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USB electrical Compliance FAQ for Cypress High-speed USB products

Last Updated: September 13, 2011

Are Cypress High-speed USB products capable of passing USB compliance? What are the frequently asked USB electrical compliance related questions when it comes to Cypress High-speed USB products?


The datasheet of Cypress USB High-speed products contain the USB-IF TID (Test ID) of the device/hub i.e. the device/hub is capable of passing USB compliance. The compliance of the device depends on the firmware/hardware as well. Hence with proper hardware/firmware, designs based on Cypress High-speed USB products should be able to pass compliance. Refer High-speed USB PCB Layout Recommendations - AN1168 for hardware guidelines. Refer EZ-USB Hubs(CY7C656XX) PCB Design Recommendations for HX2LP-specific recommendations and for FX2 specific recommendations.

Following are the USB electrical compliance FAQs with respect to Cypress High-speed USB products:

1.    Will the DVKs (Development kit)/RDKs (Reference Design Kit) pass USB compliance?

RDKs are designed to meet USB compliance provided the proper configuration file/firmware is used.

The DVKs are for development purpose. So they are designed such that they provide maximum flexibility and are not designed keeping USB compliance in mind. They will meet signal quality (eye diagram) but they are not expected to meet all the compliance parameters. For example, if you look at FX2LP DVK, the design contains LEDs which are not controlled when the device goes into suspend. This will cause the DVK to fail suspend current specification.

2.    Why is it recommended to monitor VBUS for a self-powered design? What is back-voltage test?

A self-powered design can differentiate between suspend and disconnect condition only by monitoring VBUS. As per USB specification when VBUS is removed the device must remove power from D /D- pull-up within 10s. Back-voltage test is used to test this. A self-powered design which does not monitor VBUS will fail this test.

3.    For a high-speed design what are the impedance parameters to be met at the USB interface?

The impedance parameters to be met at the USB interface are 45?±10% from D /D- to ground and a differential impedance of 90?±10% between D and D-. Non-compliance of these parameters can affect the working of the design with longer cables.

4.    The design multiplexes the D and D- line/needs more ESD/noise filtering. What is the recommended switch/diode/capacitor value that can be used on the D and D- lines?

The design is expected to meet the above mentioned impedance parameters. Even the small series impedance added due to the Switch/diode/capacitor can cause impedance mismatch between the two lines. It is also difficult to meet signal quality parameters. So we do not recommend having components on the D and D- lines.

5.    The design fails eye diagram test? What could be the possible reasons behind this issue?

The eye diagram test directly reflects the signal quality of the design. In Cypress USB High-speed products, the crystal is the major contributor of jitter on the USB lines.  It would be a good idea to check if the crystal meets the parameters specified in the datasheet. Non-compliance of impedance parameters at the USB interface and difference in trace length of D and D- lines can also contribute to this. The difference in trace length effectively introduces a phase difference between the two signals thus adding common mode noise and shift in crossover voltage (full-speed has a compliance parameter on this).

6.    Connecting a removable/non-removable device is not going to affect the working of the hub. Then why is the hub descriptor required to specify whether the device connected to a downstream port is removable or non-removable?

If the device connected to the downstream port is removable then USB-IF expects compliance tests to be run on the particular downstream port as well. This process will be tedious if the device connected is a non-removable device is connected to the downstream port. So it is recommended to properly specify the removable/non-removable property of the device connected to the downstream port.

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