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the unit falls to ground

Don't the pigs step on it then :-)

I want to use POWER DOWN but I am not able to do this. ... AT82S52 ... Who can help me how to install POWER DOWN.

I believe (check the datasheet) that your derivative can only exit power down by a reset (like all 'traditional' derivatives). You may need to use idle mode.

Another option would be to use a modern derivative which can exit power down from interrupts.

Erik