Hi Chang
Generally we make use of 8-bit data mode in USART but in case we need to accomodate larger data (greater than 256) size, we need an extra bit which will result in storing data upto 512 values. For this purpose, a 9-bit mode is avaliable in AVR's USART thing. In this kind of mode 9th bit is read/written to a specific bit of USART's register and the remaining 8 bits are computed normally.