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I am trying to find out what the board draws from the USB supply at max MHz. I really don't want to cut cables if this info is available somewhere already

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Obviously, the magnitude of current that the board draws will depends on what you're doing with the board. The STM32F3 discovery board has a jumper (JP3), where you remove the short, put in an ammeter, and you know how much current the board is using. No need to cut cables.

See page 18, http://www.st.com/web/en/resource/technical/document/user_manual/DM00063382.pdf

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Current draw is a very subjective thing - it depends on so many factors. Not just the current MHz of the CPU, but also what it is actually doing at the time. The only true way of knowing is to graph the current draw over time - you'll see it's not a flat line.

But all is not lost. You can buy a small USB current meter easily enough. That'll give you an average current draw smoothed over a short period. Kind of like using the ammeter setting of a DMM but without having to cut cables. They're reasonably cheap.

Another thing you can do is, if the USB interface conforms to the standards properly, is to examine the requested maximum current parameter of the USB descriptor. This won't be what the board is using, but will be the maximum current it requests to be allowed to use. This may be of more use to you than the actual real-time value, since it is what the board would be allowed to draw from the USB before being shut off (by a well behaved interface).

For instance, my STM32F4 Discovery board reports:

MaxPower              100mA

How you get that data depends on the OS you are using. I use Linux, so I run the lsusb -v -d <vid:pid> command. In Windows I think it can be found through the device manager somewhere. Not sure about OS X.

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  • \$\begingroup\$ Thanks. I am trying to work out if I can power my design, based on the F3 board, solely from USB2 \$\endgroup\$ – Dirk Bruere Jan 12 '15 at 11:56
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    \$\begingroup\$ Not all motherboards are going to honor the USB current cap, regardless of any USB negotiations for current. \$\endgroup\$ – Scott Seidman Jan 12 '15 at 13:31
  • \$\begingroup\$ @ScottSeidman It also depends on the type of port you plug into - data, CDP, DCP, etc... \$\endgroup\$ – Majenko Jan 12 '15 at 13:35
  • \$\begingroup\$ I suspect the USB power figure is a default, and not anything meaningful. \$\endgroup\$ – Chris Stratton Jan 12 '15 at 13:56

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