/* p6_7.c Demonstrate Nested Interrupts
 *
 * TIM3 is setup to interrupt at about 1 Hz. In the interrupt handler,
 * PB3 is turned on and a delay function of 500 ms is called.
 * PB3 is turned off at the end of the delay.
 *
 * TIM14 is setup to interrupt at about 10 Hz. In the interrupt handler,
 * PB4 and the LD2 is turned on and a delay function of 25 ms is called.
 * PB4 and the LD2 is turned off at the end of the delay.
 *
 * When TIM3 has the higher priority (the way this code is), the TIM14
 * interrupts are blocked by TIM3 interrupt handler and the LED stops
 * blinking intermittently. You can see their interactions better
 * with an oscilloscope on PB3 and PB4.
 *
 * When TIM14 has the higher priority (you need to switch the priority of
 * the two timers at the #define statements), the TIM3 interrupt handler is
 * preempted by TIM14 interrupts and the LED is blinking all the time.
 *
 * This program was tested with Keil uVision v5.23 with DFP v2.0.0
 */

#include "stm32f0xx.h"

void delayMs(int n);

/* priority of TIM3 and TIM14 should be between 0 and 3 */
#define PRIO_TIM3 1
#define PRIO_TIM14 2

int main (void) {
    __disable_irq();                /* global disable IRQs */
    RCC->AHBENR |= 0x00020000;      /* enable GPIOA clock */
    RCC->AHBENR |= 0x00040000;      /* enable GPIOB clock */
    
    GPIOA->MODER &= ~0x00000C00;    /* pin PA5 (LED) output */
    GPIOA->MODER |=  0x00000400;
    GPIOB->MODER &= ~0x000003C0;    /* pins PB3, PB4 output */
    GPIOB->MODER |=  0x00000140;
    
    /* setup TIM3 */
    RCC->APB1ENR |= 0x00000002;     /* enable TIM3 clock */
    TIM3->PSC = 1000 - 1;           /* divided by 1000 */
    TIM3->ARR = 7500 - 1;           /* divided by 7500 */
    TIM3->CR1 = 1;                  /* enable counter */

    TIM3->DIER |= 1;                /* enable UIE */
    NVIC_SetPriority(TIM3_IRQn, PRIO_TIM3);
    NVIC_EnableIRQ(TIM3_IRQn);      /* enable interrupt in NVIC */
    
    /* setup TIM14 */
    RCC->APB1ENR |= 0x00000100;     /* enable TIM14 clock */
    TIM14->PSC = 1000 - 1;          /* divided by 1000 */
    TIM14->ARR = 781 - 1;           /* divided by 781 */
    TIM14->CR1 = 1;                 /* enable counter */

    TIM14->DIER |= 1;               /* enable UIE */
    NVIC_SetPriority(TIM14_IRQn, PRIO_TIM14);
    NVIC_EnableIRQ(TIM14_IRQn);     /* enable interrupt in NVIC */

    __enable_irq();                 /* enable interrupt globally */

    while(1) {
    }
}

void TIM3_IRQHandler(void) {
    GPIOB->BSRR = 0x00000008;       /* turn on PB3 */
    delayMs(500);
    GPIOB->BSRR = 0x00080000;       /* turn off PB3 */
    TIM3->SR = 0;                   /* clear UIF */
}

void TIM14_IRQHandler(void) {
    GPIOA->BSRR = 0x00000020;       /* turn on LED */
    GPIOB->BSRR = 0x00000010;       /* turn on PB4 */
    delayMs(25);
    GPIOA->BRR  = 0x00000020;       /* turn off LED */
    GPIOB->BRR  = 0x00000010;       /* turn off PB4 */
    TIM14->SR = 0;                  /* clear UIF */
}

/* 8 MHz SYSCLK */
void delayMs(int n) {
    int i;
    for (; n > 0; n--)
        for (i = 0; i < 1142; i++) ;
}