关于定时器2通道2定时触发ADC1转换的程序，程序配置上好像有问题，请大家指教

LYC8686 提问时间：2011-8-3 14:29 /

*TIM2通道2作为ADC1转换的触发时钟200US转换一次--------------------------------- */
TIM_DeInit(TIM2);                                                //重新将Timer设置为缺省值
TIM_TimeBaseStructure.TIM_Period = 200;                          //1US*200=200US设置计数溢出大小，
TIM_TimeBaseStructure.TIM_Prescaler = 20;                        //预分频系数为20 /20=1M(1US)
TIM_TimeBaseStructure.TIM_ClockDivision = 0;                     //设置时钟分割
TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;      //设置计数器模式为向上计数模式
TIM_TimeBaseInit(TIM2,&TIM_TimeBaseStructure);                   //将配置应用到TIM2中
   //PWM设置
TIM_OCStructInit(&TIM_OCInitStructure);                          //设置缺省值
TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;                //PWM模式1输出（PWM2也行）
TIM_OCInitStructure.TIM_Pulse = 15;                              //设置占空比
TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;        //TIM输出比较极性高（LOW也行）
TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;    //使能输出状态
TIM_OC2Init(TIM2, &TIM_OCInitStructure);                         //初始化TIM2的2通道
TIM_ARRPreloadConfig(TIM2, ENABLE);                              //使能TIMx在ARR上的预装载寄存器
TIM_OC2PreloadConfig(TIM2, TIM_OCPreload_Enable);                //使能TIMx在CCR2上的预装载寄存器
ADC_InitStructure.ADC_Mode     = ADC_Mode_Independent;
ADC_InitStructure.ADC_ScanConvMode   = ENABLE;
ADC_InitStructure.ADC_ContinuousConvMode = DISABLE;
ADC_InitStructure.ADC_ExternalTrigConv = ADC_ExternalTrigConv_T2_CC2;
ADC_InitStructure.ADC_DataAlign    = ADC_DataAlign_Right;
ADC_InitStructure.ADC_NbrOfChannel   = 9;
ADC_Init(ADC1, &ADC_InitStructure);
/* ADC1 Regular Channel1 Configuration                                      */
ADC_RegularChannelConfig(ADC1, ADC_Channel_10, 1, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_11, 2, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_12, 3, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_1,   4, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_2,   5, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_1,   6, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_2,   7, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_10, 8, ADC_SampleTime_41Cycles5);
ADC_RegularChannelConfig(ADC1, ADC_Channel_16, 9, ADC_SampleTime_239Cycles5);
/* Enable ADC1 */
ADC_TempSensorVrefintCmd(ENABLE);//
ADC_Cmd(ADC1, ENABLE);
/* Enable ADC1 reset calibaration register */
ADC_ResetCalibration(ADC1);
/* Check the end of ADC1 reset calibration register */
while(ADC_GetResetCalibrationStatus(ADC1));
/* Start ADC1 calibaration */
ADC_StartCalibration(ADC1);
/* Check the end of ADC1 calibration */
while(ADC_GetCalibrationStatus(ADC1));
ADC_DMACmd(ADC1, ENABLE);             /* Enable ADC1's DMA interface        */
ADC_Cmd   (ADC1, ENABLE);             /* Enable ADC1                        */
ADC_ExternalTrigConvCmd(ADC1, ENABLE);
//ADC_SoftwareStartConvCmd(ADC1,ENABLE);/* Start ADC1 Software Conversion     */
TIM_Cmd(TIM2,ENABLE);
TIM_CtrlPWMOutputs(TIM2,ENABLE);                                 //设置TIM2的PWM输出为使能
程序下载后只能转换一次后就不再定时转换了，是不是上面配置有问题，（我是通过DMA传送ADC转换结果的，DMA配置经验证是对的）