I'm working on a project using the xmega256a3u. We built this board using an FTDI FT232R USB to Serial chip on board to get the project going quickly. We did provide for depopulating the FT232R and connecting the USB lines directly to the xmega and using the internal USB controller as our 'serial' link to the the PC host.
Currently I have the firmware configured to use the internal 32mhz RC oscillator as the CPU clock, and it also provides the PerClock at the same rate. From looking at the documentation it seems to me that I have to switch this oscillator to 48mhz for the USB clock source, and would then have to divide it down for the CPU clock, which means that I have to change all the settings for existing devices that are clocked from the F_CPU source. It looks like I'm limited to a 24mhz cpu clock if I am using the full speed USB interface.
Does anybody been there and done that with this part and know the best way to configure the clock source for both the USB and CPU to get the highest possible clock rate for the CPU and PerClock (without over clocking)? I'm still going to need to use one of the usarts and will need a clock source to generate a baud rate of 115200 baud. I'm also using the A/D converters and need to generate a clock for them too.