File content as of revision 0:d266ed1f3d53:

#include "mbed.h"

Serial pc(USBTX, USBRX); // tx, rx
Timer t;
DigitalOut led1(LED1);
DigitalOut led2(LED2);
DigitalOut led3(LED3);
DigitalOut led4(LED4);
DigitalOut start(p5);
InterruptIn stop(p6);
DigitalIn counter_start(p7);

void counter();

int main() {
    
    start   =   0;
    pc.baud(921600);
    
    led1 = 1;  led2 = 1;  led3 = 1;  led4 = 1;
    wait(0.5);    led4    =   0;
    wait(0.5);    led3    =   0;
    wait(0.5);    led2    =   0;
    wait(0.5);    led1    =   0;
    wait(0.5);
    led1 = 1;  led2 = 1;  led3 = 1;  led4 = 1;
    
    stop.rise(&counter);
    
    pc.printf("Push button!\r\n");
    while( counter_start==0 ){}
    pc.printf("Counter starts!!\r\n");
    start   =   1;
    t.start();
            
    while( start==1 ){}

    printf("The time was taken %f seconds\r\n", t.read());

}

void counter() {
    t.stop();
    start   =   0;
}