I have an MCU operated at 3 volt with an input pin that is floating by default. I have an external pull down circuit with a 10K resistor connected to the I/O pin. I also have it tied to VCC using a push button. The microprocessor is sleeping until the button is pressed (it is configured to generate an interrupt when the I/O line become high).
The solution have worked flawless, have tested it for months. Last week however I soldered a new batch of 16 units and when I inserted them inte their plastic case I noticed that some of the woke up and started processing and I did not press the button. I now have two theories and would like to know how likely they are:
ESD has damaged the MCU? Personally I do not think so, since I have seen the same problem on at least 7 devices occasionally. This combined with that they work flawless in all other aspects makes me think that this is not the case.
ESD pulls the line high occasionally when I insert it into the plastic enclosure and in this way the interrupt is triggered in the MCU. Sounds reasonable, I guess a very short ESD near the pins would actually wake up the device. Maybe the reason I see it now and not before is because of winter and a more dry air?
Am I thinking correct here?
- I would also like to know if I need to protect the pin in some way and suggestions on how to do this if required. I read that the chip (TI cc2540) is ESD protected but I don't know if it is enough, maybe it needs more protection. Maybe it has enough protection but I still need to handle short spikes in software.