Tool/software:
Hi,
Do we need to add any termination for USB lines (USBx_DP, USBx_DM)?
Regards
Soumya
Hello Soumya
Thank you for the query.
The recommendation is to connect the USB interface directly.
A CMC with provision to bypass the CMC can be optionally used.
Refer below FAQ
Refer below section in the FAQ:
Refer below explanation for connecting the USB interface signals directly:
According to the schematic checklist (processors.wiki.ti.com/.../AM335x_Schematic_Checklist of AM335x,
"USBx_DP and USB_DM should never have any series resistors or capacitance on these signals. These signals should be straight traces to the connector with no stubs or test points."
Can I know the reason why?
(observed problem)
* when connected 10 ohm series resistors (for avoiding ESD problem), error happened (at the driver) while transferring data through USB port
* when replacing it with 0 ohm, the error does not happen
* when comparing Eye-Patterns, there is no big difference (both meet the specification)
The additional 10 ohms of resistance creates an impedance mismatch in the transmission line.
This impedance mismatch causes a portion of the energy in the waveform propagating down the transmission line to be reflected. The reflection will be combined with data patterns propagating down the transmission line with the result being certain data patterns distorted beyond recognition. Some data patterns may be affected more than others due to many variables related to these reflections. It is possible the standard test packet may not be significantly affected by this impedance mismatch.
It is very important to maintain a 90 ohm differential transmission line to prevent any reflections.
USB guidelines
Follow the below guidelines.
https://www.ti.com/lit/an/spraar7j/spraar7j.pdf
Regards,
Sreenivasa
