USB PIC without USB Connection

[JEC]

With this previous suggestion, this will be only to detect whether or not the device is plugged in or not. As with this device it will always be plugged into a USB port, whether the machine is powered on or not. If the PC is off, 5v will still be on the USB ports to keep this device powered up as usual.

Hope this makes sense. Any other suggestion?

Steve

Not sure if this is for a commercial application, but stealing power from the USB bus without a properly enumerated device being attached is a blatant violation of the official USB spec...

To do this properly, you'd want either (a) the device to be enumerated and active at all times or (b) receive power from a source external to the USB bus.

John

[Tissy]

Thanks for your more than constructive feedback John.

Steve

[JEC]

You can still use Bruce's method of sensing whether the bus is active... The presence of a USB bus could allow your PIC to switch some FETs that activate a charging circuit, etc.

Or, like Bruce mentioned, you can run in non-USB datalogging mode until a voltage is sensed on that pin. Then, enumerate and dump your data.

Officially, your device is supposed to draw less than 100 mA of current during enumeration until it negotiates and is approved to receive higher power. If you're running on a laptop through a bus-powered hub, there's a chance that the computer might just say, "Sorry, no can do." There just isn't enough power available.

And if the bus is idle (which means > 3 mS of inactivity), you're not supposed to draw more than I think 500 uA. Yes, microamps.

Surf on over to usb.org, click on developers and then on discussion forums. You'll find a wealth of information and helpful engineers.

They aren't too patient with people who haven't done some basic homework before asking questions, but the forums can be searched. Many of the most popular questions have been discussed ad absurdum already...

John