File content as of revision 5:2f69b934feb0:

//
//  CLOCK OUT to PWM1[6] Sample with Freq Counter using Cap2.0 
//  For LPC1768-mbed
//
//  Reference: 5MHz Clock Out Code and Comment - http://mbed.org/forum/mbed/topic/733/
//
//  !! To Self Measurement Output Clock, Connect p21 <-> p30 with jumper wire.
//  2013.6.18 : Wrong comment about MR6 and Duty fix.
//

#include "mbed.h"

PwmOut fmclck(p21);     // for RESERVE pin21 as PWM1[6]
DigitalIn clkin(p30);   // for RESERVE pin30 as CAP2[0]

// Reset Counter and Count Start
void P30_RESET_CTR(void)
{
    LPC_TIM2->TCR = 2;             // Reset the counter (bit1<=1,bit0<=0)
    LPC_TIM2->TCR = 1;             // UnReset counter (bit1<=0,bit0<=1)
}

// Get Counter Value
int P30_GET_CTR(void)
{
    return LPC_TIM2->TC; // Read the counter value
}

// Setting p30 to Cap2.0
void P30_INIT_CTR(void)
{
    LPC_SC->PCONP |= 1 << 22;               // 1)Power up TimerCounter2 (bit22)
    LPC_PINCON->PINSEL0 |= 3 << 8;          // 2)Set P0[4] to CAP2[0]
    LPC_TIM2->TCR = 2;                          // 3)Counter Reset (bit1<=1,bit0<=0)
    LPC_TIM2->CTCR = 1;                     // 4)Count on riging edge Cap2[0]
    LPC_TIM2->CCR = 0;                                          // 5)Input Capture Disabled
    LPC_TIM2->TCR = 1;                          // 6)Counter Start (bit1<=0,bit0<=1)
}

// Clock Output From pin21(PWM6)
// Set Clock Freq with div.
// if mbed is running at 96MHz, div is set 96 to Get 1MHz.
void PWM6_SETCLK(int div)
{
    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 = div - 1;                // 3)Match Register 0 is shared period counter for all PWM1
    LPC_PWM1->MR6 = (div + 1)>> 1;          // 
    LPC_PWM1->LER |= 1;                     // 4)Start updating at next period start
    LPC_PWM1->TCR = (1 << 0) || (1 << 3);   // 5)Enable counter and PWM    
}

int main() {        
    PWM6_SETCLK(19) ; // Outout mbed's "PWM6" pin to 96MHZ/19 = 5.052MHz (Approx)
    // PWM6_SETCLK(96) ; // Outout mbed's "PWM6" pin to 96MHZ/96 = 1.000MHz (Approx)
    P30_INIT_CTR();
    while(1){
        P30_RESET_CTR();
        wait(0.1); // Gate time for count
        printf("pin30 Freq = %u (Hz)\r\n",P30_GET_CTR());
    }
}