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So this is probably a dumb question, but is there something more you have to do to use a different clock speed than just redefine the F_CPU and USB_CFG_CLOCK_KHZ values to reflect your new clock speed?

I've been working on a project with a 16mHz external clock chip. I'm trying to make more of them, but all I have is 20mHz clock chips. My code works just fine with the 16mHz chips. However, when I plug in a 20mHz clock, make clean, modify the F_CPU and CLOCK_KHZ values in usb-config.h, and recompile, the device is no longer recognized by Windows. Is there something else I have to change, or should I just order some more 16mHz chips? I'm currently running on an ATMega168 at 5v, with 3.6v zener diodes dropping the USB line voltage.

Thanks,
Alan Chatham

See datashhet to ATmega168. 20 MHz clock rate may be possible at voltage more than 5 V. Try to use self-power device at ~6 V, try to use other MCU, use stable V-USB lib drivers.
Best regards, Saimon.

These two defines should be all you need to change. And then you need to do a "make clean" because there are no dependencies on usbconfig.h and Makefile.

AVRs can usually be overclocked quite a bit, so I doubt that it's a hardware problem. I've tested the 20 MHz module myself and it worked for me (still works in one of my projects).