We are using MK60FN1M0VLQ12 with MQX 4.0 and virtual com port demo. Clock are set to these values: 60MHz - core clock, 60MHz - bus clock and 20 MHz flex bus and flash clock, PLL0 is set to 120MHz and usb is clocked from PLL0 (usb clock divider is 2,5). In MK60 documentation it is said that usb can work with cpu frequencies down to 20 MHz. But in our case, when we try to change only bus clock to 30MHz, PC can't detect our device. To change cpu clocks, we simply modify SIM_CLKDIV1 register from 0x11550000UL to 0x13550000UL in initialization function...
The question is how change of bus clock can effect usb? Because it is said that usb is clocked from pll0 and we don't change it.
And the second question is: which minimum frequencies (bus, core etc) can be set to run usb com port correctly?