USB Mass Storage
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A USB device controller enables a mass-storage device to share its data with other computers. For example, a data logger can collect data in the field and then connect to a PC, where an application reads the data from the logger’s storage media. Or a robot can attach to a PC to receive a file containing configuration data to use in robotic tasks. Flash-memory cards provide convenient storage for many small systems. Other systems function as USB hosts that can access files in off-the-shelf USB flash drives and hard drives. If you’re involved with designing or programming devices that incorporate a USB mass-storage device or host interface, this book will help you get your projects up and running. You’ll also find the book useful if you’re designing or programming devices that use flash-memory cards for data storage, whether or not the devices have USB interfaces. This book include code examples written for the Microchip PIC18F4550 microcontroller using Microchip’s MPLAB® C18 C compiler.  287 pages. Interfaces, Protocols, and Technologies  Designing and programming a USB mass-storage device or embedded USB host involves a variety of interfaces, protocols, and structures. Every USB mass-storage device must support two interfaces: A USB device interface to enable the device to communicate with a PC or other USB host. An interface between the device’s microcontroller or other CPU and the storage media. Flash-memory cards typically use the Serial Peripheral Interface (SPI), MultiMediaCard bus, SD-Card bus, or a bus derived from the ATA interface or PC-Card bus. Hard drives typically use the ATA parallel interface. A USB mass-storage device must implement these protocols and structures: Generic USB protocol. Every USB device must respond to requests sent by the USB host and other events on the bus. USB mass-storage protocol. Every USB mass-storage device must detect and respond to requests that are specific to the USB mass-storage class. SCSI commands. USB hosts access mass-storage devices via commands originally developed for devices that use the Small Computer Systems Interface (SCSI). Media-specific protocol. The storage media’s controller typically supports a command set for accessing the media’s contents. Many flash-memory cards use the MultiMediaCard protocol or the SD Card protocol. Hard drives use the ATA protocol. Reading and writing data to a mass-storage device also involves understanding logical structures in the media: Media structure. Program code accesses the storage area in drives as a series of logical blocks, or sectors. Dedicated areas in the media store information about the logical blocks and other logical structures in the media. File system. If the device firmware reads or writes to files on its own, rather than via a USB host, the device must implement a file system such as FAT16 or FAT32. This book shows how to put all of these interfaces, protocols, and structures to work in a USB mass-storage device. The book assumes you have a basic familiarity with microcontroller programming and interfacing. For more about USB, I recommend my book, USB Complete: Everything You Need to Develop Custom USB Peripherals. Contents for USB Mass Storage Introduction 1 Mass Storage Basics When to Use a Storage Device Benefits Other Considerations Requirements Devices Embedded Hosts Selecting a Media Type Drive Mechanisms Addressing Methods Reading and Writing Considerations Removable Media and Devices Hardware Interfaces Hard Drives Technology Interfaces Flash Memory Technology Options for Flash Memory MultiMediaCard SD Memory Card CompactFlash 2 Supporting USB The Interface in Brief Hosts and Devices Host Responsibilities Device Responsibilities Bus Speeds Endpoints Transfer Types Transactions The Data Toggle Descriptors Mass Storage Requirements Choosing a Device Controller Controllers with Support for Flash Memory Controllers with support for ATA/ATAPI Firmware Options Microchip PIC18F4550 Architecture Firmware Support The USB Controller 3 The USB Mass Storage Class Requirements Specifications Logical Block Addressing Mass Storage Requests Descriptors Device Descriptor Configuration Descriptor Interface Descriptor Endpoint Descriptors String Descriptors Responding to Commands The Command Block Wrapper The Command Status Wrapper Managing Communications on the Bulk Endpoints More about STALL Thirteen Cases for Any Situation PC Support Windows Linux 4 Accessing Flash Memory Cards The Interface Signals and Power Example Circuit Host Programming Configuring Hardware Ports Firmware-controlled Ports Transferring Data Default States SPI on the PIC18F4550 Configuring the Port Writing a Byte Reading a Byte 5 MultiMediaCard Protocol Command and Response Formats Commands Response Types Token Formats The Commands Classes Commands Used by Mass-storage Devices Registers Sending Commands Timing Considerations Commands with No Data Transfer Commands that Read Data from the Storage Media Commands that Write Data to the Storage Media Application Example Detecting and Selecting a Card Sending a Command Reading a Sector Writing a Sector Initializing Communications 6 SCSI Commands About the Commands Specifications Which Commands to Implement? Sense Data Fixed-format Sense Data Setting Default Values Primary Commands INQUIRY MODE SELECT MODE SENSE PREVENT ALLOW MEDIUM REMOVAL REPORT LUNS REQUEST SENSE SEND DIAGNOSTIC TEST UNIT READY Block Commands FORMAT UNIT READ READ CAPACITY START STOP UNIT SYNCHRONIZE CACHE VERIFY WRITE Multimedia Commands READ FORMAT CAPACITIES READ TOC/PMA/ATIP Handling Commands and Events Decoding Commands The UNIT ATTENTION Condition Informing the Host about Media Changes Reset Behavior 7 Media Structure A Look Inside Components of Formatted Media Drives without an MBR Sector Byte Order The Master Boot Record Sector Executable Code The Partition Table Extended Partitions The Boot Signature 8 FAT File Systems Inside a FAT16 Volume Reserved Region File Allocation Table Region Root Directory Region File and Directory Data Region Inside a FAT32 Volume Reserved Region File Allocation Table Region File and Directory Data Region Selecting a File System Cluster Sizes A Hardware Solution The File Allocation Table The First Two Entries Data Clusters Accessing the FAT Volume Information File Information Obtaining a Cluster’s Logical Block Address Reading from the FAT Writing to the FAT Finding a File’s Next Cluster Performing Sequential Reads Finding an Empty Cluster 9 Directories The Contents of an Entry File Entries Directory Entries The Volume Label Entry Subdirectory Entries Handling Long File Names LFN Entries The Checksum Creating a Short File Name Using Directories Storing an Entry Reading an Entry Getting the Main Entry Updating an Entry Updating the Time and Date 10 File Operations Cluster Operations Erasing a Cluster Reserving an Available Cluster Allocating a File’s First Cluster Allocating Additional Clusters Managing Files Obtaining File Information Finding a File Creating a Directory Entry Finding an Available Entry Creating a File Deleting a File Opening a File Reading from a File Tasks Performing a Read Operation Writing to a File Tasks Performing a Write Operation Closing a File 11 Embedded Hosts Inside an Embedded Host OTG Devices and Conventional Hosts General Host Functions Mass Storage Functions Handling Non-compliant Devices Host Options Cypress EZ-HOST Host Software A Mass-storage Host Module Index