View Full Version : USB Powered Prop Devices

07-09-2009, 05:58 AM

So I recently purchased 2 non-parallax made devices: the PSM from rayman, and the mini dev board from Hitt Consulting, both of these are powered from the usb port.

Is there a reason Parallax does not make boards that run off the usb port power? Is it generally not a good thing to do?



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Peter Jakacki
07-09-2009, 06:05 AM
I jumpered my Propeller development board to be powered off the USB because it's convenient. However, because the board has a prototyping area and is meant for development I understand that there are a lot of things that can go wrong that might also result in damage to the PC if it were powered directly from USB. The way it is at the moment you can't blame Parallax if you do something silly (connect +12V to +5V etc).


Beau Schwabe
07-09-2009, 06:27 AM

Something else to consider, and I don't know how strict this is applied. The USB port under normal circumstances is only good for a few 100 mA. A USB device that requires more current must negotiate via commands to the PC in order to be allowed more current. Again, I'm not sure how strict this rule is followed.

It was a pleasure meeting you and your wife at the EXPO.

Beau Schwabe (mailto:bschwabe@parallax.com)

IC Layout Engineer
Parallax, Inc.

Brian Carpenter
07-09-2009, 06:34 AM
Hitt Consulting's board has a self reseting fuse to protect your usb port


It's Only A Stupid Question If You Have Not Googled It First!!

07-09-2009, 07:28 AM
The PSM has FTDI chips programmed to let the PC know that it wants up to 500 mA. Also, there's a 500 mA resettable fuse (just in case)...

My demo board is hotwired for USB power and I've never had a problem...

My Prop Info&Apps: ·http://www.rayslogic.com/propeller/propeller.htm

07-09-2009, 07:39 AM
You are *allowed* to draw up to 100mA prior to enumeration. You can then *request* up to 500mA from the port, however if the port is on an unpowered hub or low-power host it has the right to deny you that power. Most PC USB ports are capable of a minimum of 500mA, and I've seen some that will supply over 1A (they are supposed to disconnect power on overload but they don't).

Given the plethora of stupid USB devices out there (lights, fans, peltier coffee warmers...) and all the USB drives that use dual port cables to increase the current available (and how they *all* violate the spec in unspeakable ways), people don't seem to let the smoke out of their motherboards. The USB power supply is pretty rugged on the PC end. You can generally short it out in all sorts of nasty ways and it will gracefully reset itself.

One of my devel boards is a usb-protoboard with the +5v rail wired straight to the USB connector. The only thing that happens is when I switch on the servo supply, the 1000uF cap causes a nice big rail sag as it charges up and the prop resets. I certainly would not do it in a production device, but for home equipment you'd be pretty safe.

Where is Paul Baker? He often has a lot of valuable input on the topic of USB and power.

Release the hounds!

07-09-2009, 04:52 PM
I figured it was more of a saftey net, especially with devices that have a prototyping or breadboard area.
I guess if you are jumping it to run off usb yourself..then the manufacturer is off the hook should anything go wrong....which is really the right way to go.

@Beau- Yeah it was a blast, hope you can make it to ohio in august.



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