File content as of revision 13:5d689f89d794:

#include "mbed.h"
#include "platform/mbed_thread.h"

using namespace std::chrono;

InterruptIn user_button(USER_BUTTON);
DigitalOut  led(LED1);

bool  executeMainTask = false;
Timer user_button_timer, loop_timer;
int   Ts_ms = 50;

/* declaration of custom button functions */
void button_fall();
void button_rise();

/* create analog input object */
AnalogIn analogIn(PA_0);
float    dist = 0.0f;

int main()
{
    user_button.fall(&button_fall);
    user_button.rise(&button_rise);
    loop_timer.start();

    while (true) {

        loop_timer.reset();

        /* ------------- start hacking ------------- -------------*/

        if (executeMainTask) {

            /* read analog input */
            dist = analogIn.read()*3.3f;
 
            /* do only output what's really necessary, outputting "Measured value in mV: "" within the loop is no good solution */
            printf("Measured value in mV: %d\r\n", (static_cast<int>(dist * 1e3)));

            /* visual feedback that the main task is executed */
            led = !led;

        } else {
            led = 0;
        }

        /* ------------- stop hacking ------------- -------------*/

        int T_loop_ms = duration_cast<milliseconds>(loop_timer.elapsed_time()).count();
        int dT_loop_ms = Ts_ms - T_loop_ms;
        thread_sleep_for(dT_loop_ms);
    }
}

void button_fall()
{
    user_button_timer.reset();
    user_button_timer.start();
}

void button_rise()
{
    int t_button_ms = duration_cast<milliseconds>(user_button_timer.elapsed_time()).count();
    user_button_timer.stop();
    if (t_button_ms > 200) {
        executeMainTask = !executeMainTask;
    }
}