USB Signal Quality Test

[中文]

Signal quality testing of the USB 2.0 interface is an important step in ensuring that USB 2.0 devices comply with the specification and qualify for the USB 2.0 certification logo. Signal quality testing mainly covers eye diagram testing, signaling rate, End-of-Packet (EOP) bit width, crossover voltage range, JK jitter, KJ jitter, consecutive jitter, rise time, and fall time. Among these, eye diagram testing is one of the most fundamental and critical test items in serial data applications.

Test Overview

Eye Diagram Templates

The eye diagram test results for USB 2.0 High-Speed signals must meet the standard eye diagram template requirements defined by USB-IF.

USB 2.0 High-Speed signal measurement planes

The eye diagram test results for USB 2.0 High-Speed signals must comply with the eye diagram templates in the USB 2.0 specification:

Transmit waveform requirements for hubs measured at TP2 and devices without captive cables measured at TP3

Transmit waveform requirements for devices with captive cables measured at TP2

Transmit waveform requirements for hub transceivers measured at TP1 and device transceivers measured at TP4

Required Materials

• A USB test fixture matching the USB mode and speed under test, available from the instrument vendor or a USB-IF-recommended test lab

• Oscilloscope (> 2 GHz), compliance test software, probes, etc.

Oscilloscope Eye Diagram Test

USB 2.0 compliance analysis software analyzes the standard signals transmitted by the interface and generates an eye diagram. The operation of compliance test software varies between oscilloscope vendors. Refer to the oscilloscope vendor’s instructions to perform the eye diagram test.

Oscilloscope eye diagram test

In USB 2.0 eye diagram testing, a wider eye opening indicates better signal quality. The test results must remain entirely within the passing region of the eye diagram template. Any waveform touching or crossing the template boundary indicates that the signal quality does not meet the requirements.

USB 2.0 High-Speed

Passing eye diagram

For an eye diagram that fails the test, check the following:

• Verify that the probes are calibrated

• Use a compliant test fixture

• Replace the cable or connector to rule out poor-quality accessories

• Check whether the PCB design follows the USB 2.0 specification. If an ESD protection device with high parasitic capacitance is used on the USB High-Speed D+/D- lines, the eye diagram test may fail. An ESD diode with parasitic capacitance below 1 pF is recommended

Failing eye diagram

USB 2.0 Full-Speed

For eye diagram testing of a Full-Speed interface, use the USB compliance test software provided by the oscilloscope vendor:

Failing USB 2.0 Full-Speed eye diagram (overshoot)

For the Full-Speed eye diagram failure shown above, add series resistors on D+/D- to reduce overshoot. Test results show that 22 Ω, 33 Ω, and 44 Ω series resistors can all improve overshoot, with 33 Ω providing the best result.

Device Mode Test

Required Materials

• Windows PC

• USB XHSETT: Compliance test tool provided by USB-IF, used to place a USB 2.0 Device into compliance test mode

Test Firmware

Download the corresponding Device mode test firmware according to the chip model and chip revision.

Device Mode Test Firmware

Chip Model	Chip Revision	USB Speed	Test Firmware
ESP32-S2	All revisions	Full-Speed	Download
ESP32-S3	All revisions	Full-Speed	Download
ESP32-P4	< v3.0	High-Speed	Download
ESP32-P4	v3.0	High-Speed	Download
ESP32-P4	>= v3.1	High-Speed	Download
ESP32-S31	All revisions	High-Speed	Download

Note

For a USB eye diagram test on the USB Serial/JTAG Port, simply put the chip into download mode and ensure that the device enumerates successfully.

Hardware Connection

Connect the PC, oscilloscope, and USB test fixture. High-Speed test fixtures differ from Full/Low-Speed test fixtures. Refer to the connection diagram in the oscilloscope compliance test software.

USB 2.0 Device signal quality test system

High-Speed Device signal quality test connection diagram

XHSETT Installation and Configuration

Check the USB port information in the PC Device Manager:

• If the PC has a USB 3.x controller, use XHSETT. xHCI supports USB 1.x/2.0/3.x

• If the PC has a USB 2.0 controller, use HSETT. HSETT supports USB 1.x/2.0

USB 3.x controller

Note

After HSETT/XHSETT is installed and launched, it takes control of all USB ports on the PC, making them temporarily unavailable. Before running HSETT/XHSETT on the test host and restarting the PC, connect a PS/2 keyboard and mouse to prevent interruption. If a PS/2 mouse is unavailable, you can use Remote Desktop or another remote control tool.

After installing HSETT/XHSETT, configure it as follows:

Open XHSETT and select Device mode:

XHSETT mode selection

Click the Enumerate Bus button to scan. After the device is detected, select the device under test and choose Device Command:

• For a High-Speed device, select TEST_PACKET under Device Command

• For a Full-Speed device, select (LOOP) DEVICE DESCRIPTOR under Device Command

XHSETT device selection

After completing the configuration above, follow the instructions in Oscilloscope Eye Diagram Test to perform the test and analyze the results.

Host Mode Test

Test Firmware

After USB Host completes enumeration of a High-Speed device, the Host mode test firmware controls the USB Host port to continuously transmit standard USB 2.0 TEST_PACKET packets. Download the corresponding Host mode test firmware according to the chip model and chip revision.

Host Mode High-Speed Test Firmware

Chip Model	Chip Revision	USB Speed	Test Firmware
ESP32-P4	< v3.0	High-Speed	Download
ESP32-P4	v3.0	High-Speed	Download
ESP32-P4	>= v3.1	High-Speed	Download
ESP32-S31	All revisions	High-Speed	Download

High-Speed Test

Required Materials

• A USB flash drive that supports USB 2.0 High-Speed, used only to let USB Host complete High-Speed enumeration and enter TEST_PACKET mode

• USB 2.0 High-Speed Host test fixture

Test Procedure

1. Flash the corresponding Host mode test firmware to the board under test, and monitor the runtime log through the serial port.

2. Connect the High-Speed USB flash drive to the USB Host port under test. After the firmware detects the flash drive, it completes device enumeration and automatically switches the USB Host port to TEST_PACKET mode.

3. Verify that the device speed in the log is High speed and that the following messages appear:

   TEST_PACKET operation successful
   Unplug the U-disk and connect the USB test fixture / oscilloscope
   The host continues sending USB 2.0 test packets
   

   If the log indicates that the device is not High-Speed, replace the USB flash drive or USB cable, or check the hardware connection. Then reset the board under test and retry.

4. Keep the board under test powered and do not reset it. Unplug the USB flash drive, then connect the USB 2.0 High-Speed Host test fixture to the same USB Host port under test. After entering TEST_PACKET mode, the firmware continuously transmits standard test packets.

5. Connect the test fixture to the oscilloscope. Following the instructions for the oscilloscope vendor’s compliance test software, select the USB 2.0 High-Speed Host test item, then follow Oscilloscope Eye Diagram Test to complete the eye diagram test and analyze the results.

Note

The Host test firmware directly controls the USB Host port to enter TEST_PACKET mode, so HSETT/XHSETT is not required. To exit test mode or restart the test, reset or power-cycle the board under test, then repeat the steps above starting with connecting the High-Speed USB flash drive.

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