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检测外部脉冲信号频率方法有三种： (1) TIM3 is configured in Input Capture Mode: the external signal is connected to TIM3 Channel2 used as input pin. To measure the frequency we use the TIM3 CC2 interrupt request, so In the TIM3_IRQHandler routine, the frequency of the external signal is computed. The "TIM3Freq" variable contains the external signal frequency: TIM3Freq = TIM3 counter clock / Capture in Hz, where the Capture is the difference between two consecutive TIM3 captures. For Low-density, Medium-density, High-density and Connectivity line devices, the minimum frequency value to measure is 1100 Hz. For Low-Density Value line, Medium-Density and High-Density Value line devices, the minimum frequency value to measure is 366 Hz. (2) TIM3 is configured in PWM Input Mode: the external signal is connected to TIM3 Channel2 used as input pin. To measure the frequency and the duty cycle we use the TIM3 CC2 interrupt request, so In the TIM3_IRQHandler routine, the frequency and the duty cycle of the external signal are computed. The "Frequency" variable contains the external signal frequency: Frequency = TIM3 counter clock / TIM3_CCR2 in Hz, The "DutyCycle" variable contains the external signal duty cycle: DutyCycle = (TIM3_CCR1*100)/(TIM3_CCR2) in %. For Low-density, Medium-density, High-density and Connectivity line devices, the minimum frequency value to measure is 1100 Hz. For Low-Density Value line and Medium-Density Value line devices, the minimum frequency value to measure is 366 Hz. （3）对于High-density devices（大容量型）如果检测脉冲的频率小于1100Hz，我们可以采用外部中断边沿的触发方式来计算脉冲的个数，通过定时器准确定时1s所测得的脉冲个数即为脉冲信号周期。
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