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JUMP ON THE BUS

Taking a Look at the USB
by Don Powrie

Start ý Speed Limit ý The Mechanics ý Destination ý Sources and PDF

Several manufacturers make silicon for implementing a USB 1.1 compliant interface. Some include built-in microcontrollers, and some are simply the serial engine. However, no matter what they have or donýt have, they all include detailed datasheets that are stuffed full of valuable information. Some even have application notes that describe in detail how to connect to a microcontroller or your target electronics. USB is a great idea in that the electronics are easy to implement. Almost all new PCs have the interface built in, and being able to connect and disconnect a USB device to a PC without shutting down is convenient.

However, the one topic that seems to be completely avoided is information about the inevitable device driver, without which USB devices simply will not operate. The device driver is the bridge between the application software running on the host PC, the USB port on the PC, and the USB silicon out at the end of the USB cable.

Device drivers are tricky pieces of software. Only a small percentage of programmers have the knowledge and experience necessary to create device drivers. A driver that is not written well can result in the blue screen of death or complete system lockup. Some operating systems assume all device drivers are well behaved and make no attempt to protect themselves from any errors in the driver.

One solution to this problem is the FT8U245AM and virtual COM port drivers from FTDI. After the virtual COM port (VCP) drivers are installed, the application software merely has to open a COM port and read or write as though it were talking over standard RS-232. The VCP drivers intercept the data that would otherwise go to the RS-232 port and feed it to the USB scheduler, which then sends it to the USB port. Setting the baud rate in the application program has no effect on the data rate. The FT8U245AM always communicates at the maximum data rate. After a FT8U245AM is connected to your system and the drivers are loaded, select which COM port your application program will access via the System Properties page.

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