Introduction -- eCos support for developing USB ethernet peripherals


The eCos USB-ethernet package provides additional support for USB peripherals that involve some sort of ethernet-style network. This can be a traditional ethernet, or it can involve some other networking technology that uses ethernet frames as a unit of transfer. It provides functions to transfer ethernet frames over the USB bus, handles certain control messages from the host, and optionally it can provide a network device driver for use by the eCos TCP/IP stack. The package comes with an example host-side device driver.

The USB-ethernet package is not tied to any specific hardware. It requires the presence of USB hardware and a suitable device driver, but not all USB peripherals involve ethernet communications. Hence the configuration system cannot load the package automatically for specific targets, in the way that a USB device driver or an ethernet driver can be loaded automatically. Instead, the package has to be added explicitly. When using the command line tools this will involve an operation like the following:

$ ecosconfig add usbs_eth

Typically, this will automatically cause the USB device driver to become active. Loading the USB-ethernet package automatically provides functionality for initialization, data transfer, and the handling of control messages and state changes. If the current configuration includes the eCos TCP/IP stack then the network device driver support will be enabled as well by default, allowing the stack to exchange ethernet frames over the USB bus.

There is a USB standard for a class of communication devices including ethernet. The package does not implement this standard, due to limitations in the hardware for which the package was first developed. Instead, the package uses its own protocol between USB host device driver and the peripheral.

Usage Scenarios

The USB-ethernet package can be used several different scenarios. In a simple scenario, the peripheral serves only to connect the USB host to a suitable network:

After initialization, and once the USB connection between host and peripheral has been established, higher-level code needs to detect packets that are intended for the host, and to forward these. This can be achieved by the low-level usbs_eth_start_tx function. Similarly, higher-level code needs to detect packets coming from the host, using usbs_eth_start_rx, and to forward these using the real network. As far as the host is concerned it is connected directly to the network. In this scenario there is no confusion about addresses: there is a single MAC address for the host/peripheral combination, corresponding to the connection to the real network, and it is this address which should be supplied during initialization.

In a more complicated scenario, there is a TCP/IP stack running inside the peripheral.

This involves the USB-ethernet package providing a service both to the host and to the eCos TCP/IP stack. It achieves the latter by acting as an eCos network device. Typically, the TCP/IP stack will be configured to act as a network bridge. The USB peripheral needs to examine the packets arriving over the real network. Some of these packets will be intended for the host, while others will be intended for the peripheral itself. To distinguish between these two scenarios, two distinct MAC addresses are needed: one for the host, and one for the peripheral. Similarly, packets sent by the host may have to be forwarded via the real network, or they may be intended for the TCP/IP stack inside the peripheral. Packets generated inside the peripheral's TCP/IP stack may need to be sent via the real network or over the USB bus. The network bridge software will have to take care of all these possibilities. Unusually for a network bridge, one of the network segments being bridged will only ever have one machine attached.

There are other possible usage scenarios. For example, the peripheral might not be attached to a real network at all. Instead it could be the USB host that acts as a network bridge, allowing a TCP/IP stack inside the peripheral to communicate with the outside world. The various details will depend on the exact type of peripheral being developed.