STM32F769NI: How to activate USART Receiver Timeout Interrupt (USART_RTOR.RTOF)?

Thank you for your reply, the IDLE interrupt does not help because it is set after one byte idle time but to be compatible with some old devices here, I need an interrupt after 10 bytes of idle time.

I also noticed the strange note and thought maybe it is meant to be without the "only". But this did not help (the note seems also to be not valid with the "only" in it).

I simply missed the RTOEN bit in USART6.CR2 Register (Receiver Timout Enable). So for activating the Timeout, you need

 LL_USART_SetRxTimeout(USART6, 960);
 LL_USART_EnableRxTimeout(USART6); // <- I missed that
 LL_USART_ClearFlag_RTO(USART6);
 LL_USART_EnableIT_RTO(USART6);

The Manual states for the USART_ISR_RTOF flag, that an interrupt is generated if RTOIE in USART_CR1 is set. It does not mention RTOEN in USART_CR2. This should be updated in the Manual by ST!

Also the Note should be "The RTOF flag needs to be cleared once per received character". For some strange reason, the RTOF flag is set directly after reception of a character (together with the RXNE flag) without a timeout. You also don't need to set the USART_RTOR_RTO timeout value for every received byte. It is enough to set it once like in my initialization above. Also it seems to do no harm if I set the timeout value twice per received character :).

So my working ISR now looks like this:

void USART6_IRQHandler(void)
{
 /* USER CODE BEGIN USART6_IRQn 0 */
 
 // Byte received:
 if (LL_USART_IsActiveFlag_RXNE(USART6))
 {
 serialHandleRXByte(LL_USART_ReceiveData8(USART6));
 
 // USART_ISR_RTOF Flag is set at reception
 // and at timeout for some reason so you need
 // to reset it to not trigger this interrupt directly
 // after reception again:
 LL_USART_ClearFlag_RTO(USART6); 
 }
 
 // RX inter-bxte timeout:
 else if (LL_USART_IsActiveFlag_RTO(USART6))
 {
 LL_USART_ClearFlag_RTO(USART6);
 
 BSP_LED_On(LED1);
 
 serialHandleRXTimeout();
 }
 /* USER CODE END USART6_IRQn 0 */
 /* USER CODE BEGIN USART6_IRQn 1 */
 
 /* USER CODE END USART6_IRQn 1 */
}