TI document SPRUFM9F explains the USB2.0 Controller used on the L138.

I have a device that the L138 is configured as a USB Device, OTG is not required

My question is what is the need for

-USB0_ID  = In a host-device application, ID pin is floating. Can this pin be left floating

-USB0_VBUS = In a host-device, is the USB transceivers powered by USB0_VDDx or VBUS?

Can VBUS be left floating in a device only configuration?

Does the VBUS plays a role in the USB detection scheme or is it solely for OTG Power source?

9 Replies

  • Hi,

    It is recommended that the ID pin be terminated as per USB spec guidelines depending upon the role your product is expected to perform.

    VBUS is sensed by the controller to detect bus conditions and is needed either in host or device modes.



  • In reply to swami:


    In a typical USB cable, there is only 4 wire, VBUS, D+, D- and Ground.

    ID pin from host will remain floating.


    What is the intended L138 application and/or user case where ID will be used?


    Also, what is the purpose for VBUS on the L138, is it an outpu or input pin?

    Does the VBUS is required to power the USB PHY?

  • In reply to tehoaislimau:


    For Host mode operation ID Pin should be grounded.

    What you describe is for Host only controllers.  As the controller is host only ID pin state does not impact the role.

    L138 USB controller can act as Host or Device depending on the ID pin state.

    VBUS line is an input line to the PHY + Controller.  It is used to sense the status of the power levels on the usb bus.  It also serves other functionalities (in OTG mode).

    VBUS line should read 5V +/- tollerrence as observed in the usb spec for a reliable usb operation.

    USB PHY is powered internally from L138



  • In reply to swami:

    In my application, L138 will be used as a device.

    Computer will be host and OTG is not supported by my device. In this condition what should be the connection for USB_ID?

    VBUS to L138 is a low current sense line or a high current sense line? What documents can I read about this application?


    Also, I have another device (Power management chip) which controls USB charging and misc. power control.

    VBUS in my application will be connected to both the Power Management, for charging and identification and also L138 for sense (?) purposes.


    Will this cause any contention? How does the VBUS sense operation work in L138

  • In reply to tehoaislimau:

    Pl. contact you TI Field engineer w.r.t your USB layout and whether that can be supported with L138 usb.



  • Genius 9460 points

    In reply to tehoaislimau:

    For a device only requirement (L138 connected to PC) ID pin should be left floating. As per USB specification L138 can draw a maximum of 500mA from host PC.


    If my reply answers your question then please click on the green button "Verify Answer"

  • In reply to Ajay:


    I have placed your questions into one email. Hopefully this addresses all your questions. If not, please feel free to reply back to this forum and we will address the part of the questions that are not answered or require more detail.

    - What is the purpose for the VBUS pin on Freon?

    [Ans]. The purpose of the VBUS pin is to insure that adequate voltage is supplied to an attached device when operating as a Host. It insures that a minimum of 4.4V is present on the VBUS at all times. When operating as a Device, it also insures that the Voltage supplied by external Host is at least 4.4V and also uses this voltage to power up internal 1.5KOhm Resistor.

    - What is the difference between VBUS and USBx_VDD? USB0_VBUS = In a host-device, is the USB transceivers powered by USB0_VDDx or VBUS?

    [Ans] USBx_VDD is for internal USB Logic. VBUS is used to power up Internal pull-up resistor when assuming the role of a device (in this case external host supplies the power). At all times (whether operating as a host or device), the controller requires the voltage on the VBUS to an acceptable range (>= 4.4V) not for its usage but this is the voltage level that is required to be present when link is active. When operating as a host, VBUS pin is used to source power to the device. If the device is a Bus Powered device, it would make use this power for its operation. It is also used to indicate the presence of a Host.

    No, the transceiver is not powered up by the VBUS; it is powered, by the USBx_VDD.

    - Is the VBUS pin on Freon a high current power line (to support USB OTG to power other devices) or low current for voltage sensing only?

    [Ans] As eluded above, the VBUS pin is used to source power to external device when operating as a Host and when operating as a Device, it is used to sense a presence of a Host (in addition it is used to power up the internal 1.5KOhm resistor pull-up on the D+ line when operating as a Device). Our device is not a bus powered device and hardly consumes current from the VBUS line when operating as a Device. However, the controller requires this voltage level to be >= 4.4V for operation. This is not necessary for our operation but it is a USB Standard requirement.

    - What is the expected current consumption (input to Freon) in all modes when VBUS is available?

    [Ans] Current consumed is almost close to nothing but would not exceed 100uA.

    -Can the D+/D- pins be shorted together with out causing damage to the PHY?

     [Ans]. D+/D- shorting will not damage the PHY.

    - What is the voltage threshold on the Freon VBUS? To indicate a USB being connected etc?

    [Ans]. For operation, the controller/PHY needs to see a voltage >= 4.4V (VBUS VALID).

    -USB0_ID = In a host-device application, ID pin is floating. Can this pin be left floating?

    [Ans]. Yes the ID pin should be left floating. The cable end will either drive the ID pin low or leave it floating. We used to recommend a weak pull-up on this pin but we are no more recommending that. Please leave this pin floating and the cable end used will drive this pin as needed.

    Best regards, Zegeye


  • In reply to Zegeye Alemu:

    Hi, Zegeye-san

    As you mentioned above, L138 was not a bus powered device.
    So, is there any description about that in the datasheet or SPRUFM9H, etc? 

    Thanks in advance for your cooperation.

    Best regards,

  • In reply to j-breeze:

    Hello J-Breeze-san,

    No, this info is not in the datasheet nor the user guide mentioned explicitly. However it is implied indirectly. USB Power related pins comes from the power supply on board not from the VBUS. Also, since it is an OTG controller, during the time it is operating as a Host, it is supposed to furnish power, and this would not be possible.

    Best regards, Zegeye