The power consumed by USB peripherals such as flash drives has typically not been much of a concern. However, with OTG support on the rise, mounting external storage via USB on mobile devices has become a common use-case. The power consumed by bus-powered devices is arguably more important compared to the performance numbers in such cases. A reader comment in our recent review of the Samsung Portable SSD T3 piqued our interest, and we set out to quantitatively measure this aspect. This article deals with some background on inline USB power measurement and our available options. This is followed by a detailed description of our hardware setup and the software infrastructure to gather the required data. In the final section, we look at some results from our evaluation of the power consumption of different flash drives followed by some concluding remarks.

Introduction

The measurement of the power consumed by USB peripherals is conceptually a simple affair. DIY solutions are also possible. Devices such as PortPilot offer a professional alternative for inline power measurement while adding management and diagnostic features. There are plenty of other inline USB power measurement gadgets, but none of them are suitable for Type-C / USB 3.1 ports.

The Chromium Twinkie USB-PD Sniffer is one of the most famous Type-C diagnostic kits, thanks to its use by Benson Leung in evaluating adherence of various Type-C cables and adapters to the specifications. Google doesn't sell Twinkie directly to end-users, but interested consumers can get the version manufactured by Plugable - the USB 3.1 Type-C Power Delivery Sniffer (USBC-TKEY).

 

 

The Twinkie / USBC-TKEY is primarily intended for developers to monitor the power delivery traffic between two USB Type-C devices. However, it has additional capabilities such as the monitoring of the VBUS and VCONN voltages and currents, injecting power delivery packets on the CC1 and CC2 pins and placing Rd/Rp/Ra resistors on CC1 and CC2. Since it is also designed to be a transparent interposer, it doesn't interfere with the data traffic between the host and the device.

The various pins on a USB 3.1 Type-C plug are brought out in the picture below

From a power measurement perspective, the interesting ones are the VBUS and VCONN. The VBUS line carries bus power for devices (and, in some cases, for the circuitry in the cable itself). The voltage may vary from 5V for typical USB 2.0 hosts / devices to up to 20V for USB-PD scenarios. VCONN, on the other hand, is always set to 5V and can supply up to 1W of power for circuits within the plug. These circuits can implement electronically marked cables or even be accessories.

Coming back to our original intent of measuring the power consumed by USB peripherals such as flash drives, it is obvious that we need to be measuring the voltage and current on the VBUS line. This aspect has suddenly become something to care about because of the number of mobile devices supporting 'OTG'. Not all mobile devices are able to provide the 900 mA output current to 'bus-power' attached devices. Note that the initial specification for USB only had provisions for 0.75 watts of power – 150 mA at 5 V. USB 2.0 bumped that to 500 mA, or 2.5 watts, and USB 3.0 specified 900 mA at 5 V, or 4.5 watts. Understanding the power consumption profile of a flash drive is essential to determine whether it can be safely used with a particular mobile device. It can also help in understanding the impact of usage on the battery life of a mobile device. From a technical viewpoint, it also gives insight into the efficiency of the USB peripheral itself.

Hardware Setup and Software Aspects
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  • serendip - Friday, March 18, 2016 - link

    How about USB hard drives? I know most won't power up because they need more than 500 mA on USB2 but it would be interesting to see exactly how much power is used. I've gotten USB hard drives to work with phones and tablets over OTG but only with a powered hub or a charging hub that allows a battery pack to be connected over micro-USB. Reply
  • jbrizz - Sunday, March 20, 2016 - link

    I can power a 320GB 7200 rpm USB 3.0 hdd from my 2012 Nexus 7 and Galaxy S5 with a USB 2.0 OTG cable. One would assume that if they can supply power across the bus they should be able to power anything that runs on a PC, 500mA is 500mA whether from a PC or phone. Reply
  • serendip - Monday, March 21, 2016 - link

    That's weird, I've never been able to get 500 GB or 1 TB USB HDD working over OTG. The drive spins up, mounts for a second and then disconnects, probably because the peak current draw exceeds 500 mA. I looked at a bunch of spec sheets from WD and Seagate and all their drives had peak current in the 1 A or higher range. They work fine when plugged into powered hubs though. Reply
  • jasonelmore - Saturday, March 19, 2016 - link

    $200 for this thing?!?! what are they smoking.. I'll just make a arduino version for $35.... Thanks for the idea tho. Reply
  • dano_spumoni - Saturday, March 19, 2016 - link

    This explains so much!!!! I was having random disconnects with a USB 3.0 to M.2 Enclosure when using the front USB 3.0 ports on my Desktop. When I use the rear ports it works perfectly and I get 400 MB/s sequential read/writes... Normal USB thumb drives and powered drives work fine in front but not USB powered enclosures. I knew something was wrong with my front ports but this is the reason. Reply
  • alin - Saturday, March 19, 2016 - link

    There are still people out there who care about power consumption for flash devices? Hmm. Very well then... :) Reply
  • woggs - Saturday, March 26, 2016 - link

    I bought one of these to play with... And changed my mind. It's on ebay if anyone wants it.
    http://my.ebay.com/ws/eBayISAPI.dll?MyEbay&gbh...
    Reply
  • woggs - Tuesday, April 19, 2016 - link

    Still on ebay...
    http://www.ebay.com/itm/141961691354?ssPageName=ST...
    Reply
  • epobirs - Sunday, March 27, 2016 - link

    Interesting. I have a SIIG USB 3.1 enclosure that I put a Patriot Blast 480GB SSD in. The performance is very nice in that it gets very close tot he limits of SATA itself. The included cable has a second host port connector to enlist more power where needed but it seems to work fine on 3.0 on a single plug.

    Is anyone developing native USB 3.1 controllers for SSD class flash memory that would exceed the SATA envelope? This would be ideal for consoles like the PS4 if the USB host was upgraded to 3.1 in later model.
    Reply
  • ggruk - Monday, April 19, 2021 - link

    Hi Ganesh

    An exciting article from 2016. I read it yesterday and found very useful.
    I am designing a new product - a special type of Monitoring Meter. Am using PIC32MM0256GPM064 in it as the main microcontroller. It comes with a USB OTG module. I then want to save data in the Meter to a USB flash drive (USB2.0). However, my meter cannot provide more than 100mA current to the USB flash drive. So, I was looking for a low power flash drive. You article shows that 128GB flash drive that you tested could be suitable. Have tested some of the many flash pen drives I have and measured the current drawn by using Windows Device Manager for the Rot Hub. One Data Traveler flash drive of 8GB showed 100mA. All others are showing 200 to 500mA. Is there a way to have a USB flash drive that would draw less than 100mA as drawn by HID devices. Would be grateful for your comments.
    I am also thinking of an option where I provide two Micro USB sockets on the Meter. One to power up the meter, say from a Mobile Phone charging Power Bank (with only the +5V and GND connected and another Micro AB socket where the flash drive is connected. Then I hope the flash drive would be supplied with power by the Power bank and data would sent by my Meter. Is this possible?
    Again would thank you very much for your comments and advice. ggrUK
    Reply

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