关于TM4C123G定时器边沿触发模式的配置

我想配置timer0B为检测计数下降沿，从t0ccp0输入，按如下配置的，为什么不能正常工作，DEBUG时发现timer0B完全没有计数

SysCtlClockSet(SYSCTL_SYSDIV_5|SYSCTL_USE_PLL|SYSCTL_XTAL_16MHZ|SYSCTL_OSC_MAIN);

SysCtlPeripheralEnable(SYSCTL_PERIPH_TIMER0);

SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOB);
TimerConfigure(TIMER0_BASE, (TIMER_CFG_SPLIT_PAIR | TIMER_CFG_A_PERIODIC |TIMER_CFG_B_CAP_COUNT));

TimerLoadSet(TIMER0_BASE, TIMER_A, 3000);

TimerControlEvent(TIMER0_BASE, TIMER_B, TIMER_EVENT_NEG_EDGE);
TimerLoadSet(TIMER0_BASE, TIMER_B, 3000);

TimerMatchSet(TIMER0_BASE,TIMER_B,2000);

IntEnable(INT_TIMER0B);

TimerIntEnable(TIMER0_BASE, TIMER_CAPB_MATCH);

IntMasterEnable();

TimerEnable(TIMER0_BASE, TIMER_B);