TM4C129ENCPDT: Unable to set timer clock to PIOSC

#include <stdint.h>
#include <stdbool.h>
#include "inc/hw_memmap.h"
#include "inc/hw_ints.h"
#include "inc/hw_types.h"
#include "inc/hw_sysctl.h"
#include "inc/hw_uart.h"
#include "inc/hw_timer.h"
#include "inc/hw_hibernate.h"
#include "driverlib/sysctl.h"
#include "driverlib/gpio.h"
#include "driverlib/timer.h"
#include "driverlib/hibernate.h"
#include "driverlib/interrupt.h"
#include "driverlib/pin_map.h"

void Timer0IntHandler(void);

uint32_t g_ui32SysClock = 0;

void main(void)
{
    // Systemclock auf 120 MHz
    g_ui32SysClock = SysCtlClockFreqSet(SYSCTL_XTAL_25MHZ | SYSCTL_OSC_MAIN | SYSCTL_USE_PLL | SYSCTL_CFG_VCO_480, 120000000);

    SysCtlPeripheralEnable(SYSCTL_PERIPH_HIBERNATE);
    while (!SysCtlPeripheralReady(SYSCTL_PERIPH_HIBERNATE)) {}
    HibernateEnableExpClk(g_ui32SysClock);
    HWREG(HIB_CC) = HIBERNATE_OUT_SYSCLK;

    // Set ALTCLK to PIOSC.
    SysCtlAltClkConfig(SYSCTL_ALTCLK_PIOSC);

    // GPIO für CCP-Ausgänge (optional für Oszilloskop)
    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOM);
    while (!SysCtlPeripheralReady(SYSCTL_PERIPH_GPIOM))
        ;
//    GPIOPinConfigure(GPIO_PM6_T5CCP0);
//    GPIOPinTypeTimer(GPIO_PORTM_BASE, GPIO_PIN_6);

    GPIOPinTypeGPIOOutput(GPIO_PORTM_BASE, GPIO_PIN_6);
    GPIOPinWrite(GPIO_PORTM_BASE, GPIO_PIN_6, 0);

    // Initialize Timer 0
    SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER0);
    while (!SysCtlPeripheralReady(SYSCTL_PERIPH_TIMER0))
        ;

//    TimerConfigure(TIMER0_BASE, TIMER_CFG_SPLIT_PAIR | TIMER_CFG_A_PWM);
    TimerConfigure(TIMER0_BASE, TIMER_CFG_A_PERIODIC);
    TimerClockSourceSet(TIMER0_BASE, TIMER_CLOCK_PIOSC);
    TimerPrescaleSet(TIMER0_BASE, TIMER_A, 255);
    TimerLoadSet(TIMER0_BASE, TIMER_A, 624);

    TimerIntRegister(TIMER0_BASE, TIMER_A, Timer0IntHandler);
    TimerIntEnable(TIMER0_BASE, TIMER_TIMA_TIMEOUT);
    IntEnable(INT_TIMER0A);
    IntMasterEnable();

    TimerEnable(TIMER0_BASE, TIMER_A);

    while(1)
    {
        __WFI();
    }
}

void Timer0IntHandler(void)
{
    TimerIntClear(TIMER0_BASE, TIMER_TIMA_TIMEOUT);

    uint8_t ui8Pin = GPIOPinRead(GPIO_PORTM_BASE, GPIO_PIN_6);
    GPIOPinWrite(GPIO_PORTM_BASE, GPIO_PIN_6, ui8Pin ^ GPIO_PIN_6);
}