/* p5_9.c TIM1 External Event Counting

 * This program configures TIM1 to use external input PA12 as the
 * counter clock source. Each rising edge of the input signal
 * increments the TIM1 counter by 1.
 * Use an external signal to drive PA12 for testing.
 * Or use a jumper between PC13 and PA12. PC13 is connected to the
 * user switch of the Nucleo board.
 *
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0
 */
#include "stm32f0xx.h"
int main(void) {
    unsigned short counter;

    // configure PA5 output for the LED
    RCC->AHBENR |=  0X00020000;         /* enable GPIOA clock */
    GPIOA->MODER &= ~0x00000C00;        /* clear pin mode */
    GPIOA->MODER |=  0x00000400;        /* set pin to output mode */

    // configure PA12 as input of TIM1 ETR
    GPIOA->MODER &= ~0x03000000;        /* clear pin mode */
    GPIOA->MODER |=  0x02000000;        /* set pin to alternate function */
    GPIOA->AFR[1] &= ~0x000F0000;       /* clear pin AF bits */
    GPIOA->AFR[1] |=  0x00020000;       /* set pin to AF2 for TIM1 ETR */

    // configure TIM1 to use external input as counter clock source
    RCC->APB2ENR |= 0x00000800;         /* enable TIM1 clock */
    TIM1->SMCR = 0x4377;                /* use ETR input as clock, no prescaler */
    TIM1->CNT = 0;                      /* clear counter */
    TIM1->CR1 = 1;                      /* enable TIM1 */

    while (1) {
        // use bit 0 to turn LED on/off
        counter = TIM1->CNT;
        if (counter & 1)
            GPIOA->ODR |= 0x20;     /* turn on LED */
        else
            GPIOA->ODR &= ~0x20;    /* turn off LED */
    }
}