PWM不出波形，谁帮我看看程序哪出问题了

void PWMInit(void)
{
ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOB);
ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_PWM0);

ROM_GPIOPinConfigure(GPIO_PB4_M0PWM2);
ROM_GPIOPinTypePWM(GPIO_PORTB_BASE, GPIO_PIN_4);
ROM_PWMGenConfigure(PWM0_BASE,PWM_GEN_1,PWM_GEN_MODE_UP_DOWN|PWM_GEN_MODE_NO_SYNC);
ROM_PWMGenPeriodSet(PWM0_BASE,PWM_GEN_1,64000);
ROM_PWMPulseWidthSet(PWM0_BASE,PWM_OUT_2,ROM_PWMGenPeriodGet(PWM0_BASE,PWM_OUT_2)/4);

ROM_PWMGenEnable(PWM0_BASE,PWM_GEN_1);
ROM_PWMOutputState(PWM0_BASE,PWM_OUT_2_BIT,true);
}

void main()
{

ROM_SysCtlClockSet(SYSCTL_SYSDIV_1|SYSCTL_USE_OSC|SYSCTL_OSC_MAIN|SYSCTL_XTAL_16MHZ);
ROM_SysCtlPWMClockSet(SYSCTL_PWMDIV_1);

PWMInit();

while(1);

}