为什么TM4C123G PF0管脚不能输出PWM？

int
PWMInit (void)
[
     //
     // PWM不分频
     //
     SysCtlPWMClockSet(SYSCTL_PWMDIV_1);
     GPIOPinTypeGPIOOutput(GPIO_PORTB_BASE, GPIO_PIN_2|GPIO_PIN_3);//配置2,3管脚为输出
     //配置PF0，PF1分别为M1PWM4，M1PWM5
     GPIOPinConfigure(GPIO_PF0_M1PWM4);
     GPIOPinConfigure(GPIO_PF1_M1PWM5);

     // 配置PF0，PF1为PWM的输出管脚
     GPIOPinTypePWM(GPIO_PORTF_BASE, GPIO_PIN_0|GPIO_PIN_1);
     //
     // Configure the PWM1 to count down without synchronization.
     //配置GEN2递减计数模式不同步模式
     PWMGenConfigure(PWM1_BASE, PWM_GEN_2,
                     PWM_GEN_MODE_DOWN | PWM_GEN_MODE_NO_SYNC);
     //设置PWM周期
     PWMGenPeriodSet(PWM1_BASE, PWM_GEN_2,100);
     //设置PWM占空比
     PWMPulseWidthSet(PWM1_BASE, PWM_OUT_4, 10);
     PWMPulseWidthSet(PWM1_BASE, PWM_OUT_5, 10);
     //使能PWM输出状态
     PWMOutputState(PWM1_BASE, PWM_OUT_4_BIT| PWM_OUT_5_BIT, true);
     GPIOPinWrite(GPIO_PORTB_BASE, GPIO_PIN_2, 0x1<<2);//H2管脚置高  对应RESETAB
     GPIOPinWrite(GPIO_PORTB_BASE, GPIO_PIN_3, 0x1<<3);//H3管脚置高 对应RESETCD
     //
     // 使能PWM发生器
     PWMGenEnable(PWM1_BASE, PWM_GEN_2);
     return(0);
]
PF1能输出PWM,而PF0不能输出PWM