TM4C123GXL Eval board does not run at 80MHz

Hello,

I have a straightforward issue that I cannot get the eval board above to operate at 80 MHz. I had my suspicions when I could not get valid debug data from the UARTs at a trivial 115200 baud rate. The device operates normally at many other rates including the second highest rate of 66.6 MHz.

Using SysCtlClockGet() to retrieve the value and display on a terminal shows completely expected operation except at 80 MHz.

Code snippets are simple for this:

SysCtlClockSet( SYSCTL_SYSDIV_3  |      // system clock divider (3 = 66.6 MHz)
                      SYSCTL_USE_PLL |      // use PLL or External osc
                      SYSCTL_XTAL_16MHZ |   // speed of external crystal
                      SYSCTL_OSC_MAIN );    // use main/intern/hibernate oscillator

As far as I can tell, to operate at 80MHz all I need to do is subsitute SYSCTL_SYSDIV_2_5 as the first parameter. Presumably the function does the rest of the work for me. 

Has anyone any ideas or experiences. Other than this the board is working fine.

Kind thanks