A pull down ?If the UART port is left unconnected (eg for service port connection to a uart terminal) then you have to add pull down to RX pin (althow I would use buffering).
In a disconnected port, I recommend that the UART peripheral is turned OFF to save energy. Periodicaly we should check if the RX pin (as TTL input) is pulled up by external device and if yes, then we should enable UART peripheral in out mC.A pull down ?
That's a bad idea - a '0' when unconnected will be interpreted as a constant "start bit".
Never recomended pulldown (neighter pull up) on the push-pull TX output pin.1. Conneting a pull down will waste energy at the Tx side when it's turned on. I=Vcc/Rpull-down will very possiblly be much more then just leaving the peripheral on.
When an external controler is connected, the RX input (will the weak pull down), will turn HIGH by the external driver. Then and only then you turn on the peripheral and it will see HIGH in the RX pin. No ambiguity expected.2. When the Rx side's UART is switched on it will immediately see '0'. It won't be able to tell right away whether the Tx isn't connected or it's connected and driving information. You'll have to solve this ambiguity with an upper layer protocol. In my opinion this makes the design unnecessarily more complex and less reliable