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JUMP ON THE BUS

Taking a Look at the USB
by Don Powrie

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

SPEED LIMIT

Version 1.1 of the USB specification outlines two data ratesılow speed, which transfers data at 1.5 Mbps, and full speed, which transfers data at 12 Mbps. The FT8U245AM transfers data at a maximum of 8 Mbps, somewhat slower than full speed but still much faster than the RS-232. In order to achieve the 8-Mb data rate, the target microcontroller must be able to read and write data every 125 ns. This is a tall order for most microcontrollers unless they are running at high clock speeds.

Another limiting factor for most USB devices is the 1-ms frame. USB devices transfer data in packets. If data is to be sent from the PC, a packet is built up by the application program and sent via the device driver to the USB scheduler. This scheduler puts a request to the list of tasks for the USB host controller to perform. This will typically take at least 1 ms to execute because it will not pick up the new request until the next 1-ms USB frame.

Therefore, there is a sizable overhead, depending on your required throughput, associated with moving the data from the application to the USB device. If data is sent one byte at a time by an application, the overall throughput of the whole system will be severely limited. The message here is that, if speed is critical, always send data in packets.

The specification for USB 2.0 is a good example of just how complicated a communications protocol can be. [1] Itıs easy to get lost in the myriad of details that outline USB communications. Thankfully, FTDIıs drivers hide most of these details and provide an easy to use interface.

With some programming languages, you can communicate via RS-232 ports (including the VCPs) by using a statically-linked communications library that handles the interrupts and provides an easy programming interface (open, read, write, etc.). Using this type of communications library presents an extra step, which you, purely for simplicityıs sake, might like to avoid.

FTDI recently released a new version of their drivers that is based on a dynamically-linked library (DLL). The application program simply requests the operating system to load the DLL at runtime, calls the open function to connect to the FT8U245AM device, and starts reading and writing data to and from the FT8U245AMıs FIFO memory. No other statically-linked communications library is required. Programming examples for how to use the DLL with various compilers are available for download from FTDIıs web site.

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