EK-TM4C129EXL: Usage of onboard oscillator for SysTick timer is producing unexpected results

Hi,

I am using the TI Launchpad EK-TM4C129EXL which has a TM4C129ENCPDT microcontroller. I have written a simple LED blink program using SysTick timer. When I use PIOSC as the clock source for the timer, I could achieve the time gap as predicted by the calculations. However, when I switch the clock source to the onboard oscillator (25MHz, according to launchpad datasheet) using STCTRL register, and by loading a value of (25000000/2 - 1) into the STRELOAD register, the time gap I observe is not 0.5 s as expected. It is off by 25 to 30%. I doubt if the frequency of onboard oscillator is 25MHz. Is there any mistake in my procedure; moreover, is there any way to check the clock frequency other than checking the datasheet?

Thanks,

Sagar