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IMPLEMENTING A SIMPLE USB INTERFACE

FOR AN EMBEDDED PROCESSOR

by Stuart Allman

Start ý Typical USB Solutions ý Required Components ý An Easier Solution ý Following the Rules ý Firmware ý Vendor-Specific Requests ý Host Transfer Mechanisms ý USB Transfers ý How Does It Work? ý The Grand Conclusion ý Sources and PDF

HOST TRANSFER MECHANISMS

Now that Iýve caught your attention and you are thoroughly salivating with creative thoughts, itýs time to think about how you are going to communicate with the host computer. Unless your platform already has a device class supported for your device, you will need to write a custom host application for your new, fabulous gizmo.

The EZ-USB driver used in this example uses the older file name mechanism for opening the driver. Newer drivers that conform to the WDM standard will use a globally unique ID (GUID) to access the driver. In order to open the USB device driver for reading and writing, the host application makes a call to bOpenDriver() with the file name specifying the device it wants to open. The system then returns a handle to the device (bOpenDriver( &hDevice, "Ezusb-0");.).

The identifier "Ezusb-0" is the first USB peripheral plugged in that uses the EZ-USB driver. If you plug in additional devices, the driver identifier will be "Ezusb-1", "Ezusb-2", and so forth for each additional device attached. If you need to randomly plug in your devices (as is practical), you need to poll the device on a WM_DEVICECHANGE message for a unique product ID or serial number.

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