THE MAGAZINE FOR COMPUTER APPLICATIONS
Circuit Cellar Online offers articles illustrating creative solutions
and unique applications through complete projects, practical
tutorials, and useful design techniques.

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

FIRMWARE

Typically, the CY7C64013 HAPI will be connected to a microprocessor data bus. For demonstration purposes, I have written firmware for an MC68331, AT91M63200 (ARM7DTMI) processor, and ADSP-21065L DSP to interface to the USB. Connection diagrams for each of these processors are shown in Figures 2, 3, and 4, respectively. The MC68331 code was developed on a proprietary system, the ARM7TDMI code was developed on an AT91EB63 evaluation kit, and the SHARC code was developed using a SHARC 21065L EZ-LAB. Because the processor interface is generic, I wonýt spend time discussing the specifics of these processors.

The USB driver that comes with the design package for this article is a generic device driver that currently comes with the Cypress EZ-USB development kit. Before you plug in the device, you need to install the ezusb.sys driver using the ezusbw2k.inf file. In the ezusbw2k.inf file, you will find a list of device names and their associated VID and PID. Anytime you modify the VID and PID of your peripheral, you need to reinstall this device driver so the main USB driver can associate your device driver with the peripheral.

Automatic driver creation tools for the Microsoft Windows platform are also available from companies such as Jungo, BSQUARE, and NuMega. Some of these tools are wizard-driven, and others allow you to just plug in your device and the software configures a driver for you. This software can help you avoid the nightmare of developing a WDM driver and will probably equal the cost of hiring a consultant to create a custom driver for you. The driver creation tools will also require that you have a Windows compiler such as Visual C++ along with the Microsoft DDK.

There is also the possibility that, with a little modification of the USB descriptors in embedinf.h, you can use the Microsoft Point of Sale (POSUSB) driver. This driver effectively allows you to interface your USB device like a RS-232 port on your USB host. For more details, check out the Resources at the end of the article.

The drawback of using this driver is that you lose the ability to form your own vendor-specific requests. You also may need to investigate throughput with this driver because it was designed to emulate serial port connectivity to point-of-sale devices.

In order to test your new, fabulous USB interface, I have written a test application that connects to the ezusb.sys device driver. The host application is shown in Photo 1, and a number of vendor-specific requests are shown in Tables 1 and 2. Included are GET_IO, SET_IO, SET_TIMEOUT, and SEND_IMMEDIATE.

PREVIOUS • NEXT