USB基础教程：SILABS公司USB开发概述

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"USB概述——SILABS公司的USB开发基础知识" 本文档是SILABS公司提供的USB开发基础知识介绍，旨在帮助读者深入理解USB技术的基本原理、优势和应用。USB（通用串行总线）是一种广泛应用于各类设备间的连接标准，具有易用性、可靠性、速度和低功耗等优点。 USB的主要优势包括： 1. **易用性**：一个接口可支持多种设备，设备可以热插拔，即插即用，自动配置，无需额外电源。设备可以从总线上抽取高达500mA的电流。 2. **可靠性**：数据传输无损失，确保数据安全快速地传递。 3. **速度**：支持三种传输速度，分别是低速（1.5Mbps）、全速（12Mbps）和高速（480Mbps）。 4. **低功耗**：支持挂起模式，设备在该模式下可以显著降低能耗，USB2.0设备功耗不超过500μA，USB3.0设备不超过2.5mA。 5. **普及性**：微软和英特尔的PC2001系统设计指南规定所有新PC必须配备USB接口，这进一步推动了USB的广泛采用。文档内容涵盖以下几个方面： 1. **USB概述**：讲解USB的基本概念和发展历史，以及它如何在各种设备中实现互连。 2. **USB低级别数据传输**：探讨数据在物理层和数据链路层的传输方式。 3. **USB协议结构**：详细阐述USB协议的层次结构，包括控制传输、中断传输、批量传输和同步传输等。 4. **USB Chapter 9 结构**：这是USB设备驱动程序必须遵循的规范，描述了设备如何与主机通信和进行枚举过程。 5. **枚举**：设备连接到USB总线时的识别和配置过程。 6. **标准类**：介绍USB设备的常见类别，如HID（人机交互设备）、CDC（通信设备类）和MSC（存储设备类）等。 7. **固件示例**：提供实际的固件代码或示例，帮助开发者理解如何在设备上实现USB功能。 8. **Silicon Labs解决方案**：介绍SILABS公司的USB产品和解决方案，如何简化USB设计并提供支持。 9. **获取更多信息的途径**：列出进一步学习USB技术的资源和社区。通过这个课程，读者将能够理解USB的核心概念，并掌握如何利用SILABS的产品和工具进行USB设计。SILABS的目标是抽象出复杂的USB技术，使开发过程更加简单和高效。

Transfer Format

Data

Token

Handshake

Data

Token

Handshake

Data

Token

Handshake

Data

Token

Handshake

Data

Token

Handshake

Data

Token

Phases (Packets)

Guaranteed delivery time of packets for data streaming

No-retransmitting of data allowed

Periodic transfers on the time base conveyed during

enumeration

Host guarantees attention before this elapsed time

Non-critical data transfers

Bandwidth allocated to the host

Good for file transfer where time critical data is not required

Enables host to read configuration information, set addresses

and select configurations

Only transfer that is required to be supported by peripherals

Has both IN and OUT transfers to a single endpoint

Comments

Data (IN or

OUT)

Isochronous

Data (IN or

OUT)

Interrupt

Data (IN or

OUT)

Bulk

Status (IN or

OUT)

Data (IN or

OUT)

(optional)

Setup

Control

Stages

(Transactions)

Transfer Type

Now that we have discussed the hierarchy of the USB protocol lets take a look at the different

transfer types. Remember, that these transfers are made up of transaction which are made up of

packets. Each of the transfer types is listed in the table along with the associated transaction and

packet types supported. The control transfer is the only bidirectional transfer allowed by the USB. It

is the transfer type used to communicate all of the configuration information between the host and a

device. Our example we provide on the previous slides illustrated the control transfer. It had the

three transactions associated with it. All devices on the bus MUST support control transfers to

endpoint 0. This is the default endpoint after reset and is at address 0. For Control transfers the status

phase is a zero length packet in the opposite direction of the transfer. For example, let’s say the host

is going to request a device descriptor which is a table we have loaded in flash. The direction of the

transfer would be IN since the data flows to the host. The status phase in this example would be a

zero length packet with an OUT token to terminate the transfer. Control transfers are used for things

like setting the device address to a value other than 0 for subsequent accesses, getting configuration

information on how the device wants to communicate using the other transfer types like Bulk.

Remembering our original example, we can see here that there are three transactions (Setup, Data,

and Status) comprised of nine packets (Token, Data, Handshake – three times) that make up the

Control transfer. The Bulk transfer uses three packets for the single transaction.

The Bulk transfer is used for non-critical data meaning data that is not required to be transferred on

any time base. It provides the highest data throughput as well so it is useful for applications like

printers, scanners or even USB to UART bridges. Remember that the host is responsible for

managing the bandwidth of the USB. One aspect of the Bulk transfer is that the host schedules

bandwidth as it becomes available, hence the non-critical nature of this type of transfer.

The interrupt transfer is used for periodic transfers where a time period is requested by the device

and the host will guarantee that the data transfer will be scheduled within that time period. It does

not guarantee that the data is transferred on a consistent time basis all the time, just that it will

schedule the transfer prior to the time period expiration. The data throughput is lower for interrupt

transfers and is useful for applications like mice, keyboards where user input is continually needing

to be sent to the host.

USB Transfers—A Closer Look

 Control Write Transfers (OUT)—contains Setup, Data (optional), and

handshake transactions

SETUP

PID=1101

DATA0

PID=0011

ACK

PID=0010

OUT

PID=0001

DATA1/0

PID=1011

ACK

PID=0010

NAK

PID=1010

STALL

PID=1110

PID=1001

0-LENGTH

DATA1

PID=1011

ACK

PID=0010

NAK

PID=1010

STALL

PID=1110

Host sends Setup Packet followed by 8 bytes of data for

the request. The device must return an ACK.

Setup Transaction

Data Transaction

Status Transaction

Host sends OUT Packet along with the data for writes.

Device responds with handshake. No response indicates

a data error. DATA1 is first and alternates DATA0 and

DATA1.

Data error

Host sends IN Packet for Status phase. Device responds

with a 0 length packet to indicate the success or failure

of the transfer. Host ACK the status. No response

indicates a data error.

Data error

Host → Device

Device → Host

In the next set of slides we are taking a look at the transactions and packets that

make up the different transfers and this slide shows the Control Transfer that is

sending data to the device. The colored boxes denote which direction the data is

flowing, whether it is from the host or the device. Keep in mind the host initiates all

transfers so the first box will always be yellow. The first transaction is the Setup

which is defined by the setup packet. Next we see the data that is sent in the request

from the host. The data sent in this phase is 8 bytes long and contains info about the

request, for example, if the host wants a device’s configuration or to set the device

address. After the data transaction the device ACKs the transaction. Next is the

data phase. Since this is an OUT control transfer the Data is yellow. The device

can either ACK, NAK or Stall the request. The ACK signifies that the transaction

was successful, the NAK means that the data either was not received correctly or

the device is not ready for the data. A Stall would mean that the device does not

support the requested feature. The Status phase for control transfers is just a 0

length packet in the opposite direction of the data phase. In this diagram the host

was sending the device information (OUT transfer) in the data transaction,

therefore, the in the status phase the host sends an IN token and the device responds

with a 0 length packet. These diagrams and the following flow diagrams like it can

be found in the book from Jan Axelson titled, “USB Complete.”

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