Version 1 (modified by trac, 6 years ago) ( diff )


Universal Serial Bus (USB)

Universal Serial Bus (USB) is an industry standard developed in the mid-90's that defines cables, connectors and communication protocols used in a bus between computers and peripherals.



  • USB 1.0 - USB LowSpeed (1.5mbps) and USB FullSpeed (12mbps)
  • USB 2.0 - adds USB HighSpeed (480mbps)
  • USB 3.0 - adds USB SuperSpeed (10gbps)

Host Controllers

The following types of USB host controllers exist:

  • Universal Host Controller Interface (UHCI): Produced by Intel for USB 1.0 and USB 1.1. Using UHCI requires a license from Intel. This controller supports both LowSpeed and Full-Speed.
  • Open Host Controller Interface (OHCI): Produced for USB 1.0 and 1.1 by Compaq, Microsoft, and National Semiconductor. Supports Low-Speed and Full-Speed and tends to be more efficient then UHCI by performing more functionality in hardware.
  • Extended Host Controller Interface (EHCI): Created for USB 2.0 after USB-IF requested that a single host controller specification be created. EHCI is used for HighSpeed transactions and delegates Low-Speed and FullSpeed transactions to an OHCI or UHCI sister controller.

USB Speeds

The following is a set of supported speeds per USB specification:

  • USB LowSpeed (1.5mbps) (USB 1.0 UHCI/OHCI) - Only supports Control and Interrupt transfer types
  • USB FullSpeed (12mbps) (USB 1.0 UHCI/OHCI)
  • USB HighSpeed (480mbps) (USB 2.0 EHCI)
  • USB SuperSpeed (10gbps) (USB 3.0 XHCI)

USB Transfer types

There are four types of transfers in USB:

  • Control Transfers Used for sending commands to the device, make inquiries, and configure the device. This transfer uses the control pipe.
  • Interrupt Transfers Used for sending small amounts of bursty data that requires a guaranteed minimum latency. This transfer uses a data pipe.
  • Bulk Transfers Used for large data transfers that use all available USB bandwidth with no guarantee on transfer speed or latency. This transfer uses a data pipe.
  • Isochronous Transfers Used for data that requires a guaranteed data delivery rate. Isochronous transfers are capable of this guaranteed delivery time due to their guaranteed latency, guaranteed bus bandwidth, and lack of error correction. Without the error correction, there is no halt in transmission while packets containing errors are resent. This transfer uses a data pipe.

USB Device Classes

USB has a list of recognized and approved USB device classes in order to simplify driver development. The most common device classes are:

  • Human Interface Device (HID):
    • Supports Control and Interrupt transfer types
    • has a max FullSpeed rate of ~64KB/s
    • standard driver in all modern OS
  • Mass Storage Device (MSD)
    • standard driver in all modern OS
  • Communication Device Class (CDC)
    • Supports Bulk and Isochronous transfer types
    • has a max FullSpeed rate of ~80KB/s
    • standard driver in all modern OS
  • Vendor (Vendor Specific)
    • Supports all four transfer types
    • has a max FullSpeed rate of ~1MB/s
    • requires custom driver (but can be userspace through libusb)

Human Interface Device

Often vendors choose HID device class to allow control via userspace applications without needing a OS specific driver (as all modern OS's have an HID driver).

The HIDAPI (open-source and cross-platform Linux/Windows/MacOS) provides simple USB HID enumeration, read and write functions.

The HID device class uses HID reports to describe details about the data input and output a device supports. The sub class can be 'Generic HID' and the report type can be 'Vendor Defined' which makes it generic. The maximum report size is 64B.

Vendor Specific

Vendors sometimes choose to implement a Vendor specific device which has an undefined class.

The libusb (open-source and cross-platform Linux/Windows/MacOS) can be used for generic USB communication. Vendor specific commands can be implemented.

Gateworks USB devices

In an effort to define these options available to developers, Gateworks has created the following table with the fields as follows:

  • Family: Board Product family.
  • SBC: The base SBC (Single Board Computer) model that the proceeding rows apply to.
  • USB Port: The particular USB port and controller in question.
  • Ref: The reference part designator that can be found on the PCB silkscreen (white lettering).
  • Reset Type: How the usb device is re-enumerated and/or reset, which will be either:
    • Soft - Both data lines of the usb bus are driven low, signalling a reset condition that is handled differently from device to device.
    • VBUS - The 5V power rail of the port is brought down (when indicated with the manual control of a gpio).
  • Bus-Device[.Port]: The bus, device, and port (if behind a hub) of the port in question.
  • Bus Steering: The conditions for the bus to be electrically steered to this port if applicable.

In all cases the miniPCIe slots get their VBUS directly from the board power rail, which has no power control.

Gateworks USB device Table:

Family SBC USB Port Ref Reset Bus-Device[.Port] Bus Steering
Ventana GW51xx Front Panel OTG J20 Soft/VBUS 1-1
miniPCIe USB EHCI J6 Soft 2-1
GW52xx Front Panel OTG J15 Soft/VBUS 1-1 gpio2 low (default)
Bot Right miniPCIe EHCI J7 Soft 2-1
Bot Left miniPCIe OTG J8 Soft 2-1 gpio2 high
GW53xx Front Panel OTG J20 Soft/VBUS 1-1
Front Panel EHCI J18 Soft/VBUS 2-1.1
Bot Left miniPCIe EHCI J9 Soft 2-1.2
Bot Center miniPCIe EHCI J8 Soft 2-1.3
Bot Right miniPCIe EHCI J6 Soft 2-1.4
GW54xx Front Panel OTG J23 Soft/VBUS 1-1
Bot Right miniPCIe EHCI J6 Soft 2-1.1
Top Right miniPCIe EHCI J9 Soft 2-1.2
Top Left miniPCIe EHCI J7 Soft 2-1.3
Front Panel EHCI J21 Soft/VBUS 2-1.4
GW551x Adapter Board OTG J3 (J9 on Adapter) Soft 1-1
Bot miniPCIe EHCI J2 Soft 2-1
GW552x Bot Left miniPCIe EHCI J6 Soft 1-1.1
Bot Right miniPCIe EHCI J5 Soft 1-1.2
Front Panel EHCI J10 (Top) Soft/VBUS 1-1.3
Front Panel EHCI J10 (Bot) Soft/VBUS 1-1.4
GW553x Front Panel OTG J8 Soft/VBUS 1-1
miniPCIe EHCI J5 Soft 2-1
Laguna GW2380 MiniPCIe EHCI J5 Soft 1-1
GW2382 Front Panel EHCI J4 Soft/VBUS (gpio9) 1-1 gpio10 high (default)
MiniPCIe EHCI J5 Soft 1-1 gpio10 low
GW2383 Front Panel OTG J4 Soft/VBUS 1-1
MiniPCIe EHCI J6 Soft 1-2
GW2387 Front Panel OTG J12 (Top) Soft/VBUS (gpio6) 1-1
Front Panel EHCI J12 (Bot) Soft/VBUS (gpio6) 2-1 gpio5 low (default)
MiniPCI EHCI J5 (Bot) Soft 2-1 gpio5 high
GW2388 Front Panel OTG J19 (Top) Soft/VBUS (gpio6) 1-1 (loading option)(1)
Front Panel EHCI J19 (Bot) Soft/VBUS (gpio6) 2-1 (loading option)(1)
MiniPCI EHCI (1) J4 (Bot) Soft 2-1 (resistor loading option)(1)
GW2391 Front Panel OTG J14 (Top) Soft/VBUS (gpio6) 1-1 gpio5 low (default)
Front Panel EHCI J14 (Bot) Soft/VBUS (gpio6) 2-1 gpio7 low (default)
MiniPCIe OTG J4 (Bot) Soft 1-1 gpio5 high
MiniPCIe EHCI J3 (Top) Soft 2-1 gpio7 high
Cambria GW2350 Front Panel EHCI J9 (Top) Soft/VBUS 1-1
Front Panel EHCI J9 (Bot) Soft/VBUS 2-1
MiniPCI EHCI (1) J4 Soft 2-1 (resistor loading option)(1)
GW2358 Front Panel EHCI J16 (Top) Soft/VBUS 1-1
Front Panel EHCI J16 (Bot) Soft/VBUS 2-1
MiniPCI EHCI (1) J8 Soft 2-1 (resistor loading option)(1)
Avila GW2348-SP107 Front Panel EHCI J18 (Top) Soft/VBUS 1-1
Front Panel EHCI J18 (Bot) Soft/VBUS 2-1
MiniPCI EHCI (1) J4 Soft 2-1 (resistor loading option)(1)
  1. This is a loading option available on specials - contact ​sales@…
    • Cambria GW2350 (Bottom MiniPCI socket J4)
      • R164, R163 = Load to route USB to MiniPCI connector J8 (bottom slot)
    • Cambria GW2358 (Bottom MiniPCI socket J8)
      • R146, R153 = Load to route USB to MiniPCI connector J8 (bottom slot)
      • R138, R140 = Load to route USB to frontpanel connector J16 (top USB port)
    • Laguna GW2388-D+ (Bottom MiniPCI socket J4)
      • R240, R244 = Load to route USB to MiniPCI connector J4 (bottom slot) (default on GW2388-4)
      • R241, R242 = Load to route USB to frontpanel connector J19 (top USB port) (default on GW2388-SP208, SP212 and SP213 w/front panel USB loaded)

USB Steering

Some boards allow USB to be steered via a software controlled gpio. Refer to the 'bus steering' column for details which show which gpio to use, what the default is set to (via bootloader), and where it can be steered to. Typically a bus may be steerable between a front-panel connector and a miniPCIe socket in which case the front-panel is always the default configured by the bootloader. You can change the steering at run-time if you whish which would be like physically removing a USB device from the bus and adding another (re-enumerates).


  • Ventana GW52xx: steer the USB OTG bus to the miniPCIe socket (device-mode only) which uses gpio2:
    # export and configure the GPIO as an output high
    echo 2 > /sys/class/gpio/export
    echo out > /sys/class/gpio/gpio2/direction
    echo 1 > /sys/class/gpio/gpio2/value # 0=front-panel 1=miniPCIe-j8
  • Laguna GW2387/GW2388/GW2391: steer USB OTG bus to miniPCIe socket (device mode only) which uses gpio 5:
    echo 1 > /sys/class/gpio/gpio5/value # 0=front-panel 1=miniPCIe

USB Reset

The best way to reset a USB device is to 'hot plug' the device by physically removing and reinserting it. However this is not typically possible and there are other options.

Note that BUS Steering can sometimes be a useful alternative to resetting a device as it forces a re-enumeration of the device.

VBUS reset

The next best thing to physically removing and re-inserting a USB device is to toggle power to that device. While this is not possible on the USB bus routed to miniPCIe sockets (which run off an always 3.3V power rail) this is possible on:

  • OTG ports with Host capability (because they must have the ability to turn on and off their VBUS)
  • Front panel ports that have over-current/fault-detect capability
  • Ports that have a gpio controlled VBUS

Ports provided by a USB OTG controller ('OTG' in USB Port name) which support host mode can have their VBUS reset by unbinding and rebinding the host controller via sysfs:

  • Ventana (where the OTG controller is '2184000.usb'):
    echo 2184000.usb > /sys/bus/platform/drivers/imx_usb/unbind
    sleep 1
    echo 2184000.usb > /sys/bus/platform/drivers/imx_usb/bind
  • Laguna (where the OTG controlleris 'dwc2.0'):
    echo dwc2.0 > /sys/bus/platform/drivers/dwc2/unbind
    sleep 1
    echo dwc2.0 > /sys/bus/platform/drivers/dwc2/bind

Ports provided by a USB Hub's (will have a '.<port>' in their bus-device.port entry in the table above) with over-current/fault detection such as those found on a GW530x/GW540x can have their VBUS reset by unbinding their upstream bus-dev from the USB bus:

echo 2-1 > /sys/bus/usb/drivers/usb/unbind
sleep 1
echo 2-1 > /sys/bus/usb/drivers/usb/bind
  • this is not possible for MiniPCIe sockets as their VBUS comes from the 3.0V rail

Ports with GPIO controlled VBUS such as the Laguna boards can be reset by turning them on/off via gpio control:

  • GW2382 (gpio9):
    echo 0 > /sys/class/gpio/gpio9/value # de-assert GW2382 VUSB_EN
    sleep 1
    echo 1 > /sys/class/gpio/gpio9/value # assert GW2382 VUSB_EN
  • GW2387/GW2388/GW2391 (gpio6):
    echo 0 > /sys/class/gpio/gpio9/value # de-assert GW2387/GW2388/GW2391 VUSB_EN
    sleep 1
    echo 1 > /sys/class/gpio/gpio9/value # assert  GW2387/GW2388/GW2391 VUSB_EN

Refer to the USB device table above for the bus-dev and/or VBUS capabilities per board.

Soft reset via sysfs bind/unbind (re-enumerate)

In some scenarios (ie Mini-PCIe modem) having command line options to reset these devices via sysfs is preferable and can be found via modern kernel drivers. Refer to the USB Device Table above for the bus-device.port reference for your board/port.

Note that commands targeting a hub cause a soft reset for all child devices which include the front panel and any pcie ports with "usb" on the silk screen. VBUS is only brought down on the front panel port(s) connected to the internal hub.


  • Reset GW54xx Front panel OTG device (bus-device 1-1). Note this will cause VBUS to be toggled for a full power reset - see above:
    echo 1-1 > /sys/bus/usb/drivers/usb/unbind
    sleep 1
    echo 1-1 > /sys/bus/usb/drivers/usb/bind
  • Reset GW54xx EHCI devices by unbinding/binding the USB hub (bus-device 2-1). The bot right, top right, top left miniPCIe sockets will get a 'soft reset' (because they power off 3P3V which can't be toggled) and front panel EHCI will get a full VBUS power cycle:
    echo 2-1 > /sys/bus/usb/drivers/usb/unbind
    sleep 1
    echo 2-1 > /sys/bus/usb/drivers/usb/bind
  • Reset GW54xx EHCI device in the top right miniPCIe socket J6 (bus-device 2-1.1):
    echo 2-1.1 > /sys/bus/usb/drivers/usb/unbind
    sleep 1
    echo 2-1.1 > /sys/bus/usb/drivers/usb/bind

Soft reset via ioctl

There are also ioctls that can accomplish the same results programmaticlly and in some cases give even finer control than the above table. An example of a simple program that uses some of these ioctls is the usbreset command (present in OpenWrt):

Further information regarding these ioctls can be found throughout the Linux source documentation.

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