File content as of revision 0:fb3e19364d48:

#include "mbed.h"

#define TICKS_PER_REVOLUTION    12.0
#define TICK_CHECK_PERIOD       0.1

DigitalOut myled(LED1);
Serial pc(USBTX, USBRX); // tx, rx
Ticker periodicTimer1, periodicTimer2;
 
float speed_rpm, speed_rps, ticks_per_second;
volatile unsigned int tick_counter, final_count;

InterruptIn freqInputPin(D2); 

void freqInputPin_Interrupt() {
    //Triggers on the rising edge of the frequency signal
    tick_counter++;
}

void check_TickCount(){
    final_count = tick_counter;
    //pc.printf("\n\rTick Count is : %d", final_count);
    tick_counter = 0;
}

void display_TickCount(){
   //pc.printf("\n\r Final Count is : %.2f", (float) final_count*(1.0/TICK_CHECK_PERIOD));
   pc.printf("\n\r Speed in RPM is  : %.2f", (float)(final_count*(1.0/TICK_CHECK_PERIOD))*(60.0/TICKS_PER_REVOLUTION));
}

int main() {
    freqInputPin.rise(&freqInputPin_Interrupt); //chain interrupt to rising edge
    periodicTimer1.attach(&check_TickCount, TICK_CHECK_PERIOD);
    periodicTimer2.attach(&display_TickCount, 1.0); //Display conut once a second
    while(1) { 
    }
}