void cfgSPI1(void)
{
  //Initialize SPI peripheral
  RCC -> APB2ENR |= RCC_APB2ENR_SPI1EN;
  //Set baud rate to fclk/128
  SPI1 -> CR1 |= SPI_CR1_BR_2 | SPI_CR1_BR_1;
  SPI1 -> CR1 |= SPI_CR1_SSM | SPI_CR1_SSI;
  //Set it as master
  SPI1 -> CR1 |= SPI_CR1_MSTR;
  SPI1 -> CR1 |= SPI_CR1_SPE;

  //Enable GPIOA peripheral clock and configure GPIO pins
  RCC -> APB2ENR |= RCC_APB2ENR_IOPAEN;
  //NSS
  GPIOA -> CRL |= GPIO_CRL_CNF4_1;
  //SCK
  GPIOA -> CRL |= GPIO_CRL_MODE5_1 | GPIO_CRL_MODE5_0;
  GPIOA -> CRL |= GPIO_CRL_CNF5_1;
  //MISO
  GPIOA -> CRL |= GPIO_CRL_MODE6_1 | GPIO_CRL_MODE6_0;
  GPIOA -> CRL |= GPIO_CRL_CNF6_1;
  //MOSI
  GPIOA -> CRL |= GPIO_CRL_MODE7_1 | GPIO_CRL_MODE7_0;
  GPIOA -> CRL |= GPIO_CRL_CNF7_1;
}

void TIM2_IRQHandler(void)
{
    TIM2->SR &= ~(TIM_SR_UIF);
    //Set slave select to low
    GPIOA -> ODR &= GPIO_ODR_ODR4;
    //Transfer 1 over SPI
    SPI1 -> DR |= 1;
    //Wait until transfer is completed
    while (!(SPI1->SR & (SPI_SR_TXE)));
    // Wait until receive complete
    while (!(SPI1->SR & (SPI_SR_RXNE)));
    // Wait until SPI is not busy anymore
    while (SPI1->SR & (SPI_SR_BSY));
    int i = SPI1 -> DR;
    GPIOA -> ODR |= GPIO_ODR_ODR4;
    // Toggle the LED output pin.
    GPIOC->ODR ^= GPIO_ODR_ODR13 ;
}

#include "Arduino.h"

ISR(SPI_STC_vect)
{
    uint8_t data_byte = SPDR;
    Serial.print(data_byte);
    Serial.print("test")
}

void setup()
{
    Serial.begin(9600);
    // Set MISO pin as output
    pinMode(MISO, OUTPUT);
    // Turn on SPI in slave mode
    SPCR |= (1 << SPE);
    // Turn on interrupt
    interrupts();
    SPCR |= (1 << SPIE);
}

void loop()
{
}