The USB Type-C® connector is very simple for consumers to use, but behind it lies a very complex system that supports multiple standards. The first group, from the USB Implementers Forum (USB-IF), specifies a multitude of USB data and power delivery. Layered on top of that are various “alternative mode” standards, such as the VESA DisplayPort over USB Type-C architecture and Thunderbolt™. An approach to debugging interoperability failures that incorporates a USB-C test coupon can put your development and test program on the road to success. This application note shows how to troubleshoot USB 3.1/3.2 link training problems using the TF-USB-C-HS with an oscilloscope installed with serial trigger and decode options, and a variety of standard probes.

Teledyne LeCroy offers two USB-C test coupons: the TF-USB-C-SB coupon for low-speed power and sideband measurements, and the TF-USB-C-HS coupon for power, sideband and high-speed measurements. As shown in Figure 1, each coupon comes with a 0.3-m cable. The insertion-loss profile of the cable and coupon combination is tuned to be the equivalent of a golden 0.8-m USB Type-C cable, so you can replace a 0.8-m cable with the coupon and not experience any difference in link performance. Each coupon also has a loop to allow a current probe to make load-current measurements. The HS version is compatible with Teledyne LeCroy DH Series probes for making high-speed differential measurements.

usb type-c cable connector

Debugging Link Training Problems


  Required Recommended
  • 4 Ch, 16 GHz min. bandwidth
  • 512 Mpts record length
  • 40 GS/s or 80 GS/s sample rate
WaveMaster/SDA 8 Zi-B
LabMaster 10 Zi-A
Software Options USB 3.2 D ProtoSync for USB(requires installation of USB Protocol Suite software)
SDAIII or SDAIII-CompleteLinQ for eye diagram analysis of the live link
Probes For probing TX/RX: 16 or 20 GHz differential (2 each) Teledyne LeCroy: DH16-PL or DH20-PL
Other   Voyager M310P, M310e or M4x USB Protocol Analyzer for USB 3.2 protocol triggering

Link training in USB 3.1/3.2 is negotiated using an LTSSM (Link Training and Status State Machine) through electrical signaling on the TX1/RX1 and TX2/RX2 connector pins. Link training must be competed on the link before high-speed data transaction can occur.

USB Type-C connector pins

One problem you might encounter during link training is a failure to train to USB 3.2 Gen 2 specifications. Teledyne LeCroy customers report that most system-interoperability problems are caused by either link-training or sideband-negotiation failures, which in turn can result from an electrical problem, a digital problem or a combination of both.

You can use the TF-USB-C-HS test coupon, DH Series differential probes and the USB 3.2 Decoder software option to investigate the root cause of link training/link management problems.

You can also incorporate the Teledyne LeCroy ProtoSync software, which runs directly on the oscilloscope to provide a protocol analyzer-like view of the decoded data, as shown in Figure 3. With ProtoSync, when you click on a packet in the protocol analyzer view, you are automatically guided to the corresponding section of the oscilloscope electrical traces. This helps you correlate link layer with electrical behaviors.

The further addition of SDAIII software enables you to do eye diagram analysis of the live link.

ProtoSync software screenshot

Probing with the TF-USB-C-HS Test Coupon

The TF-USB-C-HS is connected either between the DUT and a Known Good Device, or between the DUT and the Exerciser/Analyzer ports of a Voyager Analyzer (M310e/M310P or M4x), using the included USB-C cable. Signals are input to the oscilloscope by way of the test coupon:

  • TX1 is input to A-row Upper Deck C1 using a DH Series 16 or 20 GHz differential probe.
  • TX2 is input to A-row C2 using same.
  • Sideband signals CC1/CC2 and Vbus can also be monitored using passive probes on the B-row connectors of the oscilloscope.
  • If using a Voyager (recommended), the trigger out signal is connected to oscilloscope Ext In.

test setup for usb phy logic debug

Triggering on USB 3.2 with the Protocol Analyzer

Link training issues usually first show up while doing Link Layer or USB Type-C-specific tests using a Voyager protocol analyzer. The protocol analyzer has a rich set of Link Training Packets (LTP) and higher layer analysis of the USB 3.2 LTSSM, as shown in Figure 5. In order to trigger an oscilloscope on the area of interest, is necessary to first identify where in the protocol trace the problem is occurring, then set up the protocol analyzer to send a trigger pulse to the oscilloscope at that time. For instance, during Link training, you can send a trigger pulse on SCD1, SCD2, LBPM or an LMP (Link Management Packet). As each trigger event occurs in the signal, the event is marked on the protocol trace and a pulse is sent from the Trigger Out connector of the protocol analyzer to the Ext trigger input of the oscilloscope, enabling it to capture the electrical layer signals at virtually the “same” time.

voyager protocol analyzer screen shot

Oscilloscope Triggering and Decoding

Set up the oscilloscope for an Edge trigger using the Ext In Source.

Set up the USB 3.2 D software for Gen2x2 decoding, with One Differential Probe selection. (Note that our example uses acquisitions of Lane0 and Lane1 saved to memories M1 and M2. When probed live, these signals would be on C1 and C2.)

Setup for USB 3.2 Gen2x2 decoder

After acquiring, use the decoder Search or Filter feature to find the packet of interest, which appears in the Type column of the decoder result table.

USB 3.2 Gen2 decoding showing a Link Management Packet (LMP)

Eye Measurements

A possible source of link training errors is poor signal quality coming from the transmitter. The TF-USB-C provides a convenient way to perform serial data analysis measurements on the transmitters while in a live link. This is not a substitute for physical layer compliance testing, but it can be used to verify that the expected signal quality is coming from the two USB 3.2 transmitters, TX1 and TX2.

If you have installed the SDAIII or SDAIII-CompleteLinQ oscilloscope option, a complete set of signal integrity tools is available, including jitter and eye diagram measurements and plots.

As shown in Figure 7, you can use the eye diagrams to look at the electrical performance of the live link. If the signal is not optimized for the cable it is driving, you may see an eye diagram that has too much or too little equalization. The ISI Plot allows you to evaluate the effect of different pre-emphasis presets on the transmitted eye. Multi-lane testing with SDAIII-CompleteLinQ also allows you to see both transmitter signals side-by-side in order to determine if there are lane-dependent SI issues that might be contributing to poor link performance.

Eye diagrams screen screenshot


Teledyne LeCroy takes live-link approach to looking at the physical-layer performance of a USB Type-C system. The TF-USB-C-HS test coupon enables unprecedented ease of access in probing USB-C signals, decoding many USB-C standards, and examining eye diagrams and levels of pre-emphasis coming out of the transmitter while the link is negotiating. More information about Teledyne LeCroy USB Type-C electrical test solutions can be found on our website