In SoC I'm working on there is USB EHCI Compatible Controller.
Does all EHCI Controllers can work as Host or Device ?
Does EHCI Linux driver cover device/gadget mode for such controller ?
EHCI is a Host controller only, there is no specified way for one to function as a device, and it does not support USB OTG. Your SoC's controller most likely provides an EHCI compatible interface for the Host controller, using a different interface for the Device controller.
Related
Wondering how can the PC distinguish among multiple USBs. For example, when i plug my flash drive USB into my computer how can it know that it is a flash drive USB not a keyboard USB ?
Thanks in advance.
in the USB standard there are USB classes a memory stick is USB mass storage class, keyboard, mouse, joystick are USB HID class (Human Interface Device), cameras implement USB PTP (picture transfer protocol), USB-to-serial (virtual COM ports,...) is USB CDC ACM class ,...
the following link lists all USB classes for that windows has drivers https://msdn.microsoft.com/en-us/library/windows/hardware/ff538820%28v=vs.85%29.aspx
when the USB device is plugged in there is a communication between the MCU on the USB device (firmware) and the PC (USB host) . in this communication the device says which USB class it it and the host loads the fitting driver
you can sniff this communication using wireshark or see dmesg (on linux)
this communication is very low-level and not easily human-readable. there are tools like wireshark or lsusb -v for this
a USB device has to implement a special hierarchy of descriptors in which information of the internal structure of the USB device is contained. the hierachy is:
device descriptor -> configuration descriptor -> interface descriptor -> endpoint descriptor
EP0 is reserved for control transfers, else any transfer has to go to or come from an endpoint (OUT endpoint / IN endpoint)
see http://www.beyondlogic.org/usbnutshell/usb1.shtml
USB is a higly complex standard because it implements a wide functionality for a large number of different devices in one physical interface. i think the most complicated USB class standard is USB audio...
I have an xHCI USB host controller connected to processor with an OS that has only eHCI host driver support. Does the eHCI driver support/compatible with xHCI controller. Only USB2.0 devices are required to be connected to the host. USB3.0 device support is not necessary.
Its completely impossible as there is a huge difference between the EHCI and XHCI interface. The whole way of USB transfers have changed. The data structures changed. Only Register interface remained compatible.
Its simply impossible. I am guessing by now you already know the answer.
When I plug a USB web Camera into a PC via USB, what communications is used? I know each device COULD have its own uniques driver, but it seems as though 99% use a common built in driver now. Where is it spec'd and how do you get a linux embedded device to act like a webcam?
What you're looking for is the USB Video Class and you can download the specification for this and other standard USB device classes here:
http://www.usb.org/developers/docs/devclass_docs/
The Linux USB gadget framework has code to emulate such a device:
https://github.com/torvalds/linux/blob/master/drivers/usb/gadget/function/f_uvc.c
I want to connect a USB peripherals to a FPGA. Basically FPGA should act like an USB host. Is there a FPGA board support a USB hub so that one could connect multiple(upto 4) USB peripherals at a time.
I have a Digilent Nexys3 fpga which is based on Spartan 6. It supports only one USB device (keyboard or mouse). It doesn't support a hub. I have found Cypress host controllers, but I am not sure how to use it in a FPGA.
You need to have a USB host controller inside your FPGA, such IP is not freely available, one alternative is to use a Zynq based (for Xilinx) or Cyclone V based (for Altera) board. Those have integrated USB controllers connected to their dual ARM core.
You can find more info about those at:
Altera Cyclone V
Xilinx Zynq
You maybe able to use an external host controller but then you have to connect it to your FPGA and that is usually requires a lot of IOs and those modules are more expensive than buying a Zed board ($395) or Cyclone V board ($450).
A device has USB OTG support. Following are scenarios:
When a device is connected to a PC, the device acts as slave. (How does the device know it has to act as slave?)
When a device is connected to a printer, the device acts as master.
(How does the device know it has to act as master?)
What are the steps executed when a device is connected to OTG?
How to implement this mechanism (in brief)?
The exact behavior of USB OTG devices is described in the specification you can find at usb.org. There is a PDF inside the zip called USB_OTG.
The Host Negotiation Protocol in section 6 covers how two OTG devices decide which one is getting the embedded host. Basically this is archived by driving pull-up and pull-down resistors on the D+ line.
Note: When talking about USB the terms master/slave are not used. The master is called host and powers the bus whereas the slave is called device. In the case of OTG (in general, see specification for exceptions) both parts have the capability to be host or device. When the host was figured out by the Host Negotiation Protocol, this part becomes the so called embedded host.
In the two scenarios you mentioned the USB device is able to know whether to be a host or device by the cable. USB cables (non type-C) are not symmetrical. One side is host and the other is device. There is a pin on the connector called the ID pin which is floated on the device side and grounded on the host side. This allows the USB controller on each side to know which side of the cable it is connected to and thus which role (host or device) it should be when connected. These types of devices are called dual-role devices.
If you have a such a device, then you can plug it into a regular host (like your laptop) and it will act as a device. And you can plug it into a regular device (like a printer) and it will act as a host. This is all based on the cable.
If you plug two dual-role OTG devices to each other. Their initial roles are determined by the cable in the same way.
After the initial roles are determined, they can then swap roles from their initial cable-determined roles via the Host-Negotiation Protocol (HNP).
As far as implementing this. There is no brief way to explain it. Every controller is different and you would have to read the controller databook and programming model to implement all these procedures. As well as having a good understanding of the USB and OTG specs themselves.
A good place to start is http://usb.org where you can find all the specs.