Power Down Mode Application

I am using LPC2138 and I'd like to set it into power down mode to conserve power.
Based on the data sheet, LPC2138 is able to wake up by RTC (Interrupt Wakeup Register - INTWAKE, bit 15) What i have done is enabled RTC intterupt before i set LPC2138 into power down mode (Power Control Register - PCON, bit 1).
However i failed to wakeup the LPC from power down mode using RTC. The board hang when i executed register PCON = 0x02 (power down mode).

For your infomation, the RTC intterupt is working properly and the whole program is runing well if i disabled/mark out register PCON = 0x02 (power down mode).

Is there anyone can give a hand on this? TQ.

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