a host that falls back to a slower link mode starves both. USB3 camera fallback troubleshooting on an embedded host covers the host-side checks, and when two USB3 cameras stream together, raise the host USB buffer before blaming timestamps.
• Remember that arrival time is not exposure time when frames queue in the driver; camera buffer timestamp latency: interpret frame timing separates the two.

The decision, case by case

• One camera, wall-clock time per image: enable the chunk timestamp, anchor with latch pairs over USB, re-latch every 20 s. Expect about 1 ms absolute and sub-microsecond between frames.
• A serial peripheral at the camera: use the UART lines for exactly that, and align the data in software on the host clock.
• Several cameras aligned tighter than 1 ms: USB3 Vision has no shared hardware clock. Choose GigE Vision cameras with IEEE 1588, or share one electrical trigger timed once.
• Downstream equipment that reads camera time directly: write the counter once at start-up where the camera allows it, then keep the write honest with periodic latch pairs.

If you are specifying the station, start at the camera step of the system configurator. If the sync budget is already fixed, send the accuracy your line needs through the contact page and the interface, camera and cabling can be matched to it in one pass.