File content as of revision 0:c988614df67a:

//
//  5MHZ CLOCK OUT USING PWM1 PIN
//  (ONLY LPC1768-mbed)
//
// Code and Comment is from
// http://mbed.org/forum/mbed/topic/733/
//
// The LPC1768 clocks at 96MHz. 
// 96e6 / 5e6 gives 19.2. 
// The PWM block counts off of the system clock,
// so your period counter would have to be either 19 or 20. 
// 19 gives you a frequency of just over 5.052MHz, 
// and 20 gives you 4.8 MHz. (Note, to get these frequencies, you have to bypass the library).
//
// Assuming that the 5.052andabit is good enough, 
// here's how you get there. I'll use the library object as a starting point,
// as it does a load of the pin connection setup,
// enabling the block and all that for us.
//
// I'll just go and trample over some of its settings afterwards.
// I'm going to use pin 21 of the mbed, which is PWM output 6.
//
// The PWM block has a match register 0, 
// which is used as the period counter. 
// Then, each of the PWM outputs (1 through to 6) have their own match counters,
// which are used to determine when to change the output level.
// We're using the block in a simple "output high, until match counter matched, then output low" mode.
// Thus our value of MR0 is going to be 18 (we count from zero, match at 18),
// and our value of MR6 (if using p21) is going to be 9.
// These counters are derived from the system clock, through a peripheral clock divider and a prescaler.
// So what we need to do is:
//
//    1)Reset the PWM block
//    2)Set the peripheral clock divider to /1 (the library sets the prescaler to /1 for us)
//    3)Load the match registers
//    4)Tell the block to start at next match
//    5)Re-start the PWM block

#include "mbed.h"

PwmOut fmclck(p21);

int main() {        
    LPC_PWM1->TCR = (1 << 1);               // 1)Reset counter, disable PWM
    LPC_SC->PCLKSEL0 &= ~(0x3 << 12);  
    LPC_SC->PCLKSEL0 |= (1 << 12);          // 2)Set peripheral clock divider to /1, i.e. system clock
    LPC_PWM1->MR0 = 18;                     // 3)Match Register 0 is shared period counter for all PWM1
    LPC_PWM1->MR6 = 9;                      // 3)Pin 21 is PWM output 6, so Match Register 6
    LPC_PWM1->LER |= 1;                     // 4)Start updating at next period start
    LPC_PWM1->TCR = (1 << 0) || (1 << 3);   // 5)Enable counter and PWM

    while(1);
}