USB(1): 基礎篇

本文主要是以系統軟體開發的角度出發整理相關資料，因此對於硬體特徵不會有過多著墨! 若對硬體細節有興趣可參考 USB Made Simple 和 USB in a NutShell 等材料。

Overview

USB(Universal Serial Bus) 是連接電腦與裝置的一種序列匯流排標準，也是用於電子裝置之間通訊的一種協定。

在 USB 被制定以前，外接式裝置的傳輸介面各不相同，如印表機只能接 Parrel Port、數據機只能接 RS232、滑鼠鍵盤只能接 PS/2 等。繁雜的介面系統，加上必須安裝驅動程式並重新開機才能使用的限制，都會造成使用者的困擾。因此，創造出一個統一且支援易插拔的外接式傳輸介面，便成為無可避免的趨勢，USB 應運而生。

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Parrel Port

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RS-232

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PS/2介面

USB 版本演進

USB 規範多年來一直在不斷更新，最主要的來自對更快通訊速度的需求。如下表總結了 USB 標準版本的演進與可提供的最快速度。

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An Introduction to USB Communication

筆者撰文當下 USB 已發展至 USB4.0，傳輸速度為 40Gbit/s

Connector Types

USB 之物理接口演進則如下圖。從號稱第一次一定會差錯的 Type-A，到至今在行動裝置上常見、正反皆可的 Type-C。並且各自可能又有 mini/micro 之分。

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USB(Universal Serial Bus)

延伸閱讀:
USB 種類比較｜一篇文章搞懂 USB Type A、B、C 的差異！

Cables and Signals

標準 USB 線(USB 2.x)的內部結構，可以看到裡面有紅、白、綠、黑四根小的導線。分別對應的訊號功能為:

訊號	顏色	功能
VBUS	紅	對連接的裝置供電(4.40 – 5.25 V)
D-	白	搭配 D+ 各自使用半雙工的差分訊號協同工作，以抵消長導線的電磁干擾，傳遞 NRZI 編碼的資料信號
D+	綠	- -
GND	黑	接地

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USB Topology

USB Bus 實體上是 tiered star 拓撲架構，在一個 USB Bus 上，可以容納一個 host 以及最多 127 個 devices (這是因為封包中的 address field 長度是 7 bits，而 address 0 不使用，因為它有特殊的用途)。

在 bus 上除了具有實際功能的 USB 裝置外，另一個重要的元件是 USB hub。它允許使用 1 個 upstream port 和最多 7 個 downstream port 傳遞訊息。由於延遲考量，最多只可以將五個 hub 依序連結。這導致 USB 網路中只多只能有七層。包含 host 所在的第 1 層，以及 hub 可以從第 2 層連結到第 6 層。

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留意到 USB hub 也佔用定址。因此舉例來說，如果我們有 20 個 hub，那麼最終實際裝置的最大可能數量為 127 – 20 = 107。

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USB Topology

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Universal Serial Bus Device(ScienceDirect)

USB Protocol

USB 是一種 polled bus，意思是由 host 發起所有傳輸開頭。則為了讓 host 從 USB 裝置接收訊息，它必須向裝置做 polling。此 polling 會組織成 frame 的方式定期執行，每 1ms 啟動一個新的 frame，另外還可以有 micro frame(每 125μs 啟動一次)。

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之前曾經提過，USB bus 上的每個裝置都分配一個唯一的 7 位元位址(實際上會是 1 到 127)。而 USB bus 上發送的所有內容都被建構為 packet。

Endpoints

在 USB 協定下，對 USB 的資料傳輸透過 Endpoint 進行。一個裝置可以擁有至多 16 個 OUT 和 16 個 IN 的 Endpoints。

OUT : 從 device 輸出
IN : 輸入至 device

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每個 Endpoint 只能有一個傳輸方向，除 Endpoint 0 是例外。Endpoint 0 用於控制傳輸命令及狀態操作。像是設定裝置、取得裝置資訊、發送指令到裝置等，可以有 IN 和 OUT 的雙向傳輸。

在 USB 中的 endpoint 根據類型可分四種:

Control: 即 endpoint 0，用於裝置控制和詢問狀態。一個裝置只能有一個，而且一定要支援
Bulk: 用於大量資料傳輸，如傳給印表機或隨身碟
- 有重送機制
- 無 bandwidth/latency 的保證
Interrupt: 定期傳輸，裝置端需要聲明這個期間是多久。適合事件驅動的裝置，像是 USB 鍵盤和滑鼠
- 保證裝置提供服務的最大週期
- 如果發送失敗，重送也會在下一個周期進行
Isochronous: 需要及時資料更新時使用，可大量傳。適合例如 USB 視訊裝置、喇叭、麥克風等重視即時性且可接受少量遺失資料的應用
- 資料如果錯誤無法修正或丟失，不會重送
- 有 bandwidth/latency 的保證

USB Packet

Packet 是 USB 交換訊息的最小元素，是用幾個 field 建構而成的。Field 分別有以下幾種:

SYNC – packets 總是以此 field 為開頭，用來同步 transmitter 和 receiver 之間的傳輸
PID – 標示 packet 的種類
ADDR – packet 之目標 USB 裝置的地址
ENDP – 標示 USB device 中目標的 endpoint
DATA – 傳輸的資料
CRC – 錯誤檢測用的 CRC
- CRC5(5bits) 用於 token packets
- CRC16(16bits) 用於 data packets
EOP – packets 結尾

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如上表，每個 PID 對應到四種 packet 類型的其中，並根據實際功能可再細分。

Token Packet

Token packets 只能由 host 發出，每個類型是特定的一種 transaction type。

OUT: Host 將會從 USB 裝置獲得資料
IN: Host 將會傳送資料給 USB 裝置
SETUP: Host 接著要做 control transfer

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SOF: 此 packet 將 USB bus 上的傳輸切分成數個 time frame，如下圖。在 full-speed 下，SOF 每 1 ms 發送一次。在 high-speed 下，每個 1ms frame 被分成 8 個 micro frame。frame 號碼每 1 ms 會增加一次

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Data Packet

顧名思義用於資料傳輸，又分為 DATA0、DATA1、DATA2、DATAM 類型。這種 packet 的來源可以是 host 或 device。

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Data Packet 的 DATA 大小可為零，而其最大值則受 endpoint descriptor 中的 wMaxPacketSize 限制。

Handshake Packet

用於確認 packet 的收發狀態(ACK, NACK 等)。

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Linux USB Device Driver

關於 USB device driver 的撰寫可參考 Linux 中的 Writing USB Device Drivers。

USB Descriptor

USB Made Simple - Protocal

USB in a NutShell - USB Descriptors

USB 裝置中包含許多 descriptor，如下圖。這讓 host 可以有辨別並設定裝置的資訊。

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USB Made Simple - Protocal

Device descriptor 代表整個裝置。因此，一個 USB 裝置只能有一個。它包含有關裝置的一些基本信息，例如支援的 USB 版本、最大 packet 尺寸、供應商和產品 ID 以及裝置可以具有的可能 configuration 數量等資訊。

Configuration descriptor 提供裝置的供電方式、最大功耗以及其具有的 interface 數量等資訊。一台裝置可以有多個 configuration descriptor，但同時間只能啟用其中一個。如果要更改，整個裝置必須暫時停止運作。它也描述了其包含的 interface 數量。

一台裝置可以有一個或多個 interface，由 interface descriptor 去描述。Interface descriptor 可以被視為裝置特定功能之 endpoint 的群組。而 endpoints 固然是由 endpoint descriptor 描述，前面我們已經介紹過 endpoint 代表每個資料輸入或輸出的單元。

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A tour of USB Device Controller (UDC) in Linux

Reference