I have a board with the USB part like the SK-S7G2. If I use an external oscillator of 24 MHz, PLL div = 2 and PLL mul = 20, I get the internal clock 240 MHz. Then dividing by 5 I get the UCLK = 48 MHz. Works perfect.
*BUT* if on the same board I mount a 12 MHz oscillator, PLL div=1 amb PLL mul = 20, getting the exact internal clock of 240 MHz and UCLK = 48 MHz, USB no longer works. The board never recognizes de USB device connected (returns error 91, just as if there is nothing connected). Just to verify that the board was not damaged, restoring the 24 MHz oscillator and it's configuration it worked well again.
Any explanation? Is USB block using directly the external oscillator before the PLL or something like that?
Thank you in advance.