STM32F4 OpenCM3 USART example

#include <libopencm3/stm32/rcc.h>
#include <libopencm3/stm32/gpio.h>
#include <libopencm3/stm32/usart.h>
#include <libopencm3/cm3/nvic.h>

int _write(int fd, char *str, int len);

static void setup(void)
{
    rcc_periph_clock_enable(RCC_USART2);
    rcc_periph_clock_enable(RCC_GPIOA);
    rcc_periph_clock_enable(RCC_GPIOD);

    nvic_enable_irq(NVIC_USART2_IRQ);

    gpio_mode_setup(GPIOA, GPIO_MODE_AF, GPIO_PUPD_NONE, GPIO2 | GPIO3);
    gpio_mode_setup(GPIOD, GPIO_MODE_OUTPUT, GPIO_PUPD_NONE, GPIO12 | GPIO13);

    gpio_set_output_options(GPIOA, GPIO_OTYPE_OD, GPIO_OSPEED_25MHZ, GPIO3);

    gpio_set_af(GPIOA, GPIO_AF7, GPIO2);
    gpio_set_af(GPIOA, GPIO_AF7, GPIO3);

    usart_set_baudrate(USART2, 115200);
    usart_set_databits(USART2, 8);
    usart_set_parity(USART2, USART_PARITY_NONE);
    usart_set_stopbits(USART2, USART_STOPBITS_1);

    usart_set_flow_control(USART2, USART_FLOWCONTROL_NONE);
    usart_set_mode(USART2, USART_MODE_TX_RX);

    usart_enable_rx_interrupt(USART2);

    usart_enable(USART2);

    _write(1, "Ready, waiting for interrupts.", 30);
}

static void wait(int us)
{
    int i;
    for(i=0; i<us; i++){
        __asm__("nop");
    }
}

//syscall
int _write(int fd, char *str, int len)
{
    int i;
    for(i = 0; i < len; i++)
    {
        if(str[i] == '\n')
            usart_send_blocking(USART2, '\r');
        usart_send_blocking(USART2, str[i]);
    }
    return i;
}

static void loop()
{
    _write(1, "I am busy waiting...\n", 22);
    wait(1000000);
    __asm__("nop");
}

int main(void)
{
    setup();

    while (1) {
        loop();
    }

    return 0;
}

#define USART_RXRDY(u) \
        (((USART_CR1(u) & USART_CR1_RXNEIE) != 0) && \
         ((USART_SR(u) & USART_SR_RXNE) != 0))


void usart2_isr(void)
{
    if(USART_RXRDY(USART2))
    {
        gpio_set(GPIOD, GPIO12);
        char data = 'a';
        data = usart_recv(USART2);
        if(data == 0x10)    //CR
            data = 0x13;    //LF
        _write(1,"IRQ: Recv: ", 11);
        _write(1, &data, 1);
        _write(1, "\n", 1);
        wait(10000);
        gpio_clear(GPIOD, GPIO12);
    }
}