Launchpad misses UART Recieve Timeout interrupt

Hi !

I'am working with EM-LM4F120XL Rev A. My current aim is interrupt driven communiation via UART0 (Modbus© protocol) with FIFO. I`am using 3 UART interrupt RX, TX complete (RX/TX FIFO interrupt level select configured to default = 1/2 -> 8 bytes) and Recieve Timeout interrut. I use Recieve Timeout interrut to detect Modbus end of message event. I've used this method with LPC's Cortex -M0, so I just ported source code and rewrote some MCU-specific features. When I started to debug, I've noticed that:

if  ( UART message length ) == N*(FIFO interrupt level select register value) then Recieve Timeout interrut is not triggered.

N - integer

#define    RX_TOUT_INT                (1<<6)
#define    RX_INT                    (1<<4)
#define    TX_INT                    (1<<5)

#define RX_FIFO_EMPTY            (1<<4)
#define TX_FIFO_FULL            (1<<5)

//============================================================================================

static uint32_t FUARTStatus;

//============================================================================================

void Init_UART (){
    
    SYSCTL_RCGCUART_R |= (1<<0);                    //Enable and provide a clock to UART module 0 in Run mode
    SYSCTL_RCGCGPIO_R |= (1<<0);                    //Enable and provide a clock to GPIO Port A in Run mode

    __NOP();
    __NOP();

    GPIO_PORTA_DEN_R |= (1<<0) | (1<<1);
    GPIO_PORTA_AFSEL_R |= (1<<0) | (1<<1);            //pin A.0 and A.1 function as a peripheral signal UART0 RX/TX
    GPIO_PORTA_DR4R_R |= (1<<0) | (1<<1);            //pin A.0 and A.1 function with 4 mA drive load

    GPIO_PORTA_PCTL_R = (GPIO_PORTA_PCTL_R & 0xFFFFFF00)+0x00000011;            //pin A.0 and A.1 function as a peripheral signal UART0 RX/TX

    UART0_IBRD_R = 43;                                //UART_BAUD_RATE = 80000000/(16*115200)= 43.402777(7); Integer = 43
    UART0_FBRD_R = 26;                                //Fractional  0,4027777(7) -> UART0_FBRD_R = 0.4027777*64+0.5=26
    UART0_LCRH_R = (1<<4) | (1<<5) | (1<<6);        //none parity, 1 stop bit, 8-bit length, FIFO enabled
    UART0_CC_R = 0;                                    //UART clock from SystemClock
    UART0_CTL_R |= (1<<0) | (1<<8) | (1<<9);        //Enable UART block, enable reciever, enable transmitter
//    UART0_IFLS_R = (UART0_IFLS_R & 0xFFFFFFC0);        //Interrupt level select RXFIFO > 1/2 Full, TXFIFO < 1/2 Full
    UART0_IM_R = (1<<4) | (1<<6);                    //UART Receive Interrupt and Timeout Interrupt
    
    NVIC_EnableIRQ(UART0_IRQn);

    FUARTStatus = UART_EMPTY_STATUS;
}

//============================================================================================

void UART0_IRQHandler (){
    uint32_t _interrupt_source = UART0_MIS_R;

    if(_interrupt_source & (RX_INT | RX_TOUT_INT)){          
        UART0_ICR_R |= (1<<4);
        while(!(UART0_FR_R & RX_FIFO_EMPTY)){      
            BuffMBSlave[MBBuffIndex++] = (uint8_t)UART0_DR_R;
            if(MBBuffIndex >= SLAVE_BUF_LEN) MBBuffIndex = SLAVE_BUF_LEN;
        }
        if((_interrupt_source & RX_TOUT_INT)){
            UART0_ICR_R |= (1<<6);
            RX_INT_DISABLE;
            EndRecieve = TRUE;
            MBMessLength = MBBuffIndex;
        }
    }
    UART0_ICR_R = _interrupt_source;
}