Lpc2000

I'm playing now with CS8900A control timing.
there are some examples on the net, but they don't care about MAM,PLL 
configuration.
My question is how fast the instruction executes:
while(1){
I0SET =1
asm ("nop")
I0CLR =1
}
when CPU CLOCK is 60MHZ, and MAM = FULL
The data sheet says about rise time = 30ns.
How many "nop" should be added between to get a continous square wave?
I haven't found so far manual which says about instruction timings?

robert