Sign In
Forgot Password?
Sign In | | Create Account

USB power

Colin Walls

Colin Walls

Posted May 23, 2011

USB seems to be a subject of great interest among embedded developers. I recently delivered a couple of Web seminars on the topic and had a very good sized audience and lots of questions. If you attended, thanks for coming along. If you were not there, but are interested, you can find a recording here. Please email me if you would like a copy of the slides. There was a very wide range of questions, but a few common themes became apparent. USB 3 seems to be sparking a lot of interest and there is certainly a demand for a more detailed, specific session and we will do one in the coming months.

Another topic that seemed to stir interest and queries is power delivery over USB …

As a user of numerous electronic devices, I really like the ability that many have to get their power from the USB connection. When I want to use my scanner, I just plop it on my desk, plug in the USB and I am ready to go. As a user, I am completely insulated from the details of what is happening [as is generally the case with USB]. It just works. However, the embedded developer may need to know a bit more.

With USB 2, a device can be defined as “low power” or “high power”; this implies a limitation of 100mA and 500mA respectively. Although a host will be able to supply enough current for a high power device, a hub may not necessarily have that capacity. When a USB device is connected to a host, the enumeration process is performed, which provides the host with a selection of information about the device, including its power requirements. If insufficient power is available, the device is informed and the host may inform the user [if the host is a PC, for example]. This raises an interesting question: what if there is insufficient power for the device to operate and participate in the enumeration process? The answer is simple: this should not occur, as it is a requirement [of the USB specification] that a device be able to perform the most basic USB communications using [only] low power.

USB 3 provides increased device power: 150mA and 900mA for low and high power respectively. This is only available to devices that utilize the increased speed capability of USB 3 – SuperSpeed. Again, an interesting question arises: what are the power implications if you plug a USB 3 device into a USB 2 host/hub. The answer is that the enumeration process will reveal whether the device needs more power than USB 2 can provide; this error condition can be handled in a controlled way. Interestingly, as I mentioned before, a device must be able to participate in enumeration at low power, but what has only recently become clear to me is that this means “USB 2 low power” – i.e. no more than 100mA, not the USB 3 level of 150mA. Otherwise, backwards compatibility would be compromised and a USB 3 device might fail totally when connected to a USB 2 hub.


More Blog Posts

About Colin Walls Follow on Twitter

Colin WallsI have over thirty years experience in the electronics industry, largely dedicated to embedded software. A frequent presenter at conferences and seminars and author of numerous technical articles and two books on embedded software, I am a member of the marketing team of the Mentor Graphics Embedded Systems Division, and am based in the UK. Away from work, I have a wide range of interests including photography and trying to point my two daughters in the right direction in life. Learn more about Colin, including his go-to karaoke song and the best parts of being British: Visit The Colin Walls Blog

More Posts by Colin Walls


No one has commented yet on this post. Be the first to comment below.

Add Your Comment

Please complete the following information to comment or sign in.

(Your email will not be published)


Online Chat