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檢測外部脈沖信號頻率方法有三種： (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）對于High-density devices（大容量型）如果檢測脈沖的頻率小于1100Hz，我們可以采用外部中斷邊沿的觸發方式來計算脈沖的個數，通過定時器準確定時1s所測得的脈沖個數即為脈沖信號周期。
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