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USB_VBUS connection

Matthias Fuchs
Prodigy 170 points
Other Parts Discussed in Thread: SN74LVC1G07, SN74LVC2G07, AM3352

Hi,

our AM335x based custom board connects two on-board fixed devices to the chip (a hub and a USB-UART bridge). So no hotpluggable USB devices.

5V is far away from the chip on our board. So what do we need to connect to the VBUS pins? All reference designs connect 5V. Do we really need to connect 5V

or is 3.3V ok also? I did not find any info about the tresholds.

Matthias

  • 0
    TI__Guru**** 393215 points
    Hi Matthias,
     
    VBUS levels can be found in the AM335X TRM. Low treshold is 4.4V. The important thing to understand is how this works, since it's an OTG peripheral. VBUS is detected after USBx_DRVVBUS is activated. There is no software workaround for this, it has to be done in hardware on the PCB. Typically a power switch is used (this can be seen on the reference designs). In your case an open-drain buffer with pull-up to 5V will be enough. If 5V is applied directly to the USBx_VBUS input the USB peripheral will go into device mode.
     
    So what you need to do is:
    1. Connect USBx_ID pins to GND.
    2. Add an open-drain non-inverting buffer (e.g. SN74LVC1G07 or SN74LVC2G07), with input driven by USBx_DRVVBUS and output pulled up with 4.7kOhm to 5V and connected to USBx_VBUS.
  • 0 in reply to Biser Gatchev-XID
    Prodigy 165 points

    Hi Biser,


    Good Morning.  I want to have one small clarification with respect to USB0_VBUS connection when the USB0 controller is in Host mode. We are planning to use USB0 as a Host in AM3352 Sithara processor . So the USB is required to supply a 5V power source to an attached device through its USB0_VBUS ( ZCZ Ball : P15) line. In order to achieve this task, the USB controller requires the use of an external power distribution switch is capable of sourcing 5V power. We decided to use Micrel power distribution switch of type MIC2025-1YM.

    The power distribution switch has the following important pins,

    Pin No 7 : IN - Supply Voltage(Input) - This will be connected to 5V line

    Pin No 2 : /FLG - Flag output - This will be connected to one of the GPIO lines of the processor in order to identify the short circuit condition

    Pin No 1 : EN - Enable input ,Active High - USB0_DRVVBUS ( ZCZ Ball : F16) will be connected to this pin.

    Pin No 6 & 8 : OUT - Supply (Output) - This pin will be connected to the USB connector. And also wheteher this pin has to be connected to the USB0_VBUS (ZCZ Ball : P15)??? .

    See the Notes from AM3352 schematic check list:

    Typical connections for a USB host:

    • USBx_DP and USBx_DM are connected directly to the USB connector
    • USBx_CE is typically not used and can be left unconnected
    • USDx_ID should be grounded
    • USBx_DRVVBUS should be connected to the enable of the 5V VBUS power source.
    • USBx_VBUS should be connected to the output of the 5V VBUS power source

    In AM3352 EVB scheme the connected USB0_VBUS (ZCZ Ball : P15) pin to the power distribution switch as a signal. Let me know your view in this.

    So where this USB0_VBUS pin should be connected. Whether this has to be connected to power switch output or USB connector alone or both??

    Waiting for your reply.

    Regards,

    Saravanan.K

  • 0 in reply to Saravanan Karivandan1
    TI__Guru 58105 points

    Biser is out of the office and DK has already answered this question on your replicated post.

    Regards,
    Paul