void delay(void)
{
   SysTick->LOAD = 15999;
   SysTick->CTRL = 0x5;    /*Enable the timer and choose cclk as the clock source */

   while((SysTick->CTRL & 0x10000) == 0) /*wait until the Count flag is set */
   { }
   SysTick->CTRL  = 0; /*Stop the timer (Enable = 0) */
}