Hi,
1) page 1 says "Three External Clock modes up to 20 MHz"
2) page 2 of the datasheet: shows that the xx1503 has no USART. Either use a different microcontroler or implement a software UART.
so going through the first 2 pages already give alot of information.
to 1)
do a PDF search for "oscillator": (5th hit)
--> TABLE OF CONTENTS: tells:
5.0 Oscillator Module........................................................................................................................................................................ 42
so go to page 42 (it´s a link) to see that it does not support to connect an XTAL, but it supports an external clock. So an external oscillator may be used.
Klaus