hi,

It was said many times: read not only device`s datasheet but its Errata as well! Here you may read next documented error:

5. C51 Core – Bad Exit of Power-down in X2 Mode

When exiting power-down mode by interrupt while CPU is in X2 mode, it leads to bad execution of the first instruction run when CPU restarts.
Workaround: Set the CPU in X1 mode directly before entering power-down mode.

Regards,
Oleg