/*
 * Use TIM2_CH4 PWM function to drive the speaker on
 * the EduBase board to generate a 440 Hz sound.
 * The timer TIM2 clock is running on 16 MHz. Timer divider is
 * 16,000,000 / 440 = 36364
 */
#include "stm32f4xx.h"

int main(void) {
    RCC->AHB1ENR |=  2;             /* enable GPIOB clock */

    // configure PB2 as TIM2_CH4 PWM output pin
    GPIOB->AFR[0] &= ~0x00000F00;
    GPIOB->AFR[0] |=  0x00000100;   /* PB2 pin for TIM2 */
    GPIOB->MODER &= ~0x00000030;
    GPIOB->MODER |=  0x00000020;    /* PB2 uses alternate function */

    /* setup TIM2_CH4 PWM */
    RCC->APB1ENR |= 1;              /* enable TIM2 clock */
    TIM2->PSC = 0;                  /* prescaler divided by 1 */
    TIM2->ARR = 36364 - 1;          /* counter divided by 36364 */
    TIM2->CNT = 0;                  /* clear counter */
    TIM2->CCMR2 = 0x6000;           /* PWM mode */
    TIM2->CCER = 0x1000;            /* enable PWM Ch4 */
    TIM2->CCR4 = 18182 - 1;         /* pulse width 1/2 of the period */
    TIM2->CR1 = 1;                  /* enable timer */

    while(1) {
    }
}