dear dun han you wrote I had NOT enabled MAM before. I enabled it and the system works correctly now. Thank you for your useful advices.
same problem we have our led is 2-3 mhz.and we know we must see 30 mhz around.
which is correct about pll and MAM?
Me bad. Yes, the LPC21xx can select between legacy and fast accessing modes for the two ports but will default to the legacy access mode.