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VBUS detection needed in device mode ?

Philippe MAIRE
Intellectual 500 points
Other Parts Discussed in Thread: TPS65920, OMAP3530

Hi,

I am using the TPS65920 with an OMAP3530.

I want to use the USB either as a host, or as a device, but not the OTG features. The mode will be decided by the OMAP.

In device mode, I have system requirements that prevent me from connecting the VBUS from the external host to VBUS of the PMIC.

If the system knows in advance it has to run as a device, does the protocol/ the hardware require a +5V on VBUS, or can it handle a proper communication only with D+/D-/ GND ?

Would the absence of VBUS need driver modifications ?

Regards

Philippe

  • 0
    TI__Guru 62855 points

    Will you product be self powered or will it be powered from the USB host?

     

    The USB specification requires a USB device to remove the pull-up on the DP signal, which sources current back up to the host, when it detects a loss of VBUS.  This may be the case if a self powered product is connected to a PC and the PC was powered off.   The OMAP3530 and TPS65920 system monitors VBUS to determine if and when this occurs.  If it detects a loss of VBUS it will disconnect which turns off the DP pull-up.  The system also monitors VBUS to detect a connection to the host.

     

    Also keep in mind the USB specification has different requirements for VBUS with respect to host or device operation.  For example, VBUS on a USB device must not have more than 10uF of capacitance while a USB host must have a minimum of 120uF of capacitance.

     

    Regards,

    Paul

     

  • 0 in reply to peaves
    Intellectual 500 points

    Hi,

    If I had used OTG mode, the VBUS capacitance spec would then be 6.5µF max, and the OTG can work as host.

    Does this mean that my system should work just fine as a host even with a small cap, in a "OTG-like" way ?

    My system must be able to work sometimes as a host, sometimes as a device, and this is software-programmed. It looks complicated to have a digitally-controlled 120uF cap on the  added to VBUS when my board is acting as an host.

    Anyway, your sentence "The system also monitors VBUS to detect a connection to the host" makes me think that a device with VBUS unconnected will not work at all!

    Regards

    Philippe

  • 0 in reply to Philippe MAIRE
    TI__Guru 62855 points

    A standard USB host is required to provide a minimum of 500mA of current on VBUS.  The USB specification requires a USB device to only consume a maximum of 100mA when it is initally connected to a USB host.  A high power device, which requires more than 100mA, must get permission from the USB host before it increases its power consumption above 100mA.  If the USB host has enough budget remaining to support the high power request it will grant permission to the high power device.  This is primary used the case were bus powered hubs and devices are connected to the host. 

    A USB OTG product is required to source a minimum of 8mA of current on VBUS and if it sources more than 100mA it must meet the requirements of a standard USB host.  This allows a USB OTG product which sources current in the range of 8-100mA to operate with a very limited list of other USB products.  However, if the USB OTG product sources more than 100mA there is an expectation that it operates with any USB device.  If you design your product with the VBUS capacitor value defined in the OTG specification you may limit the types of USB devices than can be used with your product operating in host mode.

    VBUS is the only thing a USB devices sees change when it is plugged into a USB host.  So that is what it uses to detect a connection to the host.  The USB host know a USB device is connected when the DP (for high speed or full speed) or DM (for low speed) signal is pulled high.

     

    Regards,

    Paul