The Points Franc Urbanc wrote are very important! Once i had a probleme with a small 8031 design (powered with a transformator from power line). Every time the neonlights from my office turned on/off the programm crashed....

I found out, that the neonlights send "dirth" over the powerline. I connected an EMI-Filter from murata after the Transformator . I put also a Filter in all external Lines (RS232 / RS485 ...)

For all unused Interrupts i programmed an RETI at its vector
adress ==> unused EXTERNAL INT0 ==> 0003h RETI
unused Timer 1 Interrupt ==> 001Bh RETI

After all the problem was solved until today....

http://www.murata.com/

Bye, Dieter.