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Unstable signal on the reset pin of MCF51AC256.

Question asked by Jean-Herve Le Garrec on Jul 5, 2011
Latest reply on Jul 5, 2011 by Jean-Herve Le Garrec
I'm using freescale microcontroller MCF51AC256BVLKE.
I followed the instructions on page 2-7 of "MCF51AC256 Coldfire Integrated Microcontroller Reference Manual".
I already mount this microcontroller on 2 prototypes and after one or two replacement of the microcontroller, they were working fine.
I thought my prototype wasn't properly mounted. But I have the same problem on my new designs.
The probleme is :
    - I mount 3 new microcontrollers on 3 new same design => they doesn't work.
    - I can't connect my USB Multilink Interface (PE Micro) to the target on my new design.
    - I compared the signals between my working prototype and new design, the differrence is :
        - there is a signal of charge/discharge of RC circuit on the reset pin of the 3 new designs.
    - If I remove the RC circuit from the reset pin, the signal on the reset pin become a square signal.
Can you explain me what is the reason of this signal??