Don't use this internal reset and watchdog crap. Use an external reset chip containing a watchdog function, like MAX1232. Connect an additonal 4k7 pull-up at the reset line of micro to guarantee that reset signal is proper even at supply voltages lower than 2V.

Take care, that with each power-up and power-down there's a proper reset!!! Program the MAX1232 to trip at the upper threshold voltage (4.62V).

Use a PCB containing a solid ground plane. Connect all connectors in such a way, that noise and ESD cannot flow across the PCB. Choose a separate radio frequency plane, where all the connectors are mounted to and where all the cable shields are connected to. Don't have any noise currents flowing across the PCB!!!

Decouple each chip with 100nF right at the supply pins. In noisy applications use even RLC-filters: 2R2 + ferrite bead + 10µF/X5S/0805 (ceramic high cap).

Connect a scope to the micro's pins and search for any unusual spikes, dips, brown-outs, etc.

Why is it so important to have your board noise free?
Erasing and changing of contents of EEPROM and flash is done by sophisticated microroutines, which can unintentionally be invoked by noise, ESD, etc. at the micro's pins. The same can happen, if the supply voltage goes up or down without a proper reset signal!!! This was reported for many times in the past, by different manufacturers.


Could you publish your layout here?

Kai