SuperSpeed USBWireless USBOriginal USBHi-Speed USBOn-The-Go USBHi-Speed On-The-Go USB gray vertical line Express Card
Home > Developers > USB FAQ

USB Frequently Asked Questions

Compliance Issues | Signals | Power Distribution and Consumption
Cables and Long-Haul Solutions | Throughput and Bandwidth | Drivers and Power management

Compliance Issues:

1a.

1b.

Can I put the hi-speed logo on the underside of the overmold of the "A" connector?
Where can I put the hi-speed logo on a cable assembly?

2.

I would like to ship a non-captive cable with my product. What cable assemblies may I use?

3.

How can I list my product on the public product search page?
How can I remove my product on the public product search page?

4.

My product's name (or part number) has changed. How can I get this change reflected on the Integrator's List?

5.

Will a part name (or part number) change on the Integrator's List be automatically reflected on the public product search page?

6.

When do I have to retest my product?

7.

My products are building blocks, how can I be granted certification for logo use?

8.

I just went through WHQL testing. Isn't that enough? Why do we have to go through both USB-IF and WHQL testing?

9.

How much does it cost to attend a USB-IF Compliance Workshop?

10.

What about second-sourcing? I've developed a product, but another company will be also marketing it. How can they be granted logo use?

 

About the FAQ

This site is a service of the USB Implementers Forum staff. The primary charter for that staff is to support the USB Implementers Forum member companies with Developers Conferences, Spec Technical Support and Compatibility Workshops. We also provide marketing exposure for our members as a unified group of companies producing USB products, at trade shows such as CES. Our secondary charter is to provide the industry at large with any information that is available to foster understanding and development of USB.

USB

11.

Where do I get the spec for USB, EHCI or a device class?

12.

How do I get a USB VID, TID and PID?

13.

Are there design checklists I can use while building a USB device?

14.

Where do I get the USB Check/Chapter 9/Chapter 11/HidView test program?

15.

How can I find out more about Hi-Speed USB?

16.

What does it mean when a product is USB compliant, and why should I care?

17.

What is the purpose of the USB 2.0 Adopters Agreement?

18.

What is the most recent version of the USB spec?

19.

Can you point me to where I can get some information regarding micro series connectors? OTG is a new concept to our product lines and we need to get a lot of information to make product development decisions.

 

Signals:

1.

Does USB support data rates besides 12Mb/s and 1.5Mb/s?

2.

But isn't a 25ns rise or fall time out of spec?

3.

How should I test my device for signal quality?

4.

What are the most common signal quality design mistakes?

5.

What does good USB signal quality look like?

6.

What characteristic impedances should I design for in laying out my USB device's traces?

7.

But the maximum allowable termination is 44 ohms! And a 28 ohm termination is also within spec! Why?

8.

Section 7.1.14 isn't clear. How long can dribble and an EOP be?

 

Cables and long-haul solutions:

1.

Why are the cable length limits what they are?

2.

I want to build a cable longer than 5m, why won't this work?

3.

What about using USB signal repeaters to make a cable longer than 5m?

4.

I really need to put a USB device more than 30 meters away from my PC. What should I do?

 

Power Distribution and Consumption:

1.

How much power does a system in S3 need to supply to USB?

2.

In sections 7.2.3 and 7.2.4.1, allowable device actions are specified in terms of the voltage droop at the hub. How do I design for this?

3.

How should I design and test for inrush current control? What about testing steady state current draws?

 

Throughput and bandwidth:

1.

What do actual data rates on USB look like?

2.

How much bandwidth should I design my device to use?

 

Drivers and Power Management:

1.

How does one get started writing a driver?

2.

How do I support power management in my driver?

3.

Why is power management so important?

4.

Where can I learn more about these IAPC machines?

 
 

Compliance Issues

1a.   Can I put the hi-speed logo on the underside of the overmold of the "A" connector?

A:   The USB-IF does not allow the logo to be on the overmold of either the "A" or "B" connector of a cable assembly. The standard USB trident must be on the top of both plug overmolds as described in chapter 6 of the USB 2.0 specification. Any cable assembly found with the hi-speed logo on the plug overmold will be deemed non-compliant.

1b.   Where can I put the new hi-speed logo on a cable assembly?

A:   If you wish you may affix a tag to your cable assembly with the logo on one side and your corporate contact information on the other side. In this manner one could easily see if a cable assembly has passed compliance testing and where they could go for help if there were any customer support issues with the cable assembly.

top  

top

gray line

2.   I would like to ship a non-captive cable with my product. What cable assemblies may I use?

A:   You may use any cable assembly listed on the Integrators List. Please note all cables are in the "High Speed" listing, because all cables will work with low, full and high-speed products. Vendor-specific cable connections are defined as "captive cables.".

top  

top

gray line

3.   How can I list my product on the public product search page?
How can I remove my product on the public product search page?

A:   Only those products qualified to use the new USB logo are eligible for inclusion in the product search on the usb.org site. Only the USB-IF representative for each member company has administrative access to the company information displayed on this site. USB-IF member representatives should follow the simple step-by-step

instructions provided for adding or updating their company product information in the product search. If you do not know who the USB-IF representative is for your company, you can find their name in the member search.

top  top

gray line

4.   My product's name (or part number) has changed. How can I get this change reflected on the Integrator's List?

A:   The USB-IF representative for your company must complete the on-line name change request form provided in the member maintenance area and submits it to the USB-IF. All requests will be reviewed. Approved changes will occur in 5-7 business days.

top  

top

gray line

5.   Will a part name (or part number) change on the Integrator's List be automatically reflected on the public product search page?

A:   Yes. All records are updated everywhere they occur.

top  

top

gray line

6.   When do I have to retest my product?

A:   If your product was tested prior to January 2000, then you must retest now in order to have your product listed on the Integrators List. Any change (other than cosmetic) may require a retest. If you are in doubt, please send email information to

admin@usb.org. They will route the information to the appropriate people who can let you know if your product changes will require retest. Some changes made, such going from 28AWG to 25AWG wire, or from 5 meters to 3.5 meters, do not need retesting.

top  top

gray line

7.   My products are building blocks, how can I be granted certification for logo use?

A:   Please submit a testable device with your building block designed in. When it passes compliance testing, we will list your building block on the Integrators List. If this device has already passed compliance testing, we simply need the product information, especially the TID of the reference product as well as your product TID, and we can then have your building block listed on the Integrators List. You may use the logo in conjunction with your product only. Other customers you may have will need to have their designs using your building blocks certified themselves.

top  

top

gray line

8.   I just went through WHQL testing. Isn't that enough? Why do we have to go through both USB-IF and WHQL testing?

A:   WHQL testing and our compliance testing are different programs with different unrelated goals.

top  

top

gray line

9.   How much does it cost to attend a USB-IF Compliance Workshop?

A:   Compliance workshop attendance is limited to, and at no charge to, USB-IF members. The only charges you may incur would be any potential travel charges, over which we have no control.

top  

top

gray line

10.   What about second-sourcing? I've developed a product, but another company will be also marketing it. How can they be granted logo use?

A:   Please send email to

admin@usb.org letting us know what you are intending to do. Be sure to include your TID for the product to be OEM'ed, your product name and model number, and the product information, product name and number, and vendor ID from the company who will be second-sourcing for you. The USB-IF will let you know if there is any more information you will need, and will assist you in listing the second source on the Integrators List.

top  top

gray line

11.   Where do I get the spec for USB, EHCI or a device class?

A:   The

documents page provides links to the USB spec itself, host controller specifications and the device class specifications. The EHCI specification, version 1.0 can be downloaded at EHCI spec (1.0), The device class specifications are listed at---and can be downloaded from--- device class specs.

top  top

gray line

12.   How do I get a USB VID, TID and PID?

A:   Vendor IDs (VIDs) are owned by the vendor company and are assigned and maintained by the USB-IF only. Email

admin@usb.org for more information on USB-IF membership and obtaining a VID. Test IDs (TIDs) are assigned by the USB-IF or by the certified independent test lab at the time of testing. Product IDs (PIDs) are assigned by each vendor as they see fit; the USB-IF recommends each vendor set up a coordinated allocation scheme for PIDs so different teams don't inadvertently choose the same PID for different products. Duplicate numbers may cause driver error.

top  top

gray line

13.   Are there design checklists I can use while building a USB device?

A:   Yes. See

USB compliance page for more information.

top  top

gray line

14.   Where do I get the USB Check/Chapter 9/Chapter 11/HidView test program?

A:   Note: USBCheck was made obsolete by

USBCV. USBCheck will no longer be supported after March 2002 and has been removed from this site.

top  top

gray line

15.   How can I find out more about Hi-Speed USB?

A:   There is considerable information, as well as the latest copy of the USB 2.0 specification on the

www.usb.org/developers page.

top  top

gray line

16.   What does it mean when a product is USB compliant, and why should I care?

A:   Compliance means products that are called USB products are really built to match the description in the USB specification. This is important to a customer because if all products match the spec perfectly they would by definition work together perfectly. The world is not perfect and so compliance testing exists to help manufacturers measure how well their products match the specification. There are many ways to make sure products work together and many ways to see if they match the specification, but compliance testing is one of the most useful. Conscientious manufacturers do a great deal of testing on their own and use compliance test programs like the one sponsored by our organization, USB-IF. If you don't find a product on our product list it does not mean there is anything wrong with that product, but if you do find it there you know that this manufacturer has put in a lot of effort to try to make sure that product matches the USB specification and has the best chance of working properly in a variety of applications. Please visit the

www.usb.org/developers/compliance/ page for more details.

top  top

gray line

17.   What is the purpose of the USB 2.0 Adopters Agreement?

A:   The purpose of the Adopters Agreement is to facilitate the development of products that can interoperate, interconnect and/or communicate with one another in the manner defined in the USB 2.0 interface specification. Companies that sign an Adopters Agreement: (i) are obligated to license on a royalty-free and non-discriminatory basis, certain IP that would be necessarily infringed by products compliant with the final USB 2.0 interface specification; and (ii) will receive a license of the same scope from the USB 2.0 Promoters and other companies that have signed Adopters Agreements. This IP licensing obligation does not cover technologies your company may use to design or manufacture USB 2.0 products, which are not expressly set forth in the final USB 2.0 interface specification (e.g. manufacturing technology). You should carefully review the specific terms of the Adopters Agreement before signing.

top  

top

gray line

18.   What is the most recent version of the USB spec?

A:   Please check the http://www.usb.org/developers/docs/ page for the absolute latest versions of the spec.

 

top  top

gray line

19.   Can you point me to where I can get some information regarding micro series connectors? OTG is a new concept to our product lines and we need to get a lot of information to make product development decisions.

A:   Valid connectors for standard USB peripherals are the micro A-plug or micro B-receptacle.  The micro AB-receptacle is restricted to USB On-The-Go products only.  The USB-IF Integrators List and the Products Search Page can help locate a manufacturer of micro-series connectors.  The Micro USB Specification is included with the download of the USB 2.0 Specification..

top  

top

gray line  

Signals

1.   Does USB support data rates besides 12 Mb/s and 1.5 Mb/s?

A:   Yes. The new hi-speed data transfer rate is 480 Mb/s. There is, however, wide variation in edge rates. With typical line loads, full speed devices usually fall in the 12-25 ns range, and low-speed devices typically range 110-225 ns.

top  

top

gray line

2.   But isn't a 25 ns rise or fall time out of spec?

A:   If it's driven into a 50pF load, yes. If it's driven into a 1000pF load, no (the worst-case in-spec loading is actually 1070pF-two 75pF edge rate control caps, a 150pF downstream port, and a 100pF upstream port connected by a 5m cable). Take a closer look at the rise/fall time test fixtures in section 7.1.2. for the timing requirements for all three data transfer rates.

top  

top

gray line

3.   How should I test my device for signal quality?

A:   Signal Quality is a mandatory test. See the

signal quality test description.

top  top

gray line

4.   What are the most common signal quality design mistakes?

A:   Signal Quality is a mandatory test. See the

signal quality test description for details. Most problems are the result of EMI "control" components like ferrite beads mounted on the signal lines. Often, these manage to destroy the integrity of the signal as well as make emissions worse.

top  top

gray line

5.   What does good USB signal quality look like?

A:   Signal Quality is a mandatory test. See the

signal quality test description.

top  top

gray line

6.   What characteristic impedances should I design for in laying out my USB device's traces?

A:   30 ohms to ground, 90 ohms differential between the USB data lines. Be aware that many differential impedance formulas don't take into account the presence of a plane next to the data lines and tend to overestimate the reduction in single-ended impedance due to coupling between the two lines. For typical buried microstrips on spacings FR4 stackups the USB traces should be somewhere around 11 or 12mils wide with roughly a 33mil edge to edge spacing (45mils center to center). This is far enough apart that there's almost no coupling between the lines---the single-ended impedance is around 1.5O lower than what a single trace's impedance would be.

top  

top

gray line

7.   But the maximum allowable termination is 44 ohms! A 28 ohm termination is also within spec. Why?

A:   USB termination requirements are quite lenient. For example, a 0 to 3.6V signal input into a 51.75 ohm line with a 28 ohm source termination will swing between 4.6 and -1.0V-this is where the voltage levels in figure 7-1 of the USB 1.1 spec come from. In practice, most USB devices are underterminated, sometimes incorrectly.
The IF recommends sizing the series termination resistors to create a 44 ohm termination. While not enough to perfectly terminate the device when its connected to an ideal cable, it does bring the terminations well within the standard 15% impedance tolerance-most modern processes are well controlled, so the internal impedance of USB drivers is quite consistent from wafer to wafer. It is, of course, wise to characterize the drivers' impedance range and perform sampling of production parts.

top  

top

gray line

8.   Section 7.1.14 isn't clear. How long can dribble and an EOP be?

A:   Dribble is between 0 and 15ns per hub and the change in delay for the drive to J at the end of the EOP to -0/+15ns. If the delays for J/K to EOP and the EOP to J transitions are identical, the EOP's width won't change. If the EOP to J transition has greater delay than J/K to EOP, the EOP will lengthen, and it will shrink in the opposite case---the worst cases are with 0ns dribble and 15ns extra EOP to J delay and with 15ns dribble and 0ns extra EOP to J delay. Thus, the width of the EOP can change by up to 15ns per hub, or 75ns with 5 hubs in series. In shorthand, this is often referred to by saying the EOP width is between 1 and 3 bit times, though that's not technically correct. The USB-IF strongly recommends all device receivers be checked to make sure they can handle the full range of dribble and changes in EOP width-failure to handle this correctly is the leading cause of device failures on long USB busses.

top  

top

gray line  

Cables and Long-Haul Solutions

1.   Why are there cable length limits, and what are they?

A:   The cable length was limited by a cable delay spec of 26ns to allow for reflections to settle at the transmitter before the next bit was sent. Since USB uses source termination and voltage-mode drivers, this has to be the case, otherwise reflections can pile up and blow the driver. This does not mean the line voltage has fully settled by the end of the bit; with worst-case undertermination. However, there's been enough damping by the end of the bit that the reflection amplitude has been reduced to manageable levels. The low speed cable length was limited to 18ns to keep transmission line effects from impacting low speed signals.

top  

top

gray line

2.   I want to build a cable longer than 5 meters, why won't this work?

A:   Even if you violated the spec, it literally wouldn't get you very far. Assuming worst-case delay times, a full speed device at the bottom of 5 hubs and cables has a timeout margin of 280ps. Reducing this margin to 0ps would only give you an extra 5cm, which is hardly worth the trouble.

top  

top

gray line

3.   What about using USB signal repeaters to make a cable longer than 5 meters?

A:   Don't bother. The best solution is self-powered hub with a fixed 10m cable that had a one-port bus powered hub in the middle. The maximum range will still have to deal with the timeout, so any out of spec tweaking of the terminations between the two hubs and the timing budget still won't yield more than 5cm of extra distance. A better solution is described in the following question.

top  

top

gray line

4.   I really need to put a USB device more than 30 meters away from my PC. What should I do?

A:   Build a USB bridge that acts as a USB device on one side and has a USB host controller at the other end. Use a long-haul signaling protocol like Ethernet or RS-485 in the middle. Using cables or short-haul fiber, you can get ranges upwards of a kilometer, though there's no reason why the long-haul link in the middle of the bridge couldn't be a pair of radio transceivers or satellite modems.
Embedded host solutions capable of doing this already exist. Also, two PCs connected via USB Ethernet adapters are essentially a slave/slave version of this master/slave bridge.

top  

top

gray line  

Power Distribution and Consumption

1.   How much power does a system in S3 need to supply to USB?

A:   500mA per USB port. See section 7.2.3 for the details of device behavior during suspend and resume. The Device Working Group is working on power management extensions to the common class definition that should alleviate most of the S3 power supply issues with USB. Contact

admin@usb.org for more info.

top  top

gray line

2.   In sections 7.2.3 and 7.2.4.1, allowable device actions are specified in terms of voltage droop at the hub. How do I design for this?

A:   In the case of 7.2.4.1, the spec indicates the high power device's configuration can be treated as another hot plug inrush. Thus, from a hub designer's perspective, nothing needs to be done except to have the standard 120F of low ESR capacitance on the hub. For a device, up to an additional 10F can be hard started. In practice, however, very few high power devices need less than 20F to operate, so some sort of soft start circuit is almost always required to charge additional capacitance. In order to minimize the chance of a hub having to deal with a hot plug inrush and a high power device configuration inrush at the same time, the USB-IF strongly recommends this soft start circuit be designed such that the device's current draw doesn't exceed the MaxPower specified in the device's configuration descriptor. For most designs, this requires nothing more than tweaking the value of the resistor that sets the gate voltage time constant of the soft start MOSFET.
A similar recommendation extends to bus powered hubs, which are required to implement inrush current limiting when charging their port bypass capacitance in 7.2.4.1. In this case, the USB-IF recommends the soft start be designed so that the hub's current draw doesn't exceed 100mA times the number of powered ports, plus100mA for the hub itself (e.g., 200mA for one port, 300mA for two ports, etc.).
In the case of 7.2.3, with a worst-case configuration, a hub essentially sees a 500mA current step on each of its ports when the bus resumes. Thus, a hub designer typically has to design for a load step from near zero to 2-3.5A. The size of this step is comparable to many hot plug inrush currents, but handling it is considerably harder than handling an inrush because the inrush is a transient load, while the step function persists indefinitely and usually requires the hub's power supply to swing across most of its range. To prevent the hub from being overloaded on a resume, the USB-IF recommends devices be designed such that they do not exceed their MaxPower when sending a remote wakeup or when resuming.

top  

top

gray line

3.   How should I design and test for inrush current control? What about testing steady-state current draws?

A:   This is discussed fully in the

current test description.

top  top

gray line  

Throughput and Bandwidth

1. What do actual data rates on USB look like? 

A:   USB's actual throughput is a function of many variables. Typically, the most important ones are the target device's ability to source or sink data, the bandwidth consumption of other devices on the bus, and the efficiency of the host's USB software stack. In some cases, PCI latencies and processor loading can also be critical.
Assuming only the target endpoint consumes a significant amount of bus bandwidth-and both the target and the host are able to source or sink data as fast as USB can move it-the maximum attainable bandwidth is a function of the transfer type and signaling rate. These bandwidths are given in chapter 5 of the USB Specification. In practice, most hosts can reach the maximum isochronous and interrupt bandwidths with a single target endpoint. With bulk transfers, typical transfer rates are around 900kb/s to a single endpoint, increasing to near ideal transfer rates with multiple endpoints.

top  

top

gray line

2. How much bandwidth should I design my device to use? 

A:   For devices like most low speed devices (keyboards, mice), latency tolerant bulk devices (printers, scanners, still cameras), or latency intolerant devices with low bandwidth (POTS modems, speakers) bandwidth allocation isn't usually of much concern because the device can tolerate data rates that are a small part of USB's total bandwidth. For higher data rate devices (video cameras, xDSL modems) the amount of bandwidth available can be critical.
A well-designed peripheral handles a shortage of bandwidth gracefully. For instance, many USB cameras provide several different interfaces, each using a different amount of isochronous bandwidth, and allow the user to select which interface is used. Thus, the user can choose whatever tradeoff between available bus bandwidth and frame rate best suits their needs. Another example is a bulk device that requires a certain number of packets per frame to work correctly. An application could monitor the data transfer rate, notify the user if the data rate became too low, and suggest ways to bring it back up.
As a general rule of thumb, a device should be able to operate with as little as 30 to 40% of the bus's total bandwidth. This allows two relatively high bandwidth devices to work together (such as a DSL modem and video camera in a teleconferencing setup) while still leaving bandwidth available for must-have devices like keyboards and mice.

top  

top

gray line  

Drivers and Power Management

1.   How does one get started writing a driver?

A:   Some sort of device driver development kit (DDK) is needed. Microsoft has their Windows Server 2003 DDK available via MSDN subscription or via CD that can be purchased at

http://www.microsoft.com/whdc/ddk.

top  top

gray line

2.   How do I support power management in my driver?

A:   There is a white paper called Understanding WDM Power Management provided in the white papers section at

http://www.usb.org/developers/whitepapers/. This whitepaper provides a primer for understanding power management, source code for supporting power management, as well as covering the issues regarding power management and providing source code solutions.

top  top

gray line

3.   Why is power management so important?

A:   ACPI compliant systems are now being shipped. There is a new initiative for PC's to be instantly available called the Instantly Available PC (IAPC). IAPC systems allow for the user to place their system in a deep sleep state rather than turning it off so that it may be resumed to a fully powered on state quickly. The consequences of this are that device drivers must handle the power management messages and must be free of memory leaks.

top  

top

gray line

4.   Where can I learn more about these IAPC machines?

A:   To learn more about the Instantly Available PC (IAPC) initiative, visit

http://developer.intel.com/technology/iapc/index.htm.

top  top

gray line