Lpc2000

I used Internal Register "Sec" in Keil Simulator for tracing execution 
time of my program and it gave precise execution time comparing with 
the time I measured on the target, Ashling's LPC2100 EVBA7, board. But 
I found it's not precise when I changed CPU speed on the fly, by 
changing PLL multiplier, such as 10 MHz to 60 MHz. The execution time 
that I got from the target board greater than from simulator, around 1 
us from simulator but 10 us from target board.

Does anyone know this problem?

Teera.