Cypress EX-USB FX3 Programmer'S Manual Download

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FX3 Programmers Manual, Doc. # 001-64707 Rev. *C

Introduction to USB

2.1.5

Power Distribution and Management

Power management refers to the part of the USB Specification that spell out how power is allocated
to the devices connected downstream and how different communication layers operate to make best
use of the available bus power under different circumstances.

USB 2.0 supports both self and bus powered devices. Devices indicate this through their descriptors.
Devices, irrespective of their power requirements and capabilities are configured in their low power
state unless the software instructs the host to configure the device in its high power state. Low power
devices can draw up to 100 mA of current and high power devices can draw a maximum of 500 mA.

The USB host can 'suspend' a device to put it into a power-down mode. A 3 ms 'J' state (Differential
'1' indicated by D+ high D– low) on the USB bus triggers the host to issue a suspend request and
enter into a low power state. USB devices are required to enter a low power state in response to this
request.

When necessary, the device or the host issues a Resume. A Resume signal is initiated by driving a
'K' state on the USB bus, requesting that the host or device be taken out of its low power 'suspended'
mode. A USB device can only signal a resume if it has reported (through its Configuration
Descriptor) that it is 'remote wakeup capable', and only if the host has enabled remote wakeup from
that device.

This suspend-resume mechanism minimizes power consumed when activity on the USB bus is
absent

2.1.6

Device Classes

In an attempt to simplify the development of new devices, commonly used device functions were
identified and nominal drivers were developed to support these devices. The host uses the
information in the class code, subclass code, and protocol code of the device and interface
descriptors to identify if built-in drivers can be loaded to communicate with the device attached. The
human interface device (HID) class and mass storage class (MSC) are some of the commonly used
device classes.

The HID class refers to interactive devices such as mouse, keyboards, and joy sticks. This interface
use control and interrupt transfer types to transfer data because data transfer speeds are not critical.
Data is sent or received using HID reports. Either the device or the interface descriptor contains the
HID class code

The MSC class is primarily intended to transfer data to storage devices. This interface primarily uses
bulk transfer type to transfer data. At least two bulk endpoints for each direction is necessary. The
MSC class uses the SCSI transparent command set to read or write sectors of data on the disk
drive.

Details about other classes can be found at the Implementers forum website

http://www.usb.org

2.2

USB 3.0: Differences and Enhancements over USB 2.0

2.2.1

USB 3.0 Motivation

USB 3.0 is the next stage of USB technology. Its primary goal is to provide the same ease of use,
flexibility, and hot-plug functionality but at a much higher data rate. Another major goal of USB 3.0 is
power management. This is important for "Sync and Go" applications that need to trade off features
for battery life.

The USB 3.0 interface consists of a physical SuperSpeed bus in addition to the physical USB 2.0
bus. The USB 3.0 standard defines a dual simplex signaling mechanism at a rate of 5 Gbits/s.