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Introduction -- eCos support for USB slave devices


The eCos USB slave support allows developers to produce USB peripherals. It consists of a number of different eCos packages:

  1. Device drivers for specific implementations of USB slave hardware, for example the on-chip USB Device Controller provided by the Intel SA1110 processor. A typical USB peripheral will only provide one USB slave port and therefore only one such device driver package will be needed. Usually the device driver package will be loaded automatically when you create an eCos configuration for target hardware that has a USB slave device. If you select a target which does have a USB slave device but no USB device driver is loaded, this implies that no such device driver is currently available.

  2. The common USB slave package. This serves two purposes. It defines the API that specific device drivers should implement. It also provides various utilities that will be needed by most USB device drivers and applications, such as handlers for standard control messages. Usually this package will be loaded automatically at the same time as the USB device driver.

  3. The common USB package. This merely provides some information common to both the host and slave sides of USB, such as details of the control protocol. It is also used to place the other USB-related packages appropriately in the overall configuration hierarchy. Usually this package will be loaded at the same time as the USB device driver.

  4. Class-specific USB support packages. These make it easier to develop specific classes of USB peripheral, such as a USB-ethernet device. If no suitable package is available for a given class of peripheral then the USB device driver can instead be accessed directly from application code. Such packages will never be loaded automatically since the configuration system has no way of knowing what class of USB peripheral is being developed. Instead developers have to add the appropriate package or packages explicitly.

These packages only provide support for developing USB peripherals, not USB hosts.

USB Concepts

Information about USB can be obtained from a number of sources including the USB Implementers Forum web site. Only a brief summary is provided here.

A USB network is asymmetrical: it consists of a single host, one or more slave devices, and possibly some number of intermediate hubs. The host side is significantly more complicated than the slave side. Essentially, all operations are initiated by the host. For example, if the host needs to receive some data from a particular USB peripheral then it will send an IN token to that peripheral; the latter should respond with either a NAK or with appropriate data. Similarly, when the host wants to transmit data to a peripheral it will send an OUT token followed by the data; the peripheral will return a NAK if it is currently unable to receive more data or if there was corruption, otherwise it will return an ACK. All transfers are check-summed and there is a clearly-defined error recovery process. USB peripherals can only interact with the host, not with each other.

USB supports four different types of communication: control messages, interrupt transfers, isochronous transfers, and bulk transfers. Control messages are further subdivided into four categories: standard, class, vendor and a reserved category. All USB peripherals must respond to certain standard control messages, and usually this will be handled by the common USB slave package (for complicated peripherals, application support will be needed). Class and vendor control messages may be handled by an class-specific USB support package, for example the USB-ethernet package will handle control messages such as getting the MAC address or enabling/disabling promiscuous mode. Alternatively, some or all of these messages will have to be handled by application code.

Interrupt transfers are used for devices which need to be polled regularly. For example, a USB keyboard might be polled once every millisecond. The host will not poll the device more frequently than this, so interrupt transfers are best suited to peripherals that involve a relatively small amount of data. Isochronous transfers are intended for multimedia-related peripherals where typically a large amount of video or audio data needs to be exchanged continuously. Given appropriate host support a USB peripheral can reserve some of the available bandwidth. Isochronous transfers are not reliable; if a particular packet is corrupted then it will just be discarded and software is expected to recover from this. Bulk transfers are used for everything else: after taking care of any pending control, isochronous and interrupt transfers the host will use whatever bandwidth remains for bulk transfers. Bulk transfers are reliable.

Transfers are organized into USB packets, with the details depending on the transfer type. Control messages always involve an initial 8-byte packet from host to peripheral, optionally followed by some additional packets; in theory these additional packets can be up to 64 bytes, but hardware may limit it to 8 bytes. Interrupt transfers involve a single packet of up to 64 bytes. Isochronous transfers involve a single packet of up to 1024 bytes. Bulk transfers involve multiple packets. There will be some number, possibly zero, of 64-byte packets. The transfer is terminated by a single packet of less than 64 bytes. If the transfer involves an exact multiple of 64 bytes than the final packet will be 0 bytes, consisting of just a header and checksum which typically will be generated by the hardware. There is no pre-defined limit on the size of a bulk transfer. Instead higher-level protocols are expected to handle this, so for a USB-ethernet peripheral the protocol could impose a limit of 1514 bytes of data plus maybe some additional protocol overhead.

Transfers from the host to a peripheral are addressed not just to that peripheral but to a specific endpoint within that peripheral. Similarly, the host requests incoming data from a specific endpoint rather than from the peripheral as a whole. For example, a combined keyboard/touchpad device could provide the keyboard events on endpoint 1 and the mouse events on endpoint 2. A given USB peripheral can have up to 16 endpoints for incoming data and another 16 for outgoing data. However, given the comparatively high speed of USB I/O this endpoint addressing is typically implemented in hardware rather than software, and the hardware will only implement a small number of endpoints. Endpoint 0 is generally used only for control messages.

In practice, many of these details are irrelevant to application code or to class packages. Instead, such higher-level code usually just performs blocking read and write, or non-blocking USB-specific calls, to transfer data between host and target via a specific endpoint. Control messages are more complicated but are usually handled by existing code.

When a USB peripheral is plugged into the host there is an initial enumeration and configuration process. The peripheral provides information such as its class of device (audio, video, etc.), a vendor id, which endpoints should be used for what kind of data, and so on. The host OS uses this information to identify a suitable host device driver. This could be a generic driver for a class of peripherals, or it could be a vendor-specific driver. Assuming a suitable driver is installed the host will then activate the USB peripheral and perform additional application-specific initialisation. For example for a USB-ethernet device this would involve obtaining an ethernet MAC address. Most USB peripherals will be fairly simple, but it is possible to build multifunction peripherals with multiple configurations, interfaces, and alternate interface settings.

It is not possible for any of the eCos packages to generate all the enumeration data automatically. Some of the required information such as the vendor id cannot be supplied by generic packages; only by the application developer. Class support code such as the USB-ethernet package could in theory supply some of the information automatically, but there are also hardware dependencies such as which endpoints get used for incoming and outgoing ethernet frames. Instead it is the responsibility of the application developer to provide all the enumeration data and perform some additional initialisation. In addition, the common USB slave package can handle all the standard control messages for a simple USB peripheral, but for something like a multifunction peripheral additional application support is needed.

Note: The initial implementation of the eCos USB slave packages involved hardware that only supported control and bulk transfers, not isochronous or interrupt. There may be future changes to the USB code and API to allow for isochronous and interrupt transfers, especially the former. Other changes may be required to support different USB devices. At present there is no support for USB remote wakeups, since again it is not supported by the hardware.

eCos USB I/O Facilities

For protocols other than control messages, eCos provides two ways of performing USB I/O. The first involves device table or devtab entries such as /dev/usb1r, with one entry per endpoint per USB device. It is possible to open these devices and use conventional blocking I/O functions such as read and write to exchange data between host and peripheral.

There is also a lower-level USB-specific API, consisting of functions such as usbs_start_rx_buffer. A USB device driver will supply a data structure for each endpoint, for example a usbs_rx_endpoint structure for every receive endpoint. The first argument to usbs_start_rx_buffer should be a pointer to such a data structure. The USB-specific API is non-blocking: the initial call merely starts the transfer; some time later, once the transfer has completed or has been aborted, the device driver will invoke a completion function.

Control messages are different. With four different categories of control messages including application and vendor specific ones, the conventional open/read/write model of I/O cannot easily be applied. Instead, a USB device driver will supply a usbs_control_endpoint data structure which can be manipulated appropriately. In practice the standard control messages will usually be handled by the common USB slave package, and other control messages will be handled by class-specific code such as the USB-ethernet package. Typically, application code remains responsible for supplying the enumeration data and for actually starting up the USB device.

Enabling the USB code

If the target hardware contains a USB slave device then the appropriate USB device driver and the common packages will typically be loaded into the configuration automatically when that target is selected (assuming a suitable device driver exists). However, the driver will not necessarily be active. For example a processor might have an on-chip USB device, but not all applications using that processor will want to use USB functionality. Hence by default the USB device is disabled, ensuring that applications do not suffer any memory or other penalties for functionality that is not required.

If the application developer explicitly adds a class support package such as the USB-ethernet one then this implies that the USB device is actually needed, and the device will be enabled automatically. However, if no suitable class package is available and the USB device will instead be accessed by application code, it is necessary to enable the USB device manually. Usually the easiest way to do this is to enable the configuration option CYGGLO_IO_USB_SLAVE_APPLICATION, and the USB device driver and related packages will adjust accordingly. Alternatively, the device driver may provide some configuration options to provide more fine-grained control.