AT83C51RC2_14 Datasheet, PDF (3/4 Pages) ATMEL Corporation – Variable Length MOVX for Slow RAM/Peripherals

12. 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.
3
4242B–8051–03/08