XM radio service is an emerging satellite digital audio radio service suitable for automotive environment, it offers users high-quality radio programs with better signal coverage by adopting communication satellite broadcasting technology. In order to incorporate XM radio function into a digital car audio system based on the embedded system industry standard ITRON, we analyze both software and hardware requirements of the XM function module in this work. Architectural design that separates the XM radio function module into five layers is given. The operating system selected for the microcontroller used in the car audio system is introduced. Implementation details of the XM function module, such as control commands and device communication protocol of the XM tuner, the state transition matrix layer and the application layer are also presented in this work.