#include <stm32l476xx.h>

// Initializes the ADC for reading from channel one.
void adc_init(void);
// Does an ADC conversion on channel one and returns the result.
unsigned int adc_read(void);

int main(void)
{
    adc_init();

    // DAC initialization:
    // Enables DAC channel one on PA4
    // Does not configure a hardware or software trigger, so DAC updates
    // when its data register is written.
    RCC->AHB2ENR |= RCC_AHB2ENR_GPIOAEN;
    RCC->APB1ENR1 |= RCC_APB1ENR1_DAC1EN;
    DAC1->CR = DAC_CR_EN1;

    // Execute a first-order IIR filter in a loop: y[n] = a*y[n-1] + b*x[n]
    const float a = 0.5f;
    const float b = 0.5f;
    unsigned int yprev = 0; // y[n-1]
    while (1) {
        unsigned int x = adc_read();
        unsigned int y = a * yprev + b * x;

        DAC1->DHR12R1 = y;
        yprev = y;

        // Add some delay between samples
        for (int i = 0; i < 100; ++i);
    }
}

void adc_init(void)
{
    // Use the CCIPR register to select the system clock as a source for the
    // ADC clock, then enable the ADC clock through the AHB.
    RCC->CCIPR &= ~(RCC_CCIPR_ADCSEL_Msk);
    RCC->CCIPR |= 3 << RCC_CCIPR_ADCSEL_Pos;
    RCC->AHB2ENR |= RCC_AHB2ENR_ADCEN;

    // Enable the ADC's internal voltage regulator, with a delay for its
    // startup time.
    // ADVREGEN requires some bits in the CR register to be zero before being
    // set, so we clear CR to zero first.
    ADC1->CR = 0;
    ADC1->CR = ADC_CR_ADVREGEN;
    for (int i = 0; i < 100; ++i);

    // Clear DIFSEL to do single-ended conversions.
    ADC1->DIFSEL = 0;

    // Begin a single-ended calibration and wait for it to complete.
    // If the clock source for the ADC is not correctly configured, the
    // processor may get stuck in the while loop.
    ADC1->CR &= ~(ADC_CR_ADCALDIF); // Select single-ended calibration
    ADC1->CR |= ADC_CR_ADCAL;
    while ((ADC1->CR & ADC_CR_ADCAL) != 0);

    // Enable the ADC and wait for it to be ready.
    ADC1->CR |= ADC_CR_ADEN;
    while ((ADC1->ISR & ADC_ISR_ADRDY) == 0);

    // GPIO pin PC3 will be used as our ADC input.
    // In order: enable the GPIOC clock, set PC3 to analog input mode,
    // connect ADC channel one to its GPIO pin (PC3).
    RCC->AHB2ENR |= RCC_AHB2ENR_GPIOCEN;
    GPIOC->MODER |= 3 << GPIO_MODER_MODE0_Pos;
    GPIOC->ASCR |= 1 << 0;

    // Make channel one the first in the ADC's read sequence.
    // Using '=' here also clears the L field of the register, giving the
    // sequence a length of one.
    ADC1->SQR1 = 1 << ADC_SQR1_SQ1_Pos;
}

unsigned int adc_read(void)
{
    ADC1->CFGR &= ~(ADC_CFGR_CONT);         // Single conversion
    ADC1->CR |= ADC_CR_ADSTART;             // Start converting
    while ((ADC1->ISR & ADC_ISR_EOC) == 0); // Wait for end-of-conversion
    ADC1->ISR &= ~(ADC_ISR_EOC);            // Clear the status flag
    return ADC1->DR;                        // Read the result
}