USBHost - Arduino Sketches: Tools and Techniques for Programming Wizardry - page 323

connectors, but if the user mistakenly inverted the keyboard and mouse, they
would not function. Simply unplugging and replugging into the correct port
resolved this. This left a serial connector free for other peripherals: modems,
PC-to-PC connectors, software dongles, joysticks, circuit programmers, parallel
port switchers, to name but a few—still far too many. Also, another interesting
event was occurring; some users wanted something that the designers hadn't
anticipated: two interface options, like one standard mouse for day-to-day
operations and either a track-pad or a mouse specialized for graphical work.
To simplify everything, Universal Serial Bus (USB) was created, which is a way
of connecting devices to a computer using a standard interface. Keyboards, mice,
scanners, modems … just about anything could be connected to the computer
using USB. Even better, if the amount of USB ports on the computer weren't
enough to add another peripheral, a USB hub could be used. A single USB con-
troller can have as many as 127 different ports by using hubs.
USB Protocol
For USB to work, it requires at least one USB host. This is the device that controls
USB devices, and USB devices communicate with the host. For a standard PC
setup, the PC is the USB host.
Devices connect to the host, and when they do, an enumeration occurs. Each
device connected is given a number from 1 to 127. When enumerated, the device
description is read, so the USB host knows what this device can do. Sometimes
drivers are needed to fully use a USB device; some devices do not require drivers
because the computer already knows what the device's function is. Several USB
classes exist, and one of them is called HID, short for Human Interface Device.
HID devices include keyboards and mice.
USB devices are “ hot pluggable ”; they can be connected while the system is
running. They can also be disconnected without the need for rebooting; when
unplugging your keyboard and plugging it into another USB port takes only a
few seconds for the computer to recognize the new port.
For desktop and laptop computers, the USB mechanism is simple; the computer
acts as the USB host, and a connected peripheral is a USB device. The computer
enumerates the USB device, and a connection is established. For some devices,
like mobile phones, this is a more complicated process.
Mobile phones lack the connectivity possibilities of a computer. They have a
single USB port, no disk drive, no CD drive, and limited capabilities for physical
input. Some phones can be used as USB drives; plug in the right kind of smart-
phone to a computer, and the telephone can use an internal SD card as a disk,
allowing the computer access to the i les. This is great when you want to copy
multimedia i les onto a telephone, but it isn't always practical. What happens
when you are far from your computer, when you have taken the perfect photo