Details:

Performing receive sensitivity tests (EL_16 and EL_17) as defined in the USB-IF USB 2.0 Electrical Compliance Test Specification may invoke the problem described in Advisory i2091.

The issue was originally found while performing these tests using automation software, which increased USB signal amplitude while sending packets. The software was sweeping the amplitude from a value less than 100 mV to a value greater than 150 mV while verifying the device under test (DUT) NAK’d all packets below 100 mV and NAK’d no packets above 150 mV. However, increasing the amplitude through the squelch threshold while sending valid packets may lock the PHY as described in Advisory i2091.

Workaround(s):

It may be necessary to perform the receive sensitivity test manually by breaking it into two parts. The first part begins the same as described above, with the initial amplitude set to a value less than 100 mV, verify the DUT NAK’d all packets while increasing the amplitude until it reaches 100 mV. The other part of the test begins by setting the amplitude above 150 mV, verify the DUT NAK’d no packets while decreasing the amplitude until it reaches 150 mV. This confirms the squelch threshold lies between 100 mV and 150 mV as required by the USB specification without sweeping the amplitude through the squelch threshold which can lock the PHY.