VBUS Pin of USB Connector

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    Question: In a self-powered USB device what should VBUS pin of the USB connector connected to? Can it be left unconnected?

     

    Answer:

    As per the USB specification, "Devices may not provide power to the pull-up resistor on D+/D- unless VBUS is present. When VBUS is removed, the device must remove power from the D+/D- pull-up resistor within 10 seconds” (section 7.2.1). The only way for a self-powered device to differentiate between a disconnect and a suspend is to monitor VBUS line. This can be done by monitoring VBUS at the USB connector through a GPIO. Following is simplified form of the implementation in CY4611 reference design (based on FX2)

    // If VBUS is present we need to connect to the bus else we disconnect 
    if (!VBUS_PRESENT) 

    USBCS |= bmDISCON; 

    else 

    USBCS &= ~bmDISCON; 
    }

    Here VBUS is monitored through a GPIO and VBUS_PRESENT get constantly updated based on the presence of VBUS.

    For more details on how this can be handled in FX2LP please refer Monitoring the EZ-USB FX2LP(TM) VBUS - AN15813.