STM32-Arduino UART trouble

// --- ARDUINO CODE ---
void loop() {
    digitalWrite(4, 0); // Clear EXTI11 line.

    if (mySerial.available() && received < 480) {   // STM32 sending data and is not done.
        buff[received] = mySerial.read();           // Append received data to the buffer.
        received++;
    }
    if (received >= 480) {                          // If the buffer is full
        received = 0;                               // transmit it to PC.
        Serial.println(buff);
    }


    if (Serial.available()) {
        if (Serial.read() == 'r') {     // PC requests data from the STM32
            digitalWrite(4, 1);         // Triggers STM32 EXTI11 line.
            while (Serial.available())  // Empty the buffer.
                Serial.read();
        }
    }
}
// --- ---------- ---

// --- STM32 CODE ---
void EXTI15_10_IRQHandler(void)
{
    // Make sure that the interrupt is the good one.
    if (HAL_GPIO_ReadPin(data_req_IRQ_GPIO_Port, data_req_IRQ_Pin)) {
        if (is_sending_data == FALSE)   // If no transmission is happening
            should_send_data = TRUE;    // raise transmission flag.
    }

    // IRQ handling stuff...
}

void HAL_UART_TxCpltCallback(UART_HandleTypeDef * huart) {
    is_sending_data = FALSE;    // Transmition is completed, unblock requests.
}

void main(void){
    // Init and other stuff...

    while (1) {
        if (should_send_data == TRUE) { // If data was requested
            HAL_GPIO_WritePin(LD5_GPIO_Port, LD5_Pin, GPIO_PIN_RESET);
            HAL_UART_Transmit_IT(&huart3, matrice, 480);    // Start transmission by interrupt.
            is_sending_data = TRUE;                         // Block requests.
            should_send_data = FALSE;                       // Clear flag.
        }
    // matrice acquisition stuff here
    }
}