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TM4C129DNCPDT: USB Interface: Connection of USB0VBUS

Part Number: TM4C129DNCPDT


The MicroController is configured as USB Device. The board is self-powered. 

With the initial Version we had a defect with the USB communication at customers site. We do not know what happend exactly.

By having a look into the design guidelines (spma056, page 37ff) we saw, that a resistor to USB0VBUS is recommended.

Can you please have a look in the 2 schematics attached:

- Initial Version: with that one a defect occurred

- optimized version: with that version we wanna go into redesign.

Are there other recommendations? Is the optimized Version enough protected?


Best regards


Cortex M4 USB initial.pdf

7028.Cortex M4 USB optimized.pdf

  • HI,

      Per the system design guideline (spma056) the recommendation is to add a 100ohm series resistor if you are using the USB as a device. 

    4.3.1 USB Device Only For TM4C129x devices that are used in a device-only configuration, the only signal used in addition to USB0DM and USB0DP is USB0VBUS, which is located on port PB1. PB1 is 5-V tolerant. In USB deviceonly mode, USB0VBUS is used to detect when voltage has been applied to or removed from the USB connector, which triggers software to manage the internal USB PHY accordingly. For a USB device-only configuration, a 100Ω resistor should be placed in series between VBUS on the USB connector and PB1 (or alternate GPIO) on the microcontroller in order to limit damage caused by any ESD events.

      What I'm seeing in your optimized schematic is an addition of R31 1k series resistor and a inductor. Can you explain why you would choose this combination rather than the system design guideline recommendation? Have you completed the testing of your optimized design that it is working?

      Your original version:

      Your optimized version:

  • Charles,

    Thanks for the quick answer!

    I saw the 100 ohm in the guideline. I thought it might be a bit low. 

    I checked the impedance of the pin USB0VBUS and figured out something around 60 kOhm.

    So a 1k should be good also.

    The L - C I've foreseen against fast transient (ESD/Surge). The L-C will act as a low pass filter.

    Please let me know, If there are any concerns. 

  • Hi Reto,

      If you have tested your design with 1k series resistor on VBUS pin and proves that it is robust then I don't have any comments and concerns against it.  My understanding is that if you are self-powered, you don't even need the VBUS pin. You can use another GPIO instead to monitor that the host removes the VBUS.

      This below post may be also helpful.