File content as of revision 0:3e987e5f426b:

// analog input p20 (ADC0[5], pin 3) test

#include "mbed.h"

int main() {

    // power on, clk divider /4
    LPC_SC->PCONP |= (1 << 12);          
    LPC_SC->PCLKSEL0 &= ~(0x3 << 24);    
        
    // software-controlled ADC settings
    LPC_ADC->ADCR = (0 << 0) // SEL: 0 = no channels selected
              | (25 << 8)    // CLKDIV: PCLK max ~= 25MHz, /25 to give safe 1MHz
              | (0 << 16)    // BURST: 0 = software control 
              | (0 << 17)    // CLKS: not applicable 
              | (1 << 21)    // PDN: 1 = operational
              | (0 << 24)    // START: 0 = no start
              | (0 << 27);   // EDGE: not applicable

    // setup P1_31 as sel 3 (ADC), mode 2 (no pull)    
    LPC_PINCON->PINSEL3 &= ~((unsigned int)0x3 << 30);
    LPC_PINCON->PINSEL3 |= (unsigned int)0x3 << 30;
    
    LPC_PINCON->PINMODE3 &= ~((unsigned int)0x3 << 30);
    LPC_PINCON->PINMODE3 |= (unsigned int)0x2 << 30;

    while(1) {
        // Select channel and start conversion
        LPC_ADC->ADCR &= ~0xFF;
        LPC_ADC->ADCR |= 1 << 5; // ADC0[5]
        LPC_ADC->ADCR |= 1 << 24;
    
        // Repeatedly get the sample data until DONE bit
        unsigned int data;
        do {
           data = LPC_ADC->ADGDR;
        } while ((data & ((unsigned int)1 << 31)) == 0);

        // Stop conversion    
        LPC_ADC->ADCR &= ~(1 << 24);
    
        printf("0x%3X\n", data);   
    }
}