Using SysCtlClockGet() to generate different clock; error

1. I am using tm4c123gh6pm& want to generate different freq.

2. Case 1: Internal 16Mhz crystal as system clock. What value should I write here, value I had put is wrong

SysCtlClockSet(SYSCTL_OSC_INT );

 freq = SysCtlClockGet();                /* freq returns: 66666666 */

3. Case 2: 80Mhz using internal 16Mhz

  SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_OSC_INT);

   freq = SysCtlClockGet();                /* freq returns: 66666666 */

4. case 3: External 16Mhz clock as system clock

SysCtlClockSet(SYSCTL_OSC_MAIN | SYSCTL_XTAL_16MHZ );
freq = SysCtlClockGet(); /* freq returns: 66666666 */

5. case 4: 80Mhz using external 16Mhz oscillator

  SysCtlClockSet(SYSCTL_SYSDIV_2_5 | SYSCTL_USE_PLL | SYSCTL_OSC_MAIN | SYSCTL_XTAL_16MHZ );
  freq = SysCtlClockGet();                /* freq returns: 66666666 */

6. basically whatever setting I am doing, the  SysCtlClockGet() always return 66.67Mhz.

However if I call this function before configuring the clock it retusn correct value ie 16Mhz