#include "stm32f4xx.h"
#include "LCD2x16.c"

int IRQcounter = 0;             // declare & initialize IRQ counter

void main (void){   

  RCC->AHB1ENR |= 0x10;         // Clock for PortE
  RCC->APB2ENR |= 0x00004000;   // Clock for SYSCFG - system configuration controller

  GPIOE->MODER |= 0x55550000;   // upper half port E == outputs
  
  LCD_init();                            // init LCD
  LCD_string("Number of IRQs:", 0x00);   // display title string

  NVIC_EnableIRQ(EXTI3_IRQn);            // Enable IRQ for external signals, line EXTI3_IRQn

  SYSCFG->EXTICR[0] = 0x4000;            // select PE to make IRQ EXTI3
  EXTI->RTSR |= 0x00000008;              // allow positive edge interrupt for EXTI3
  EXTI->IMR  |= 0x00000008;              // enable interrupt on EXTI3  
  
  while (1) {
    LCD_uInt16(IRQcounter,0x40,1);       // write IRQ count to LCD
    LCD_uInt16(GPIOE->IDR & 0x3f,0x48,1);       // write keycode to LCD
    for (int i = 0; i<100000; i++) {};   // waste some time
  };
}

void EXTI3_IRQHandler (void)   {
  IRQcounter++;                          // do the actual work
  EXTI->PR = 0x08;                       // clear interrupt flag for EXTI3
  GPIOE->ODR |=  0x0100;                 // added to signal the execution of the IRQ function
  GPIOE->ODR &= ~0x0100;                 // end of execution, about 300ns later
}