I have been working on replacing an old serial protocol with USB. I have been using a dev board but today learned the real system runs at 1.8V with a CPU frequency of 12MHz. 12MHz is fine for FS-USB but the voltage being 1.8V prevents me from using the 32MHz internal oscillator for the USB if I am reading the documentations correctly. The Nice voltage vs frequency graph in the XMega A1U datasheet is referenced to for the CPU clocks but can I presume it is true for all clock sources? I read the only other discussion I could find about this, but making hardware changes to switch voltages when the USB is used is not really an option. AVR1017 (USB Hardware Design Recommendations) just states that you should use 3.3V and 48MHz clock. I looked at just doing USB Low Speed since I presume it can use a lower clock speed but that would mean I am limited to HID/control transfers that are significantly slower than the current serial.
Is 48MHz the minimum clock speed I can use for FS-USB?
Am I reading the datasheet correctly that there is no way to get a 48MHz clock at 1.8V? What about an external source?
Any Ideas or questions I am not even thinking of?
I'm new to XMega but it's been fun so far. I'm just hoping all the USB code I've been writing for the past week isn't for nothing, thanks!