File content as of revision 14:73923b07ae4a:

// https://devzone.nordicsemi.com/question/44628/maximum-clock-possible-on-gpio-pin-nrf51/

// Mbed doesn't support high speed clock output, so had to hack this using direct calls to the Nordic API

#include "mbed.h"

// Not to be confused with the output pin
#define GPIOTE_CHANNEL_NUMBER 0

/**
 * @enum nrf_gpiote_polarity_t
 * @brief Polarity for GPIOTE channel enumerator.
 */
typedef enum
{
  NRF_GPIOTE_POLARITY_LOTOHI = GPIOTE_CONFIG_POLARITY_LoToHi,       ///<  Low to high
  NRF_GPIOTE_POLARITY_HITOLO = GPIOTE_CONFIG_POLARITY_HiToLo,       ///<  High to low
  NRF_GPIOTE_POLARITY_TOGGLE = GPIOTE_CONFIG_POLARITY_Toggle        ///<  Toggle
} nrf_gpiote_polarity_t;


 /**
 * @enum nrf_gpiote_outinit_t
 * @brief Initial output value for GPIOTE channel enumerator.
 */
typedef enum
{
  NRF_GPIOTE_INITIAL_VALUE_LOW  = GPIOTE_CONFIG_OUTINIT_Low,       ///<  Low to high
  NRF_GPIOTE_INITIAL_VALUE_HIGH = GPIOTE_CONFIG_OUTINIT_High       ///<  High to low
} nrf_gpiote_outinit_t;


                        
static void timer2_init()
{
    // Start 16 MHz crystal oscillator.
    NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
    NRF_CLOCK->TASKS_HFCLKSTART    = 1;

    // Wait for the external oscillator to start up.
    while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0)
    {
        // Do nothing.
    }
    NRF_TIMER2->MODE        = TIMER_MODE_MODE_Timer;       // Set the timer in Timer Mode.
    NRF_TIMER2->PRESCALER   = 0;                          
    NRF_TIMER2->BITMODE     = TIMER_BITMODE_BITMODE_16Bit; // 16 bit mode.
    NRF_TIMER2->TASKS_CLEAR = 1;
    NRF_TIMER2->CC[0]       = 1;
    NRF_TIMER2->EVENTS_COMPARE[0] = 0;
    NRF_TIMER2->SHORTS    =
        (TIMER_SHORTS_COMPARE0_CLEAR_Enabled << TIMER_SHORTS_COMPARE0_CLEAR_Pos);
}

static void ppi_init(void)
{
    // Configure PPI channel 0 to toggle GPIO_OUTPUT_PIN on every TIMER2 COMPARE[0] match (200 ms)
    NRF_PPI->CH[0].EEP = (uint32_t)&NRF_TIMER2->EVENTS_COMPARE[0];
    NRF_PPI->CH[0].TEP = (uint32_t)&NRF_GPIOTE->TASKS_OUT[GPIOTE_CHANNEL_NUMBER];

    // Enable PPI channel 0
    NRF_PPI->CHEN = (PPI_CHEN_CH0_Enabled << PPI_CHEN_CH0_Pos);
}


/**
 * @brief Config GPIOTE channel as output, setting the properly desired output level.
 *
 *
 * @param channel_number specifies the GPIOTE channel [0:3] to configure as an output channel.
 * @param pin_number specifies the pin number [0:30] to use in the GPIOTE channel.
 * @param polarity specifies the desired polarity in the output GPIOTE channel.
 * @param initial_value specifies the initial value of the GPIOTE channel input after the channel configuration.
 */
static __INLINE void nrf_gpiote_task_config(uint32_t channel_number, uint32_t pin_number, nrf_gpiote_polarity_t polarity, nrf_gpiote_outinit_t initial_value)
{
    /* Check if the output desired is high or low */
    if (initial_value == NRF_GPIOTE_INITIAL_VALUE_LOW)
    {
        /* Configure channel to Pin31, not connected to the pin, and configure as a tasks that will set it to proper level */
        NRF_GPIOTE->CONFIG[channel_number] = (GPIOTE_CONFIG_MODE_Task       << GPIOTE_CONFIG_MODE_Pos)     |
                                             (31UL                          << GPIOTE_CONFIG_PSEL_Pos)     |
                                             (GPIOTE_CONFIG_POLARITY_HiToLo << GPIOTE_CONFIG_POLARITY_Pos);                                    
    } 
    else 
    {
        /* Configure channel to Pin31, not connected to the pin, and configure as a tasks that will set it to proper level */
        NRF_GPIOTE->CONFIG[channel_number] = (GPIOTE_CONFIG_MODE_Task       << GPIOTE_CONFIG_MODE_Pos)     |
                                             (31UL                          << GPIOTE_CONFIG_PSEL_Pos)     |
                                             (GPIOTE_CONFIG_POLARITY_LoToHi << GPIOTE_CONFIG_POLARITY_Pos);
    }

    /* Three NOPs are required to make sure configuration is written before setting tasks or getting events */
    __NOP();
    __NOP();
    __NOP(); 

    /* Launch the task to take the GPIOTE channel output to the desired level */
    NRF_GPIOTE->TASKS_OUT[channel_number] = 1;

    /* Finally configure the channel as the caller expects. If OUTINIT works, the channel is configured properly. 
       If it does not, the channel output inheritance sets the proper level. */
    NRF_GPIOTE->CONFIG[channel_number] = (GPIOTE_CONFIG_MODE_Task << GPIOTE_CONFIG_MODE_Pos)     |
                                         ((uint32_t)pin_number    << GPIOTE_CONFIG_PSEL_Pos)     |
                                         ((uint32_t)polarity      << GPIOTE_CONFIG_POLARITY_Pos) |
                                         ((uint32_t)initial_value << GPIOTE_CONFIG_OUTINIT_Pos);

    /* Three NOPs are required to make sure configuration is written before setting tasks or getting events */
    __NOP();
    __NOP();
    __NOP(); 
}

void init_clock(PinName clkPin) {
 
    // configure TIMER2
    timer2_init();

    // configure PPI
    ppi_init();

    nrf_gpiote_task_config(GPIOTE_CHANNEL_NUMBER, clkPin, \
                           NRF_GPIOTE_POLARITY_TOGGLE , NRF_GPIOTE_INITIAL_VALUE_LOW);
  
   NRF_TIMER2->TASKS_START = 1;  // Start event generation.
}