startup.c

extern void usart_sends(USART_TypeDef * USART, const char * buffer);

/* timer4 irq handler */

void tim4_irq_hanlder(void) {

    if (TIM4->SR & TIM_SR_UIF) {

        /* switch leds */

        LED_PORT->ODR ^= RED_LED;

        /* send test byte */

        usart_sends(USART1, "Testing string\r\n");

        /* clear status bit */

        TIM4->SR &= ~TIM_SR_UIF;

    }

}