Lpc2000

Hi, I am new to this LPC group and I would like to say 'hello' to 
everyone who is reading this message! 

Currently I am working with a LPC2106 processor and my first task is 
to get the LEDs on the board to blink at a correct frequency.

I am using a 20MHz crystal osillator with the setting of PLL 
multiplier value of 3 (which gets me 60MHZ for system clock rate), a 
VPB divider of 2 (gets me 30MHz for pclk frequency). 

I am using vectored IRQ interrupts to increment the cntr (counter)  
in my C main() function, in order to set that: if the counter has 
been counted for 500 times (cntr > = 500), the LEDs would be turned 
on (and turned off after another 500 counts therefore it blinks by 
using IOSET and IOCLR). 

The value of Match Register on Timer0 has been set as 60000 (MR0 = 
60000). 

So the question I want to ask is that: How do I know wether the LEDs 
are blinking at the correct time interval of 1 second? And if I want 
to make the LEDs blinking at a rate of EXACTLY 2 seconds or 1/2 
seconds (or at any other frequency), how should I set the MR value 
or the cntr value in order to do that? Is there a formula for 
calculating the relationships between Match Register value and the 
pclk? 

Thank you for reading my post! ^^