Simple timer not working

Question

I coded a simple timer using app_timer and tested it on my nRF51822-DK:

#ifndef NRF51
#error NRF51 must be defined.
#endif

#include <nrf.h>
#include <app_timer.h>

#include <nrf_delay.h>
#include <nordic_common.h>

#include "nrf_drv_config.h"
#include "leds.h"

uint8_t flag;

APP_TIMER_DEF(timer);

/**
 * @brief Function for error handling, which is called when an error has occurred.
 *
 * @param[in] error_code  Error code supplied to the handler.
 * @param[in] line_num    Line number where the handler is called.
 * @param[in] p_file_name Pointer to the file name.
 */

void app_error_handler(uint32_t error_code, uint32_t line_num, const uint8_t * p_file_name)
{
    // On assert, the system can only recover with a reset.
#ifndef DEBUG
    // On release mode we simply reset the device.
    NVIC_SystemReset();
#else
    // This call can be used for debug purposes during application development.
    // @note CAUTION: Activating this code will write the stack to flash on an error.
    //                This function should NOT be used in a final product.
    //                It is intended STRICTLY for development/debugging purposes.
    //                The flash write will happen EVEN if the radio is active, thus interrupting
    //                any communication.
    //                Use with care. Uncomment the line below to use.
    //ble_debug_assert_handler(error_code, line_num, p_file_name);
    // The following variable helps Keil keep the call stack visible, in addition, it can be set to
    // 0 in the debugger to continue executing code after the error check.
    volatile bool loop = true;
    UNUSED_VARIABLE(loop);

    uint32_t m_error_code = error_code;
    uint32_t m_line_num = line_num;
    const uint8_t * m_p_file_name = p_file_name;

    UNUSED_VARIABLE(m_error_code);
    UNUSED_VARIABLE(m_line_num);
    UNUSED_VARIABLE(m_p_file_name);
    __disable_irq();

    led_green_on();
    led_red_off();

    for (;;)
    {
        nrf_delay_ms(100);
        led_green_toggle();
        led_red_toggle();
    }
#endif // DEBUG
}

uint32_t event_handler(app_timer_timeout_handler_t timeout_handler,
                       void *                      p_context)
{
    flag = 0;
    return NRF_SUCCESS;
}

void timeout()
{
     led_red_on();
}

int main()
{
    uint32_t retval = 0;

    leds_init();

    // Initialize app_timer.
    APP_TIMER_INIT(TIMER_PRESCALER, OP_QUEUES_SIZE, event_handler);

    retval = app_timer_create(&timer, APP_TIMER_MODE_SINGLE_SHOT, timeout);
    app_timer_start(timer, APP_TIMER_TICKS(2000, TIMER_PRESCALER), NULL);

    flag = 1;
    do
    {
        nrf_delay_ms(200);
        led_green_toggle();
    } while(flag);

    // This should never be reached. If it is app_error_hanlder will be called.
    APP_ERROR_CHECK_BOOL(false);
    return retval;
}

Where the "green" led is LED1 and the "red" one is LED2, TIMER_PRESCALER is 0, OP_QUEUES_SIZE is 8.

I would expect the green led to blink for ~2seconds and then the blink will stop, the red led will turn on and after that both will be toggling fast.

But it seems I never get a timeout. What am I missing here?

Simple timer not working

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