Spurious mbed reset on PC wakeup

28 Aug 2010

Hi

I am using my mbed as a home automation controller and it is connected via the USB to a home theatre PC that spends most of its time suspended. It appears that when I wake the PC, the mbed is reset. Is there some way I can avoid this and still leave the PC and mbed connected physically (in case of remote programming needs).

Note I'm running some software on the PC called LEDSdriver which may scan the serial ports, but I wouldn't expect it to cause a break.

Thanks
Daniel

28 Aug 2010

Try using a seperate power supply for the mbed (i.e. power it from Vin). This could solve your problem.

29 Aug 2010

Hi Igor

Thanks for the suggestion, I already am powering the mbed from a 12V-5V DC converter (the 12V coming from the house alarm panel), so the USB can be unplugged.

And I just did an experiment and left the USB unplugged from the computer, and I still got a reset, so I can't blame the PC after all.

Is there a software problem that could cause a reset (as opposed to a hard fault where I have a handler that will pause the mbed until reset)?

Or would it have to be a hardware issue eg. supply instability or noise? I have a pushbutton connected to the reset pin.

Thanks
Daniel

PS If you are wondering how I know there is a reset, I am updating a counter every few seconds and writing it to pachube. I am resetting it at midnight, but if it goes to zero at other times then it must be because of a reset of the mbed. The mbed carries on counting after the reset though, and does not freeze.

29 Aug 2010

Hey Daniel,

It very well can be a noise issue. When I touch the metal part of the reset button of the mbed it sometimes resets before I press the actual button. You can try adding an additional pullup resistor to the reset line to lower the input impedance. Try a 1k, it should make it immune to noise or static..

30 Aug 2010

Hi Igor

Thanks for the suggestion, I will give it a go. I had also noticed that the mbed could reset with a light touch on the button, but didn't think too much of it.

Regards
Daniel

17 Sep 2010

Hi Igor

I've added that resistor to my circuit now, and it looks like I'm not getting the spurious resets anymore. Thanks very much for the advice, back to sorting out software issues now!

Is it worth adding anything to the wiki documentation on this?

Regards
Daniel

17 Sep 2010

Glad to be of help :)