#include <stdio.h>
#include "stm32f3xx.h"

// The current clock frequency
uint32_t SystemCoreClock=8000000;

// Counts milliseconds
volatile uint32_t systick_count=0;

// Interrupt handler
void SysTick_Handler()
{
    systick_count++;
}

// Delay some milliseconds.
// Note that effective time may be up to 1ms shorter than requested.
void delay_ms(int ms)
{
    uint32_t start=systick_count;
    while (systick_count-start<ms);
}

// Redirect standard output to the serial port
int _write(int file, char *ptr, int len)
{
    for (int i=0; i<len; i++)
    {
        while(!(USART2->ISR & USART_ISR_TXE));
        USART2->TDR = *ptr++;
    }
    return len;
}

// Change system clock to 72 MHz using 8 MHz crystal
void init_clock()
{
    // Because the debugger switches PLL on, we may need to switch
    // back to the HSI oscillator before we can configure the PLL

    // Enable HSI oscillator
    SET_BIT(RCC->CR, RCC_CR_HSION);

    // Wait until HSI oscillator is ready
    while(!READ_BIT(RCC->CR, RCC_CR_HSIRDY)) {}

    // Switch to HSI oscillator
    MODIFY_REG(RCC->CFGR, RCC_CFGR_SW, RCC_CFGR_SW_HSI);

    // Wait until the switch is done
    while ((RCC->CFGR & RCC_CFGR_SWS_Msk) != RCC_CFGR_SWS_HSI) {}

    // Disable the PLL
    CLEAR_BIT(RCC->CR, RCC_CR_PLLON);

    // Wait until PLL is fully stopped
    while(READ_BIT(RCC->CR, RCC_CR_PLLRDY)) {}

    // Flash latency 2 wait states
    MODIFY_REG(FLASH->ACR, FLASH_ACR_LATENCY, FLASH_ACR_LATENCY_1);

    // Enable HSE oscillator
    SET_BIT(RCC->CR, RCC_CR_HSEON);

    // Wait until HSE oscillator is ready
    while(!READ_BIT(RCC->CR, RCC_CR_HSERDY)) {}

    // 72 MHz using the 8 MHz HSE oscillator with 9x PLL, lowspeed I/O runs at 36 MHz
    WRITE_REG(RCC->CFGR, RCC_CFGR_SWS_HSE + RCC_CFGR_PLLSRC_HSE_PREDIV + RCC_CFGR_PLLMUL9 + RCC_CFGR_PPRE1_DIV2);

    // Enable PLL
    SET_BIT(RCC->CR, RCC_CR_PLLON);

    // Wait until PLL is ready
    while(!READ_BIT(RCC->CR, RCC_CR_PLLRDY)) {}

    // Select PLL as clock source
    MODIFY_REG(RCC->CFGR, RCC_CFGR_SW, RCC_CFGR_SW_PLL);

    // Update variable
    SystemCoreClock=72000000;

    // Disable the HSI oscillator
    CLEAR_BIT(RCC->CR, RCC_CR_HSION);
}


void init_io()
{
    // Enable clock for Port A
    SET_BIT(RCC->AHBENR, RCC_AHBENR_GPIOAEN);

    // PA5 = Output for the LED
    MODIFY_REG(GPIOA->MODER, GPIO_MODER_MODER5, GPIO_MODER_MODER5_0);

    // Use system clock for USART2
    SET_BIT(RCC->APB1ENR, RCC_APB1ENR_USART2EN);
    MODIFY_REG(RCC->CFGR3, RCC_CFGR3_USART2SW, RCC_CFGR3_USART2SW_0);

    // PA2 (TxD) shall use the alternate function 7 (see data sheet)
    MODIFY_REG(GPIOA->AFR[0], GPIO_AFRL_AFRL2, 7UL<<GPIO_AFRL_AFRL2_Pos);
    MODIFY_REG(GPIOA->MODER, GPIO_MODER_MODER2, GPIO_MODER_MODER2_1);

    // PA3 (RxD) shall use the alternate function 7 (see data sheet)
    MODIFY_REG(GPIOA->AFR[0], GPIO_AFRL_AFRL3, 7UL<<GPIO_AFRL_AFRL3_Pos);
    MODIFY_REG(GPIOA->MODER, GPIO_MODER_MODER3, GPIO_MODER_MODER3_1);

    // Set baudrate, assuming that USART2 is clocked with
    // the same frequency as the CPU core (no prescalers)
    USART2->BRR = (SystemCoreClock / 115200);

    // Enable transmitter of USART2
    USART2->CR1 = USART_CR1_UE + USART_CR1_TE;
}


void SystemInit()
{
    // Switch the FPU on
    SCB->CPACR = 0x00F00000;
}


int main()
{
    init_clock();
    init_io();

    // Initialize the timer for 1 ms intervals
    SysTick_Config(SystemCoreClock/1000);

    printf("Start\n");
    delay_ms(100);

    while (1)
    {
        // LED an
        WRITE_REG(GPIOA->BSRR, GPIO_BSRR_BS_5);

        uint32_t start=systick_count;

        float f=1.0;
        for (int i=0; i<10000; i++)
        {
            f=f*1.001;

            if (f>100000.0)
            {
                f=1.0;
            }
        }

        // LED aus
        WRITE_REG(GPIOA->BSRR, GPIO_BSRR_BR_5);

        printf("f=%f, duration=%ld ms\n",f, systick_count-start);
        delay_ms(500);
    }
}