File content as of revision 0:bc53a5d93a32:

#include "mbed.h"
 
#define SHT21_REG_DATA  (0x00) // Data Register
#define SHT21_REG_CONF  (0x01) // Configuration Register
#define SHT21_ADDR      (0xAA) // SHT21 address
 
I2C i2c1(D14, D15); // This is I2C1 SDA, SCL
//PwmOut attiny(D3);    // This is PWM2/2
DigitalOut attiny(D3);  // This is PWM2/2
Serial usart1(D10, D2); // This is USART1 TX, RX
InterruptIn button(USER_BUTTON);    // This is USERBUTTON (PC_13)

Ticker flipper;
DigitalOut led(LED1);

volatile bool button_pressed = false;

//void usart1_RxIrq()
//{
//  if(usart1.readable())
//  {
//  }
//}

void pwmon()
{
    button_pressed = true;
}

void pwmoff()
{
    button_pressed = false;
}

void flip() 
{
  led = !led;
    attiny = !attiny;
}

int main()
{
    bool set_pwm = false;
    int data = 0;
    char data_write[2];
    char data_read[1];
    
    // Configure the Temperature sensor device SHT21:
//  data_write[0] = SHT21_REG_CONF;
//  data_write[1] = 0x00;
//  i2c1.write(SHT21_ADDR, data_write, 2, 0);
    
    // Configure serial port
    usart1.baud(115200);    // Baud rate
    //usart1.attach(&usart1_RxIrq, Serial::RxIrq);      // RX interrupt handler
    
    // Configure button interrupt
    button.fall(&pwmon);
    button.rise(&pwmoff);
    
    while(1)
    {
        if(button_pressed)
        {
            data = 0;
            
            if(!set_pwm)
            {
                // Configure pwm
//              attiny.period_ms(50);   // choose the period of the pwm    
//              attiny.write(0.50f);    // 50% duty cycle

                set_pwm = true;
                led = 1;
                attiny = 1;
                
                flipper.attach(&flip, 3.0); // the address of the function to be attached (flip) and the interval (3 seconds)
            }
        }
        else
        {
//          attiny.write(0);    // Turn off the pwm         
            flipper.detach();
            
            set_pwm = false;
            attiny = 0;
            led = 0;
            
            // Write and Read from the slave
            data_write[0] = SHT21_REG_DATA;
            data_write[1] = data;
            i2c1.write(SHT21_ADDR, data_write, 2, false);
            i2c1.read(SHT21_ADDR, data_read, 1, false);
            
            usart1.printf("Value = %d\n", data_read);
            wait(2.0);
            
            data++;
        }
    }
}