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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

Deciding how to connect all of the modules of your embedded system together can be similar to playing a game of "would you rather" with your coworkers. Do you design a new interface or use an industry standard and accept a learning curve? How do you know that an industry standard interface will meet all of your needs before you design it in? What becomes an even worse predicament is, how do you test the interface if you have only your system to connect it to? The universal serial bus (USB) just may be the elixir that takes away some of your development pains.

Yes, itýs sad but true, Intel plans to phase out your old trusty RS-232 serial port on the back of your PC in favor of the USB. I, along with many of you, used the old trusty RS-232 serial interface for years. It is incredibly simple, and you have complete control over everything that goes on the bus. So, the question becomes, what do you use now that will be simple, allow complete control, and be immediately understandable?

The answer I have come to in recent years has been the USB. In this article, Iýll show how getting up and running on the USB can be painless and almost a direct drop in replacement for RS-232 solutions. Iýll show you three example processors that I have developed firmware for and how they can transfer data on the USB.

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