Understanding the USB 2.0 “TEST J, K, SE0” Tests

Explains pass/fail criterion, setup, and has specification excerpts.

Contents
Test J/K/SE0_NAK pass criteria
USB 2.0 High-speed signaling. 1
Setup Considerations. 1
USB Compliance Updates
Specification Excerpts. 3


Test J/K/SE0_NAK pass criteria

Test	Name	D+	D-	Margin	Notes
EL_8	TEST_J	400 mV	N/A	±20 mV	deprecated
EL_9	TEST_J	N/A	0 mV	±20 mV	Normative (Required)
EL_8	TEST_K	N/A	400 mV	±20 mV	deprecated
EL_9	TEST_K	0 mV	N/A	±20 mV	Normative (Required)
EL_9	SE0_NAK	0 mV	0 mV	±20 mV	Normative (Required)

EL_8 is the 400 mV ±20 mV measurements (data line voltage driven high), which is informative now.
EL_9 is the 0 mV ±20 mV measurements (data line voltage not driven), which is still required.

USB 2.0 High-speed signaling

State	D+	D-	aka	notes
J	high	low	Differential 1
K	low	high	Differential 0
SE0	low	low
SE1	low	high		Should never happen

 
Setup Considerations
These test use the High Speed Signal Quality Test Fixtures.
The tests require the signals to be driven into the ideal, High Speed termination, for both upstream facing and downstream facing ports. The ideal termination is achieved when both of the following are true:

1. the fixture is in "Test" mode - both yellow and green LEDs on
2. the SMA breakouts on the fixture are terminated with 50 Ω.

It is best to perform his measurement with calibrated DMM. If using an oscilloscope, the best way to do this is to use coax cables to the fixture, having the oscilloscope in 50 Ω mode, at a small vertical scale.
 
USB Compliance Updates
http://compliance.usb.org/index.asp?UpdateFile=Electrical#67
The Driven Lines of High-speed Test_J and Test_K are Removed as Certification Requirements
Mandate: Optional
Effective Date: January, 2010
The USB-IF no longer requires EL_8: Test_J and Test_K to be performed as a condition for USB Certification.  USB certification of high-speed USB devices (hosts, hubs and peripherals) no longer requires execution or passing of EL_8: Test_J and Test_K tests.  EL_8 is defined in the USB 2.0 Electrical Test Specification and measures the data line voltage when driven high.  The nature of this test measures the DC voltage level of the data line when driven, which is not a real-world representation of the functionality of the data-line.  Violations of driven data-lines should be detectable in signal quality tests.
 
Measurement of EL_9: Test_J, Test_K and SE0 is still a requirement for certification.  EL_9 is defined in the USB 2.0 Test Specification and measures the data line voltage when not driven.  EL_9 allows measurement of SE0 and common mode voltage violations.  Consequently, all USB controllers must continue to support the Test_J, Test_K and SE0_NAK commands as described in Section 7.1.20 "Test Mode Support."
 
http://compliance.usb.org/index.asp?UpdateFile=Electrical#92
Maximum allowed voltage on non-driven lines of Test_J and Test_K are relaxed
Mandate: Required
Effective Date: April, 2014
The High-speed Electrical Test Specification, EL_9, measures the voltage on data lines that are not driven during test modes Test_J, Test_K and SE0_NAK. For the purposes of certification, when either D+ or D- is not being driven, the output voltage must be 0V ±20mV when terminated with precision 45 Ω resistors to ground.

Prior to this update, the limits were 0 V ±0 mV.
Specification Excerpts