Problems with UART on STMF4 Discovery Board

Problems with UART on STMF4 Discovery Board

I have some problems with UART comunication. My platform is STM32f4 Discovery Board with STM32f407VG.
First my system setup:
void clock_setup(void)
{
    RCC->PLLCFGR = (uint32_t)0x27412A04;
    RCC->CFGR = (uint32_t)0x00081000;
    RCC->CR |= (uint32_t)0x00010000;
    while((RCC->CR & 0x00020000) != 0x00020000);
    RCC->CR |= (uint32_t)0x01000000;   
    while((RCC->CR & 0x02000000) != 0x02000000);
    FLASH->ACR = (uint32_t)0x00000702;
    RCC->CFGR |= (uint32_t)0x00000002;
    while(!(RCC->CFGR & 0x00000008));
}
void system_setup(void)
{
    SCB->CPACR |= 0x00f00000; //fpu can be configured
    clock_setup();
}
Main and button interrupt handler:
int main(void)
{
    int i = 0;
    __enable_irq();
    NVIC_SetPriority(EXTI0_IRQn,0x31);
    NVIC_EnableIRQ(EXTI0_IRQn);
    SYSCFG->EXTICR[0] = 1;
    EXTI->IMR = 0x00000001;
    EXTI->RTSR = 0x00000001; //rising edge on
    EXTI->FTSR = 0x00000000; //falling edge off
    RCC->AHB1ENR |= (uint32_t)0x00000009;
    GPIOD->MODER = (uint32_t)0x55000000;
    GPIOD->OTYPER = (uint32_t)0x0000000;

    USART1_Config();

    while(1){
    }
    return 1;
}
void EXTI0_IRQHandler(void)
{
    if(EXTI->PR & (1<<0))
    {
        USART1_puts("Test\n");
        enableBlink = !enableBlink;
    }
    EXTI->PR |= (1<<0);
}
USART configuration and functions:
void USART1_puts(volatile char* s)
{
    while(*s)
    {
        while(!(USART1->SR & 0x00000040)){
        }
        USART_SendData(USART1,*s);
        *s++;
    }
}
void USART1_Config(void)
{
    USART_InitTypeDef USART_InitStruct;
    GPIO_InitTypeDef GPIO_InitStruct, GPIO_InitStruct2;
    NVIC_InitTypeDef NVIC_InitStruct;

        /* Enable clocks */
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_USART1,ENABLE);
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA,ENABLE);

    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_9 | GPIO_Pin_10;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_AF;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_50MHz;
    GPIO_InitStruct.GPIO_OType = GPIO_OType_PP;
    GPIO_InitStruct.GPIO_PuPd = GPIO_PuPd_NOPULL;//GPIO_PuPd_UP;
    GPIO_Init(GPIOA,&GPIO_InitStruct);
    GPIO_PinAFConfig(GPIOA,GPIO_PinSource9,GPIO_AF_USART1);
    GPIO_PinAFConfig(GPIOA,GPIO_PinSource10,GPIO_AF_USART1);

    USART_InitStruct.USART_WordLength = USART_WordLength_8b;
    USART_InitStruct.USART_BaudRate = 9600;
    USART_InitStruct.USART_StopBits = USART_StopBits_1;
    USART_InitStruct.USART_Parity = USART_Parity_No;
    USART_InitStruct.USART_HardwareFlowControl = USART_HardwareFlowControl_None;                                                                      
    USART_InitStruct.USART_Mode = USART_Mode_Tx | USART_Mode_Rx;
    USART_Init(USART1,&USART_InitStruct);
    USART_ITConfig(USART1,USART_IT_RXNE,ENABLE);

    NVIC_InitStruct.NVIC_IRQChannel = USART1_IRQn;
    NVIC_InitStruct.NVIC_IRQChannelPreemptionPriority = 0;
    NVIC_InitStruct.NVIC_IRQChannelSubPriority = 0;
    NVIC_InitStruct.NVIC_IRQChannelCmd = ENABLE;
    NVIC_Init(&NVIC_InitStruct);

    USART_Cmd(USART1,ENABLE);
}
void USART1_IRQHandler(void)
{
    int i = 0;
    if(USART_GetITStatus(USART1, USART_IT_RXNE) != RESET)
    {
            static uint8_t cnt = 0;
        char t = USART_Recei