PCB Design
Software Execution
The software running in the MCU must be written to support the USB specified protocol and command set. This is verified during USB compliance testing by using the USB Command Verifier (CV), and is often referred to as the “Chapter9” test. See USB.org to download the Command Verifier software. If the USB system design passes the CV tests, it can then be tested for compliance with the signal quality specifications.USB SQ Compliance Tests
The USB committee has published detailed test procedures for the signal SQ compliance tests on their web site, USB.org. Read and follow this test procedure to produce MAX3420E/1E eye diagrams.{Universal Serial Bus Implementers Forum, Full and Low Speed Electrical and Interoperability Compliance Test Procedure, Revision 1.3, February, 2004.}
Description of how the FS SQ test is performed:
The USB test procedure indicates the correct SQ test set up. Use all the mandated USB HUBs, cables and devices. A functioning USB system is formed,and included in the USB system is a special test fixture that provides attachment of two single-ended active probes to the USB D+ and D- signal lines. These probes form the inputs to a digital oscilloscope. A third probe is attached to a specified “trigger” source, which is an adjacent USB channel. This trigger connection is crucial with full speed and low speed USB SQ testing so that the oscilloscope captures data from the desired device under test, and with the correct signal flow direction. (USB is a bi-directional bus, so triggering on the desired portion of the traffic is simplified by using this method to condition the trigger event.) For USB full speed signals, the adjacent trigger channel uses the D+ signal.A USB Host PC executes a special USB.org provided program, “High Speed Electrical Test” (HSET) to enumerate the USB system, and then repeatedly sends the USB command to the FS device under test, “Get_Device_Descriptor”, in a continuous loop. Once the FS device (MAX3420E/1E in this case) is continuously transmitting the Device Descriptor response, the up-stream USB signal is captured by the digital oscilloscope. Special software is then used to do the signal analysis and construct the full speed eye diagram. Tektronix DSA oscilloscopes with an optional USB 2.0 analysis package installed will do this analysis automatically and display the results. When the desired signal quality results are captured, the reports are saved in files for use at a later time. The reports can be printed or inserted into a full lab report as desired.
If all of this is too confusing, consider hiring a USB test laboratory to conduct the full USB compliance tests. See the USB.org web site for information about existing USB test laboratories around the world.