使用ls3S9B90使用timer配置PWM编译通过下载到板子测试为何不出现波形？

SysCtlClockSet(SYSCTL_SYSDIV_1 | SYSCTL_USE_OSC | SYSCTL_OSC_MAIN | SYSCTL_XTAL_16MHZ);
   
    SysCtlPeripheralEnable(SYSCTL_PERIPH_tiMER1);

    SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOE);

    GPIOPinConfigure(GPIO_PE4_CCP2);

    GPIOPinTypeTimer(GPIO_PORTE_BASE, GPIO_PIN_4);



    TimerConfigure(TIMER1_BASE, TIMER_CFG_16_BIT_PAIR | TIMER_CFG_A_PWM);

    TimerControlLevel(TIMER1_BASE, TIMER_A, true);

    TimerLoadSet(TIMER1_BASE, TIMER_A, 20000);

    TimerMatchSet(TIMER1_BASE, TIMER_A,  10000);

    TimerEnable(TIMER1_BASE, TIMER_A);