Hi,
I use a Atmel 89S8252 in a medical device
operated by batteries in a voltage range 4.7-5.4V.
There´s a 4700µ Capacitor connected to the supply.
The 89S8252 runs at 24 MHz. On Port 1 I use a 4k7
Resistor Network as pullup. The resisitive load on
the other ports is min. 10k on each Pin.

The Problem: Sometimes when I connect the supply to
the circuit the 8252 hangs or some of the ports
swing at max.40 Hz. This condition cannot be reset.
The only way to bring this MCU back to normal work
is by reprogramming it. The problem most often occurs
when the supply is disconnected and immediately
reconnected to the circuit.

Any help appreciated !
Karsten