The SIE requires a 48MHz clock. The PLL runs at 96MHz and divides by 2 to provide the SIE clock. The PLL can be driven by many different frequencies (including 20MHz) through the use of the divider. This is assuming full-speed operation. Chris On Wed, 28 Oct 2009 23:00:26 -0400 "Bob Ammerman" wrote: > Haven't looked at it for some time, but doesn't he need to use a 6MHz > XTAL with PLL to do USB? Or a 24MHz XTAL, I guess. > > 20 MHz won't work because the USB SIE requires a 24MHz clock, IIRC. > > -- Bob Ammerman > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist