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USB Device MSC Class

• USB Device MSC Class Overview
• USB Device MSC Class Resource Needs from Core
• USB Device MSC Class Configuration
• USB Device MSC Class Programming Guide
• USB Device MSC Class Storage Drivers

This section describes the mass storage device class (MSC) supported by Silicon Labs USB Device. MSC is a protocol that enables the transfer of information between a USB device and a host. The information being transferred is anything that can be stored electronically, such as executable programs, source code, documents, images, configuration data, or other text or numeric data. The USB device appears as an external storage medium to the host, enabling the transfer of files via drag and drop.

A file system defines how the files are organized in the storage media. The USB mass storage class specification does not require any particular file system to be used on conforming devices. Instead, it provides a simple interface to read and write sectors of data using the Small Computer System Interface (SCSI) transparent command set. As such, operating systems may treat the USB drive like a hard drive, and can format it with any file system they like.

The USB mass storage device class supports two transport protocols, as follows:

• Bulk-Only Transport (BOT)

• Control/Bulk/Interrupt (CBI) Transport (used only for floppy disk drives)

The mass storage device class implements the SCSI transparent command set using the BOT protocol only, which signifies that only bulk endpoints will be used to transmit data and status information. The MSC implementation supports multiple logical units.

The MSC implementation is in compliance with the following specifications:

• Universal Serial Bus Mass Storage Class Specification Overview, Revision 1.3 Sept. 5, 2008.

• Universal Serial Bus Mass Storage Class Bulk-Only Transport, Revision 1.0 Sept. 31, 1999.

USB Device MSC Class Overview

• Protocol

• Endpoints

• Class Requests

• Small Computer System Interface (SCSI)

Protocol

In this section, we will discuss the Bulk-Only Transport (BOT) protocol of the Mass Storage Class. The Bulk-Only Transport protocol has three stages:

• The Command Transport

• The Data Transport

• The Status Transport

Mass storage commands are sent by the host through a structure called the Command Block Wrapper (CBW). For commands requiring a data transport stage, the host will attempt to send or receive the exact number of bytes from the device as specified by the length and flag fields of the CBW. After the data transport stage, the host attempts to receive a Command Status Wrapper (CSW) from the device that details the status of the command as well as any data residue (if any). For commands that do not include a data transport stage, the host attempts to receive the CSW directly after CBW is sent. The protocol is detailed in Figure - MSC Protocol.

Figure - MSC Protocol

Figure 12 MSC Protocol

Endpoints

On the device side, in compliance with the BOT specification, the MSC is composed of the following endpoints:

• A pair of control IN and OUT endpoints called default endpoint.

• A pair of bulk IN and OUT endpoints.

The table below indicates the different usages of the endpoints.

Table - MSC Endpoint Usage

Class Requests

There are two defined control requests for the MSC BOT protocol. These requests and their descriptions are detailed in the table below.

Table - Mass Storage Class Requests

Small Computer System Interface (SCSI)

At the programming interface level, the MSC device implements one of the standard storage-media communication protocols, like SCSI and SFF-8020i (ATAPI). The "Programming Interface" specifies which protocol is implemented, and helps the host operating system to load the suitable device driver for communicating with the USB storage device. SCSI is the most common protocol used with USB MSC storage devices. We provide an implementation for MSC SCSI subclass that our GSDK users can use out of the box.

SCSI is a set of standards for handling communication between computers and peripheral devices. These standards include commands, protocols, electrical interfaces and optical interfaces. Storage devices that use other hardware interfaces, such as USB, use SCSI commands for obtaining device/host information and controlling the device’s operation and transferring blocks of data in the storage media.

SCSI commands cover a vast range of device types and functions and as such, devices need a subset of these commands. In general, the following commands are necessary for basic communication:

• INQUIRY

• READ CAPACITY(10)

• READ(10)

• REQUEST SENSE

• TEST UNIT READY

• WRITE(10)

USB Device MSC Class Resource Needs from Core

Each time you add an MSC class instance to a USB configuration via the function sl_usbd_msc_add_to_configuration(), the following resources will be allocated from the core.

Note that those numbers are per configuration. When setting up your SL_USBD_INTERFACE_QUANTITY, SL_USBD_ALT_INTERFACE_QUANTITY, SL_USBD_INTERFACE_GROUP_QUANTITY and SL_USBD_DESCRIPTOR_QUANTITY configuration values, don't forget to take into account on how many configurations the class will be added. For the SL_USBD_OPEN_ENDPOINTS_QUANTITY configuration value, since endpoints are opened only when a configuration is set by the host, you just need to take into account the number of needed endpoints for a class instance.

USB Device MSC Class Configuration

Two groups of configuration parameters are used to configure the MSC class:

• USB Device MSC Class Application-Specific Configurations

• USB Device MSC Class Logical Unit Configuration

USB Device MSC Class Application-Specific Configurations

• Class Compile-Time Configurations

• Class Instance Creation

Class Compile-Time Configurations

Silicon Labs USB Device MSC class and SCSI subclass are configurable at compile time via #defines located in the sl_usbd_core_config.h file.

Table - Generic Configuration Constants

Class Instance Creation

Creating a USB Device MSC SCSI class instance is done by calling the sl_usbd_msc_scsi_create_instance() function. This function takes one configuration argument that is described below.

p_scsi_callbacks

p_scsi_callbacks is a pointer to a configuration structure of type sl_usbd_msc_scsi_callbacks_t. In addition to the common usb device class callbacks connect/disconnect, it provides the MSC class with a set of optional callback functions that are called when an event occurs on the logical unit. A null pointer (NULL) can be passed to this argument if no callbacks are needed.

The table below describes each configuration field available in this configuration structure.

Table - sl_usbd_msc_scsi_callbacks_t Configuration Structure

USB Device MSC Class Logical Unit Configuration

Adding a logical unit to an MSC class instance is done by calling the function sl_usbd_msc_lun_add(). This function takes one configuration argument that is described below.

p_lu_info

p_lu_info is a pointer to a structure of type sl_usbd_msc_scsi_lun_info_t. Its purpose is to provide the information on the logical unit to the MSC class.

The table below describes each configuration field available in this configuration structure.

Table - sl_usbd_msc_scsi_lun_info_t Configuration Structure

USB Device MSC Class Programming Guide

This section explains how to use the MSC class.

• Initializing the USB Device MSC Class

• Adding a USB Device MSC SCSI Class Instance to Your Device

• USB Device MSC Class Logical Unit Handling

Initializing the USB Device MSC Class

To add MSC SCSI class functionality to your device, first initialize the MSC base class and the SCSI subclass by calling the function sl_usbd_msc_init() and sl_usbd_msc_scsi_init().

The example below shows how to call sl_usbd_msc_init() and sl_usbd_msc_scsi_init().

Example - Calling sl_usbd_msc_init() and sl_usbd_msc_scsi_init()

sl_status_t  status;

status = sl_usbd_msc_init();
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

status = sl_usbd_msc_scsi_init();
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

Adding a USB Device MSC SCSI Class Instance to Your Device

To add MSC SCSI class functionality to your device, first create an instance, then add it to your device's configuration(s). You must add at least one logical unit to your instance.

Creating an MSC SCSI Class Instance

Create a MSC SCSI class instance by calling the function sl_usbd_msc_scsi_create_instance().

The example below shows how to call sl_usbd_msc_scsi_create_instance() using default arguments. For more information on the configuration arguments to pass to sl_usbd_msc_scsi_create_instance(), see USB Device MSC Class Application Specific Configurations .

Example - Calling sl_usbd_msc_scsi_create_instance()

uint8_t        class_nbr;
sl_status_t    status;

sl_usbd_msc_scsi_callbacks_t app_usbd_msc_scsi_callbacks = {
  .enable = NULL,
  .disable = NULL,
  .host_eject = NULL
};

status = sl_usbd_msc_scsi_create_instance(&app_usbd_msc_scsi_callbacks,0
                                          &class_nbr);
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

Adding the MSC Class Instance to Your Device's Configuration(s)

After you have created an MSC class instance, you can add it to a configuration by calling the function sl_usbd_msc_add_to_configuration().

The example below show how to call sl_usbd_msc_scsi_add_to_configuration() using default arguments.

Example - Calling sl_usbd_msc_scsi_add_to_configuration()

sl_status_t  status;

status = sl_usbd_msc_scsi_add_to_configuration(class_nbr,                             (1)
                                               config_nbr_fs);                        (2)
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

(1) Class number to add to the configuration returned by sl_usbd_msc_scsi_create_instance().

(32) Configuration number (here adding it to a Full-Speed configuration).

USB Device MSC Class Logical Unit Handling

• Adding a Logical Unit

• Attaching/Detaching a Storage Medium

Adding a Logical Unit

When adding a logical unit to your MSC SCSI class instance, it must be bound to a storage medium (RAMDisk, SD card, flash memory, etc). The MSC class uses a storage driver to communicate with storage media. This driver will need to be supply when adding the logical unit.

The example below shows how to add a logical unit via sl_usbd_msc_scsi_lun_add().

Example - Adding a Logical Unit via sl_usbd_msc_scsi_lun_add()

sl_usbd_msc_scsi_lun_t       *lu_object_ptr = NULL;
sl_usbd_msc_scsi_lun_info_t  lu_info;
sl_status_t                  status;

lu_info.sl_usbd_msc_scsi_lun_api_t = &app_usbd_scsi_storage_block_device_api;
lu_info.vendor_id_ptr              = "Silicon Labs";
lu_info.product_id_ptr             = "block device example";
lu_info.product_revision_level     =  0x1000u;
lu_info.is_read_only               =  false;

status = sl_usbd_msc_scsi_lun_add(class_nbr,
                                  &lu_info,
                                  &lu_object_ptr);
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

Attaching/Detaching a Storage Medium

After the logical unit has been added, a storage medium must be attached to be available from the host side.

The MSC class offers two functions to control the storage media association to the logical unit: sl_usbd_msc_scsi_lun_attach() and sl_usbd_msc_scsi_lun_detach(). These functions allow you to emulate the removal of a storage device in order to re-gain access from the embedded application if necessary.

The example below shows how to use the function sl_usbd_msc_scsi_lun_attach() and sl_usbd_msc_scsi_lun_detach().

Example - Media Attach/Detach

sl_status_t  status;

status = sl_usbd_msc_scsi_lun_attach(lu_object_ptr);
if (status != SL_STATUS_OK) {
  /* An error occurred. Error handling should be added here. */
}

...                                                                      (1)

status = sl_usbd_msc_scsi_lun_detach(lu_object_ptr);
if (status != SL_STATUS_OK) {
    /* An error occurred. Error handling should be added here. */
}

...                                                                      (2)

status = sl_usbd_msc_scsi_lun_attach(lu_object_ptr)
if (status != SL_STATUS_OK) {
    /* An error occurred. Error handling should be added here. */
}

...                                                                      (3)

(1) From this moment, if the MSC device is connected to a host, the storage media is accessible.

(2) If the MSC device is connected to a host, the media will now appear as unavailable. At this moment, operations can be performed on the media from the embedded application.

(3) Again, if the MSC device is connected to the host, the storage media will appear as connected.

USB Device MSC Class Storage Drivers

The USB Device MSC Class needs a storage driver to communicate with a storage medium. For the moment, Silicon Labs doesn't offer drivers.

The driver API is defined by typedef sl_usbd_msc_scsi_lun_api_t. Your sl_usbd_msc_scsi_lun_api_t variable must be included to your sl_usbd_msc_scsi_lun_info_t variable, passed as argument when you add a logical unit with sl_usbd_msc_scsi_lun_add(). See section USB Device MSC SCSI API for more details on the structures.

The storage driver implementation can be as simple as an array of sectors in RAM. Typical sector size (i.e., block size) is 512 for mass storage devices, and 2048 for CD-ROMs.