How USB device is detected, identified and ready to use for applications?

-
What is USB device enumeration?

USB device enumeration is the process of detecting, identifying and loading drivers for USB device. 

USB host controller queries the device in order to decide what type of device it is in order to attempt to assign an appropriate driver for it. 

Some of the basic commands issued by the host to the device are:
  • Set Address – Instructs the device change it’s current address settings
  • Get Device Descriptor – Overall information about the device (manufacture, firmware version …)
  • Get Configuration Descriptor – How the endpoints will be used
  • Get Interface Descriptor – Various different interface that the device may use
  • Get String Descriptor – Unicode strings for Manufacture and Product

This process is a crucial for every USB device and without it the device would never be able to be used by the OS.


We will go step by step to understand the whole process.


Detection of device
As you may know that USB interface has 4 wires.
  1. Power
  2. Ground
  3. D+
  4. D-
USB host port with no device connected uses 15kohm resistors to connect D+ and D- to GND.


Whenever an USB device is attached to an USB host port, there is status change on these data lines and this status change is used to detect a device by USB host. This change also helps USB host to determine speed of device.

Determining the device speed 
Low speed(1.5Mbps) uses 1.5k pull-up register to Vcc on the D- line 
Full speed(12Mbps) uses 1.5k pull-up register to Vcc on the D+ line 

A high speed USB device (480Mbps) will initially appear as a full speed device to the host. The first thing the USB host does is to attempt to send /receive packets at high speed to the USB device. This is known as J and K chirp and if communication is successful it will be assumed that the USB device is a high speed device. If this initial communication fails then the USB host assumes that the device is a full speed device.

This means a high speed device has a 1k5 pull up resistor on USB DP that can be switched in / out of circuit.

A J state is defined as a differential signal on USBD+ and USB D- >= +300mV.
A K state is defined as a differential signal on USBD+ and USB D- >= -300mV. 


Identification of device
Devices are identified by device descriptor.

Once the USB host has determined that device is connected and at what speed it should communicate, USB host resets the device and try to read the device descriptors to identify the device. 

Device descriptor has following fields: 
  • bLength
  • bDescriptorType
  • bcdUSB
  • bDeviceClass
  • bDeviceSubClass
  • bDeviceProtocol
  • bMaxPacketSize
  • idVendor
  • idProduct
  • bcdDevice
  • iManufacturer
  • iProduct
  • iSerialNumber
  • bNumConfigurations
After receiving device descriptor information USB host will ask for configuration and interface descriptor. I am not covering these in this post.

Loading the driver
When the USB device has been fully identified by the USB host, then the host PC needs a driver to control the USB device. Matching the USB device to the driver is usually done by matching up the VID and PID.

When the driver has been loaded, then the USB device is available for applications to access.

References:

Comments

Post a Comment

Popular posts from this blog

MBR partitioning with 'parted' utility

-
Image
As you might know, linux provides a very handy utility i.e. parted to create, destroy, resize, move and copy(and much more) partitions. So, in this post I will explain how parted is used to create partitions for MBR scheme. Let's commence with some pre-requisites, Machine running linux to perform operations Any storage device like HDD, USB etc. on which we will perform various operations I am using USB stick for this purpose and will carry out all operations on that. I strongly recommend you to use a external disk which doesn't have any important data and if it has, you must take backup before performing any of the following operations. If you do not have much idea about MBR partitioning schemes, Please refer this post first to get basic understanding of disk partitioning. NOTE: You need to have root permission to carry out all the following operations. First thing we do here is to check what's there inside the storage disk. We insert USB drive t...
Read more

Disk Partitioning: MBR vs GPT

-
Image
What is disk partitioning? Disk Partitioning is a process of creating different regions or blocks, commonly referred as partitions of mass storage devices like fixed disks and removable disks. Why disk partitioning is required? The whole purpose of Disk Partitioning is to let Operating System(OS) manage information/data in each region/partition separately. There are two different disk partitioning schemes as follows: MBR (Master Boot Record) GPT (GUID Partition Table) MBR (Master Boot Record) MBR is a special type of boot sector resides in partitioned mass storage device. The concept of MBR was publicly introduced in 1983 with PC DOS 2.0 (From Wikipedia). Conventionally, sector size is 512 bytes and first sector of any storage drive contains MBR if that drive is partitioned with MBR scheme. MBR holds minimum boot loader code, primary/logical/ extended partition entry information, file system type, boot signature etc. The structure of classical MB...
Read more

Replace default splash screen in Yocto

-
Image
On boot, if you have observed a splash screen with yocto logo appears in core-image-sato. This post is all about to change that splash screen with your own customized image.  Splash screen is handled by a recipe called psplash found under "poky/meta-poky/recipes-core" directory of the source tree.  Since psplash expects an image to be in header file format, first you need to convert your image into a header file format by using script called "make-image-header.sh" Here is what  make-image-header.sh contains, source -  http://git.yoctoproject.org/cgit/cgit.cgi/psplash/tree/make-image-header.sh You could replace your generated image header in  poky/meta-poky/recipes-core/psplash/files with existing one but I would recommend to create a recipe in your own layer and add this image file there. Steps to be followed, Get your own splash screen image(I took .jpg image) Convert that into header file by using script described above ./ mak...
Read more