You could either poll the (software) RX pin from the PC at defined intervals, or you could use a pin with an 'interrupt-on-change (IOC)' function. Once the pin is active you can immediately jump to the SERIN/SERIN2 with a timeout (in case there is some junk on the line).

You have to take care when using the IOC function so that it does not disturbs the HSERIN, more important the other SEROUT (to LCD). If you go with the polling method, you should poll it at every tenth of a bit time (IIRC).