USB-C ports have to handle a variety of functions including data transfer, power delivery, and DisplayPort (DP) Alt Mode video output. Device manufacturers need to perform factory End-of-Line (EOL) testing to make sure that their products meet specifications In this blog post, we'll be looking at how to automate and speed up EOL testing of DisplayPort (DP) Alt Mode with Acroname's USB-C-Switch.
The USB-C-Switch is an industrial 10 Gbps USB-C port selector able to connect one of up to four devices to one host, or one device to one of up to four hosts. Unlike a USB hub, the selected ports form a direct 1:1, bidirectional connection that appears “like a cable” to connected devices, supporting alt modes like DP Alt Mode (up to 4k at 60 Hz). When combined with an Acroname Universal Orientation Cable (UOC), the switch can emulate a cable flip, allowing tests of all connections on a port without the need to manually reinsert the cable.
What is USB-C DisplayPort Alt Mode?
DP Alt Mode lets USB-C ports transmit DP video and audio signals over the same connector used for data and power, allowing a device to connect directly to a DisplayPort monitor or adapter.
In this mode, the USB 3.2 SuperSpeed data lanes are taken over to carry DP video signals. The link can be configured to use two or all four high-speed lanes, reducing or completely disabling the USB 3 data connection (The USB 2.0 connection is unaffected).
The Sideband Use (SBU) lines provide low-speed communication between the USB-C device and the connected display, carrying configuration data and link training signals. This ensures the correct lane mapping, resolution, and refresh rate are established before video transmission begins.
How to test DP Alt Mode
We have observed a variety of methods for EOL DP Alt mode testing.
- Port Under Test (PUT) to DP monitor: Operator verifies correct test image on screen
- PUT to Video Capture card + auto image analysis: Like to (1) but programmatic
- PUT to USB-HDMI adapter to EDID monitor emulator: Pass when DUT OS recognizes connected monitor
- PUT to USB-HDMI adapter to EDID monitor emulator: Pass when SBU communication is observed
USB-C-Switch works with any of these method, including directly monitoring SBU traffic, making it easy to integrate DP Alt Mode tests into a single automated USB-C port test sequence. In some cases we have seen total port test times drop from minutes to seconds by removing slow manual steps.
USB-C-Switch test example configuration
USB-C-Switch can automatically connect the PUT to multiple test peripherals. In this example, the PUT is connected to the USB-C-Switch common port via UOC and is switched to the appropriate mux port for each stage of the test. Using similar automations, customers have seen total test times go from minutes to seconds versus their previous manual methods.
Figure 1: USB-C-Switch test solution with EDID emulator
Connections
Control Port:
- Test Control Host PC: Controls and powers the USB-C-Switch via USB through the Control port.
Common Port:
- PUT: connected via UOC to the USB-C-Switch Common port
Mux Ports:
- USB-C Alt Mode HDMI adapter with EDID emulator
- USB-PD charger
- USB-PD sink emulator and electronic load
- USB 3.0 flash drive (USB 2.0 compatible)
Network Connection:
- Allows the DUT to communicate test results (enumeration, power states, and monitor detection) with the test control host.
Fast Alt-Mode test with direct SBU detection
Some customers have found that their device DP Alt Mode failures occur due to PCBA assembly issues, either through open or bridged solder connections of the SBU and USB SS pins. For these devices, verification of USB SS function (part of the separate USB SS test) and presence of SBU signals is sufficient to test DP Alt Mode.
The redriver version of USB-C-Switch is capable of directly measuring voltage on the SBU lines, and the values can be read by the Test Control Host PC. Alt-Mode communication begins on those lines well before the OS detects the monitor emulator.
Switching to SBU signal detection resulted in significant speedup, bringing the mean detection time from from 9.6 s to 1.8 s. The biggest change is in the long outliers -- in the above example, the longest OS detection took almost 20 s versus a maximum SBU detection time of 2.5 s)
Relevant APIs
stem.usb.getSBU1Voltage(channel)
stem.usb.getSBU2Voltage(channel)
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