We use external 16MHz crystal oscillator for LPC11U68JBD48E clock:
unsigned int j;
SystemCoreClockUpdate();
Chip_Clock_EnablePeriphClock(SYSCTL_CLOCK_IOCON);
// start SYSOSC
Chip_IOCON_PinMuxSet(LPC_IOCON, 2, 0, (IOCON_FUNC1 | IOCON_MODE_INACT)); // PIO2_0 = XTALIN
Chip_IOCON_PinMuxSet(LPC_IOCON, 2, 1, (IOCON_FUNC1 | IOCON_MODE_INACT)); // PIO2_1 = XTALOUT
Chip_SYSCTL_PowerUp(SYSCTL_POWERDOWN_SYSOSC_PD);
Chip_Clock_SetPLLBypass(false, false);
for (j = 0; j < 0x400; ++j); // 500us delay is needed after SYSOSC is powered up
// MAINCLK = 64MHz, from PLL
Chip_Clock_SetMainClockSource(SYSCTL_MAINCLKSRC_IRC);
Chip_SYSCTL_PowerDown(SYSCTL_POWERDOWN_SYSPLL_PD);
Chip_Clock_SetSystemPLLSource(SYSCTL_PLLCLKSRC_MAINOSC);
Chip_Clock_SetupSystemPLL(3, 1); // FCLKIN = 16, FCLKOUT = 64, FCCO = 256MHz
Chip_SYSCTL_PowerUp(SYSCTL_POWERDOWN_SYSPLL_PD);
while (!Chip_Clock_IsSystemPLLLocked());
Chip_Clock_SetSysClockDiv(2); // system clock at 32Mhz
Chip_Clock_SetMainClockSource(SYSCTL_MAINCLKSRC_PLLOUT);
SystemCoreClockUpdate();
// so far so good, turning off IRC:
Chip_SYSCTL_PowerDown(SYSCTL_POWERDOWN_IRC_PD);
Chip_SYSCTL_PowerDown(SYSCTL_POWERDOWN_IRCOUT_PD);
// now lets try set up USB clock:
Chip_SYSCTL_PowerDown(SYSCTL_POWERDOWN_USBPLL_PD);
Chip_Clock_SetUSBPLLSource(SYSCTL_USBPLLCLKSRC_MAINOSC);
Chip_Clock_SetupUSBPLL(2, 1); // FCLKIN = 16, FCLKOUT = 48, FCCO = 192MHz
Chip_SYSCTL_PowerUp(SYSCTL_POWERDOWN_USBPLL_PD);
while (!Chip_Clock_IsUSBPLLLocked()); // hangs here indefinitely
// and this code is not reached ...
Chip_Clock_SetUSBClockSource(SYSCTL_USBCLKSRC_PLLOUT, 1);
Chip_Clock_EnablePeriphClock(SYSCTL_CLOCK_USB);
Chip_Clock_EnablePeriphClock(SYSCTL_CLOCK_USBRAM);
Chip_SYSCTL_PowerUp(SYSCTL_POWERDOWN_USBPAD_PD);
So, could USB PLL be used when IRC is turned off?
