ARTICLES USB 2.0 Printed Circuit Board Design Electrical Compliance Testing Understanding the types of electrical tests that are performed on USB devices helps to correctly design a PCB. This article will focus on high-speed devices, as this is where the challenge in PCB design is manifest. Since the authors' experience is primarily in the design and testing of peripherals, we will focus primarily on device testing, though the principles discussed here are also applicable to host-side design. High-speed devices are required to pass the following electrical tests: high-speed and full-speed signal integrity, high-speed chirp, high-speed receiver sensitivity, DC voltage levels on signal lines, and inrush current on VBUS. A description is provided below of the tests particularly useful in PCB design. High-speed and Full-speed Signal Integrity The purpose of signal integrity testing is to verify that D+ and D-, the signal lines, meet specified timing and electrical requirements. Part of this testing is accomplished by placing the device in a test mode defined in the USB 2.0 specification. In this mode a device repeatedly transmits a predetermined data packet. This packet is measured with an oscilloscope. The waveform is parsed into individual bit times with the bits overlaid. The result is a plot known as an "eye pattern" (see Figures 1, 2), which is a concise representation of the device transmitter's rise and fall times, signal levels, jitter, etc. This plot makes it easy to see at a glance whether or not the transmitter meets the required electrical specifications. Other measurements are taken to verify parameters such as EOP width and signaling rate. These tests are performed in both high-speed and full-speed modes to validate that the device functions correctly at both signaling speeds. Figure 1: eye pattern The high-speed eye pattern is measured using a 10 GS/s (gigasamples / s) digital oscilloscope with a high-bandwidth differential probe. The data is processed for compliance to the USB 2.0 specification in MATLAB. The shaded areas indicated specification violations; if the eye pattern waveform crosses into a shaded region, the device fails. Figure 2: eye pattern The full-speed eye pattern is measured using a digital oscilloscope with high-bandwidth single-ended active probes. The USB-IF provides the test scripts for MATLAB. To ARTICLE Top page <<Former / Next>> To DESIGN WAVE MAGAZINE's Homepage (English) To CQ Publishing's Homepage (Japanese) Copyright(C) 1996-2001 CQ Publishing Co., Ltd.