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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

TYPICAL USB SOLUTIONS

Unlike RS-232, the USB is a master-slave configured bus that must have a central host and can have up to 127 peripherals attached. In other words, only the USB host initiates data transfers, and your device must be willing to share the bandwidth. Currently there are only two speeds available for V.1.1 of the USB, 12 Mbps for full speed and 1.5 Mbps for low speed, with a third (480 Mbps) on the way for USB 2.0. Even with the fastest microcontrollers on the market today, itýs impractical to try to bit-bang USB traffic and remain within the USB timing specifications.

USB peripheral connectivity solutions generally exist in three forms. First is the standalone serial interface engine (SIE). Many vendors sell these so that an embedded processor can set up packets at the transaction level and have the SIE format the packet and send the data at the physical level. An SIE solution has the drawback of making the embedded processor firmware have to understand and communicate at the transaction level. Data cannot be simply transferred without understanding the USB protocol.

The second common solution is to integrate the SIE into a microcontroller. Literally dozens of silicon vendors provide anywhere from bare-bones to high-power microcontroller solutions for the USB. Typically these are used in peripherals in which a microcontroller can act as the main system controller. This high level of integration allows for cost-competitive solutions in a tiny package, however, the microcontroller firmware still has to understand and interpret data at the USB transaction level.

And, for this article, I will be examining a third solution for systems that already have a central microprocessor and want to easily integrate USB connectivity, similar to RS-232 connectivity. This solution involves letting a separate USB microcontroller act as a peripheral handling the transaction level, freeing the central microprocessor to simply send and receive bytes at the application level. The only drawback of this scheme seems to be a moderate hit to throughput because the microcontroller must handle the transaction level. Still, much larger throughput than RS-232 can easily be achieved.

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