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USB waveform abnormality

Question asked by kenji noda on May 8, 2018

Noise was applied to the board equipped with the STM32F302RC6, and the 0 potential level of USB_D + was raised. The applied point is not a USB terminal, and the USB cable is not connected yet. (The connection destination of the other port was not abnormal. * The PUP and the PD resistor are mounted on the pattern.)

Voltage level on oscilloscope (vs. GND voltage)

USB_D +: 3.0 V - 960 mV * Floating, USB_D -: 3.3 V - 0 mV

USB 2.0, FULL SPEED (D + 1.5 kΩ PUP)

The port setting is AF + PP. Why does the USB waveform become such a symptom when noise is applied to the board? Is it better to set the USB port setting as AF + PP + PD?

Advice please.