Thomas Walsh

EMI from Cell Phone Triggering IRQs in QG4!

Discussion created by Thomas Walsh on Jun 29, 2011
Latest reply on Jul 11, 2011 by Robert Bailey

Hi All,


I am using a QG4 in a new product that we have already produced a few thousand of.  The device is a battery-powered LED lamp with three brightness settings.  We've already gone through lots of field trials and months of testing.  But today I was just brushing my teeth, with one of these lamps sitting in the bathroom (where I had been using it in my own high-humidity "field trial").  The lamp was off, but all of a sudden, it turned on to mode one... then mode two... then mode three... all over the course of 10 seconds!  It was acting as if a ghost were pushing the button and turning the lamp on.  


I about freaked when I saw this as I thought it was a code malfunction that I had just randomly been present for.  But then I realized that I had set down my cell phone right next to the lamp.  After some investigation, I find that if I put my phone right next to the device, I can occasionally activate the lamp just by calling my phone.  Afterwards, even after many tries, the code is still running fine, and I can tell that no reset has occurred.


I use an IRQ with subsequent switch debounce polling to wake the device from sleep and then switch lighting modes.  I am really surprised that EMI from a cell phone could even trigger the IRQ to begin with, much less get through the debounce routine.  I do not have a cap on the IRQ pin, but it is not like I have a long antenna going into that pin - just a 5 cm wire going to a tactile switch.  


Can I prevent this IRQ triggering by just adding a 100 nF cap across the IRQ pin?


Beyond fixing the IRQ line itself, I am quite worried because I assume that if a cell phone can trigger the IRQ, it could also screw up the code execution completely.   Can anybody point me to comprehensive advice on this?  I have used QD4 for years and never experienced anything like this.  I have not yet checked whether this is just one device that is susceptible or all of the devices.