stm32f205 定时器2 ETR功能读不出数据

[修行者-121]

stm32f205 定时器2 ETR功能读不出数据
一下是配置（ETR重映射到PB3口）
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOB, ENABLE);
RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
TIM_TimeBaseStructInit(&TIM_TimeBaseStructure);

//配置TIMER2作为计数器
TIM_DeInit(TIM2);
TIM_TimeBaseStructure.TIM_Period = 0xFFFF;
TIM_TimeBaseStructure.TIM_Prescaler = 0x0;
TIM_TimeBaseStructure.TIM_ClockDivision = 0x0;
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure); // Time base configuration
TIM_ETRClockMode2Config(TIM2, TIM_ExtTRGPSC_OFF, TIM_ExtTRGPolarity_NonInverted, 0x0f);
TIM_SetCounter(TIM2, 0);
TIM_Cmd(TIM2, ENABLE);

GPIO_PinAFConfig(GPIOB,GPIO_PinSource3,GPIO_AF_TIM2);

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
GPIO_InitStructure.GPIO_Speed = GPIO_Speed_2MHz;
GPIO_Init(GPIOB, &GPIO_InitStructure);


秒中断里面 TIM_GetCounter(TIM2); 总是零，大家帮忙看一下哪里有问题！