30-05-2013, 04:07 PM
USB - Universal Serial Bus
USB.ppt (Size: 924 KB / Downloads: 30)
Motivations
Connection of the PC to Telephone
The USB provides a ubiquitous link that can be used across a wide range of PC-to-telephone interconnects.
Ease of use
Hot plug
Port expansion
The lack of a bi-directional, low-cost, low-to-mid speed peripheral bus has held back the creative proliferation of peripherals such as telephone/fax/modem adapters, answering machines, scanners, PDA’s, keyboards, mice, etc.
USB
Fast
Bi-directional
Isochronous
low-cost
dynamically attachable serial interface
consistent with the requirements of the PC platform of today and tomorrow
Goals for USB
The following criteria were applied in defining the architecture for the USB:
Ease-of-use for PC peripheral expansion
Low-cost solution that supports transfer rates up to 12Mb/s
Full support for real-time data for voice, audio, and compressed video
Protocol flexibility for mixed-mode isochronous data transfers and asynchronous messaging
Integration in commodity device technology
Comprehension of various PC configurations and form factors
Provision of a standard interface capable of quick diffusion into product
Enabling of new classes of devices that augment the PC’s capability.
Feature list
Easy to use for end user
Single model for cabling and connectors
Electrical details isolated from end user (e.g., bus terminations)
Self-identifying peripherals, automatic mapping of function to driver, and configuration
Dynamically attachable and re-configurable peripherals
Wide range of workloads and applications
Suitable for device bandwidths ranging from a few kb/s to several Mb/s
Supports isochronous as well as asynchronous transfer types over the same set of wires
Supports concurrent operation of many devices (multiple connections)
Supports up to 127 physical devices
Supports transfer of multiple data and message streams between the host and devices
Allows compound devices (i.e., peripherals composed of many functions)
Lower protocol overhead, resulting in high bus utilization
USB Interconnect
Bus Topology: Connection model between USB devices and the host.
Inter-layer Relationships: In terms of a capability stack, the USB tasks that are performed at each layer in the system.
Data Flow Models: The manner in which data moves in the system over the USB between producers and consumers.
USB Schedule: The USB provides a shared interconnect. Access to the interconnect is scheduled in order to support isochronous data transfers and to eliminate arbitration overhead.
Implementer Viewpoints
USB Physical Device: A piece of hardware on the end of a USB cable that performs some useful end user function.
Client Software: Software that executes on the host, corresponding to a USB device. This client software is typically supplied with the operating system or provided along with the USB device.
USB System Software: Software that supports the USB in a particular operating system. The USB System Software is typically supplied with the operating system, independently of particular USB devices or client software.
USB Host Controller (Host Side Bus Interface): The hardware and software that allows USB devices to be attached to a host.
Signals
Low and Full speed devices:
A differential ‘1’ is transmitted by pulling D+ over 2.8V with a 15K ohm resistor pulled to ground and D- under 0.3V with a 1.5K ohm resistor pulled to 3.6V.
A differential ‘0’ on the other hand is a D- greater than 2.8V and a D+ less than 0.3V with the same appropriate pull down/up resistors.
The receiver defines a differential ‘1’ as D+ 200mV greater than D- and a differential ‘0’ as D+ 200mV less thanD-.
The polarity of the signal is inverted depending on the speed of the bus. Therefore the terms ‘J’ and ‘K’ states are used in signifying the logic levels. In low speed a ‘J’ state is a differential 0. In high speed a ‘J’ state I s a differential 1.