Before you use TXD & RXD as normal ports on your FLASH uC, consider if there would be advantage in programming the uC inside the production model. I don't know the specifics of your quoted chip, but the 89~RD2 series from Philips and Temic/Atmel allow you to do In-System-Programming. This means you can load trays of blank microcontrollers into the boards as they go through the pick & place production line. And the boards come out of the solder process you can program the individual boards when they pass though test stations.

ISP involves more than the two mentioned signals. Its something important to consider for streamlining your production line.

aka j