/* p11_4.c Variable duty cycle PWM to control LED on duration
 *
 * This program is the same as p11_3.c except the frequency is much
 * slower so that the change of the duration when the LED is on
 * can be observed by naked eyes.
 * 
 * This program was tested with Keil uVision v5.24a with DFP v2.11.0
 */
 
#include "stm32f4xx.h"

void delayMs(int n);

int main(void) {
    RCC->AHB1ENR |= 1;              /* enable GPIOA clock */
    
    GPIOA->AFR[0] |= 0x00300000;    /* PA5 pin for TIM8 */
    GPIOA->MODER &= ~0x00000C00;
    GPIOA->MODER |=  0x00000800;
    
    /* setup TIM8 */
    RCC->APB2ENR |= 2;              /* enable TIM8 clock */
    TIM8->PSC = 16000 - 1;          /* divided by 16000 */
    TIM8->ARR = 1000 - 1;           /* divided by 1000 */
    TIM8->CNT = 0;
    TIM8->CCMR1 = 0x0068;           /* PWM mode */
    TIM8->CCER = 4;                 /* enable PWM Ch1N */
    TIM8->CCR1 = 10;                /* pulse width */
    TIM8->BDTR |= 0x8000;           /* enable output */
    TIM8->CR1 = 1;                  /* enable timer */

    while(1) {
        TIM8->CCR1 = TIM8->CCR1 * 110 / 100;
        if (TIM8->CCR1 > 1000)
            TIM8->CCR1 = 10;
        delayMs(400);
    }
}

/* 16 MHz SYSCLK */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 3195; i++) ;
}