You will need to contrive a way to get the RXD0 signal to be valid prior to the point at which the chip samples the data input pin. If they say it works this way then that is just the way you have to work with it. One way you can get the source of the data to present its data in time is to have it clock out on the previous edge of the TXD0 clock signal. Sometimes it works out that it is necessary to invert the TXD0 clocking signal so that it lets the source clock out on the opposite edge that that to which the microcontroller clocks.

Michael Karas