When I disconnect the ALE from the strobe , I found that the RST is oscilating between 2.0V and 4.9V,i.e. it is not a good clean reset , like I get if I if I use the push button or a brownout condition. Any clues why this could happen ?

Hallo Thomas,

that's just the action of watchdog! If you do not reset the watchdog, it will reset the micro. That's what a watchdog is used to do.

Kai