[FAQ] AM625: USB 2.0 voltage swing is exceeding 400mv +/- 10%

Hello Prema, 

Thank you for the query.

You might have to verify the Eye diagram.

Please refer below thread.

(+) AM623: Is there USB Host Eye Diagram Test Report? - Processors forum - Processors - TI E2E support forums

Regards,

Sreenivasa

Prema,

It looks like your plots are taken in differential mode. The USB2.0 specs are of +/-10mV on the low side and +360mV to +440mV on the high side. I believe this is meant to be measured at "DC" with reference to ground.

Can your retake the plots as DC plots referenced to ground?

Thanks,

Stuart

Hi Stuart,

We understand they are differential signals as per usb2.0 standard and hence used differential probe(snapshot from standard below). However we will measure in single ended as you had suggested and share the waveforms.

also,we want to know if drive strength can be modified to reduce overshoot? pls let us know.

Hello Prema,

Please note that we do not have mechanism to adjust the drive strength.

Regards,

Sreenivasa

Hi Stuart & Sreenivasa

We tried to check the differential USB signals in single ended mode referenced to GND. The voltage levels are within 440mV now.

In our next spin of boards we are planning to add provision for series termination on these lines to reduce this overshoot. PLs let us know if this is ok.

USB lines captured w.r.t GND(max values are 427.4mV and 415.4mV for DP and DM lines).1GHZ(3.9pf) probe used.

When using differential probe(3.5GHz) ,the same lines are showing 446.9mV max

Hello Prema, 

Thank you for the note.

We do not recommend adding any series resistors on the High Speed USB signals.

Please help me understand  the reflections that you mentioned above.

Regards,

Sreenivasa

Hi Sreenivasa

The USB signals(DP and DM) are within 440mV when captured w.r.t GND(single ended mode), as suggested by Stuart.

However in differential mode capture they are slightly exceeding. We want to understand if we can bring down the max voltage so that its not touching the max limit of 440mV.

PLs let us know regarding this.