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TM4C123GH6PM: USB0 VBUS connection

Shitij Avlani
Prodigy 180 points
Part Number: TM4C123GH6PM
Other Parts Discussed in Thread: EK-TM4C123GXL

I want to use the USB VBUS for monitoring whether the device has connected and disconnected from the host. I'm a little confused after going through all the posts on the forum.

I am designing my own custom board and need a few clarifications. The silicon revision of the microcontroller is 7.

To connect the VBUS pin to PB1, I just need to add an RC filter to limit the rise time to below 2ns right? And for the times when the host is connected but the pin hasn't been configured in the USB mode, the voltage has to be limited to 3.3V, so I'm planning on using a simple voltage divider.

In summary, by using a voltage divider and RC filter I can connect the VBUS pin to PB1? Am I correct in inferring this?

Regards,

Shitij Avlani

  • 0
    TI__Guru**** 190806 points
    Hi Shitij,
    PB1 is 5V tolerant. You do not need a voltage divider.
  • 0 in reply to Charles Tsai
    Prodigy 180 points
    Hi Charles,

    The datasheet states that "All GPIO signals are 5-V tolerant when configured as inputs
    except for PD4, PD5, PB0 and PB1, which are limited to 3.6 V." Now is VBUS is present when the device is powered up, and before the code changes the state of PB0 and PB1 to its USB function, if I am not wrong the 5V on VBUS can damage that pin right?

    Regards,
    Shitij
  • 0 in reply to Shitij Avlani
    TI__Guru 72500 points

    If used in "Device Only" mode, PB1 can be protected with a 100 Ohm resistor. An alternate 5-V tolerant Fail-Safe GPIO should be selected to do the VBUS detection if there is a chance 5V could be applied to the input prior to PB1 on the TM4C123x device being configured for VBUS, such as during power-up, while the device is in reset, and the while the initial boot sequence executes. See section 4.1.1 of the System Design Guidelines.

  • 0 in reply to Bob Crosby
    Prodigy 180 points

    Hi Bob,

    Thank you for your input, I have gone through the system design guidelines and I have a small doubt as to whether an ESD suppressor is necessary on D+, D- lines. The EK-TM4C123GXL board does not have one and it seems to work perfectly well. Can I do without it?

    Secondly, if I incorporate a level shifter at PB1, permanently shifting VBUS voltage level to 3.3V as shown above, it will still allow me to monitor the device connection status right? The controller is going to be used in device only mode.

    Regards,

    Shitij Avlani

  • 0 in reply to Shitij Avlani
    TI__Guru 72500 points
    Your level shifter seems overly complicated. As mentioned in the design guidelines, in USB device only mode a 100 Ohm series resistor will do. Also, as mentioned in the layout design guide, ESD protection for D+ and D- is recommended. ESD protection protects the part from permanent damage caused by voltage spikes outside of the maximum recommended specifications. It does not affect the functionality of the circuit.