/* p5_9.c TIM2 External Event Counting

 * This program configures TIM2 to use external input PB8 as the
 * counter clock source. Each rising edge of the input signal
 * increments the TIM2 counter by 1.
 * Use an external signal to drive PB8 for testing or use a jumper
 * between PC13 and PB8. PC13 is connected to the user switch.
 * 
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
#include "stm32f4xx.h"
int main(void) {
    // configure PA5 output for the LED
    RCC->AHB1ENR |=  1;	            /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;    /* clear pin mode */
    GPIOA->MODER |=  0x00000400;    /* set pin to output mode */

    // configure PB8 as input of TIM2 ETR
    RCC->AHB1ENR |=  2;             /* enable GPIOB clock */
    GPIOB->MODER &= ~0x00030000;    /* clear pin mode */
    GPIOB->MODER |=  0x00020000;    /* set pin to alternate function */
    GPIOB->AFR[1] &= ~0x0000000F;   /* clear pin AF bits */
    GPIOB->AFR[1] |= 0x00000001;    /* set pin to AF1 for TIM2 ETR */
    
    // configure TIM2 to use external input as counter clock source
    RCC->APB1ENR |= 1;              /* enable TIM2 clock */
    TIM2->SMCR = 0x4377;            /* use ETR input as clock, no prescaler */
    TIM2->CNT = 0;                  /* clear counter */
    TIM2->CR1 = 1;                  /* enable TIM2 */

    while (1) {
        // use bit 0 to turn LED on/off
        if (TIM2->CNT & 1)
            GPIOA->ODR |= 0x20;     /* turn on LED */
        else
            GPIOA->ODR &= ~0x20;    /* turn off LED */
    }
}