STM32的TIM输入信号结构
The input stage samples the corresponding TIx input to generate a filtered signal TIxF. Then, an edge detector with polarity selection generates a signal (TIxFPx) which can be used as trigger input by the slave mode controller or as the capture command. It is prescaled before the capture register (ICxPS).
从引脚接收一个信号TIx，经过数字滤波得到TIxF信号，经过边沿检测选择得到TIxFPx信号，这个信号可以用于从机模式，也可以直接使用这个信号用于编程。
输入捕获模式
In Input capture mode, the Capture/Compare Registers (TIMx_CCRx) are used to latch（锁存） the value of the counter after a transition detected by the corresponding ICx signal. When a capture occurs, the corresponding CCXIF （输入捕获中断标志）flag (TIMx_SR register) is set and an interrupt or a DMA request can be sent if they are enabled. If a capture occurs while the CCxIF flag was already high, then the over-capture flag CCxOF (TIMx_SR register) is set. CCxIF can be cleared by software by writing it to 0 or by reading the captured data stored in the TIMx_CCRx register.（通过读取CCRx中的数据可以清除CCxIF捕获中断标志） CCxOF is cleared when you write it to 0.
接着“STM32的TIM输入信号结构”的内容，上面最后说到用于编程的TIxFPx信号是检测到边沿跳变产生的，此时就产生ICx信号，一旦有ICx信号产生,CCRx寄存器就锁存此时计数器中的值，同时，中断标志CCxIF就会置位，如果CCxIF处于置位的时候又发生了捕获事件，那么过捕获标志CCxOF就置位，特别地，程序员读取CCRx中的数据后硬件清除CCxIF中断标志。
Input capture configuration code example
/* (1) Select the active input TI1 (CC1S = 01),
program the input filter for 8 clock cycles (IC1F = 0011),
select the rising edge on CC1 (CC1P = 0, reset value)
and prescaler at each valid transition (IC1PS = 00, reset value) */
/* (2) Enable capture by setting CC1E */
/* (3) Enable interrupt on Capture/Compare */
/* (4) Enable counter */
TIMx->CCMR1 |= TIM_CCMR1_CC1S_0 \
| TIM_CCMR1_IC1F_0 | TIM_CCMR1_IC1F_1; /* (1 */
TIMx->CCER |= TIM_CCER_CC1E; /* (2) */
TIMx->DIER |= TIM_DIER_CC1IE; /* (3) */
TIMx->CR1 |= TIM_CR1_CEN; /* (4) */
//上面配置TIM的模式寄存器TIMx capture/compare mode register TIM->CCMR1
//CC1S用于选择信号来自于TI1还是TI2，这里的理解要回到第一张图，可以看到CCMR1这个配置寄存器的作用。
//这段代码就是第一张图的配置，程序员可以根据的需要去配置CCMR1寄存器。
PWM输入模式
This mode is a particular case of input capture mode. The procedure is the same except:
1）Two ICx signals are mapped（映射） on the same TIx input.
2）These 2 ICx signals are active on edges with opposite（相反的） polarity（极性）.
3）One of the two TIxFP signals is selected as trigger input and the slave mode controller
is configured in reset mode.
这是输入捕获模式的特例，stm32TIM上某一个引脚的TIx信号可以产生两个IC信号IC1和IC2（这里以IC1和IC2举例，读者可以使用IC3和IC4，没有IC2和IC3，这是规定），这两个IC信号极性相反，这样刚好可以构成一个PWM，这两个信号中的一个用于触发，另一个处于从模式，被配置成复位的模式。
捕获PWM的步骤，用于测量占空比
\1. Select the active input for TIMx_CCR1: write the CC1S bits to 01 in the TIMx_CCMR1
register (TI1 selected).
\2. Select the active polarity for TI1FP1 (used both for capture in TIMx_CCR1 and counter
clear): write the CC1P to ‘0’ and the CC1NP bit to ‘0’ (active on rising edge).
\3. Select the active input for TIMx_CCR2: write the CC2S bits to 10 in the TIMx_CCMR1
register (TI1 selected).
\4. Select the active polarity for TI1FP2 (used for capture in TIMx_CCR2): write the CC2P
bit to ‘1’ and the CC2NP bit to ’0’(active on falling edge). （注意：极性跟IC1信号的相反）
\5. Select the valid trigger input: write the TS bits to 101 in the TIMx_SMCR register
(TI1FP1 selected). （选择IC1触发，则IC2沦为从模式）
\6. Configure the slave mode controller in reset mode: write the SMS bits to 100 in the
TIMx_SMCR register.
\7. Enable the captures: write the CC1E and CC2E bits to ‘1 in the TIMx_CCER register.
这是PWM的一个例子，看回PWM输入模式的介绍，这个例子首先选择信号来至于STM32的哪个TIM引脚，TI1，TI2或者TI3，这里又用到配置寄存器CCMR1了，配置IC1和IC2极性相反，一个作为触发源，另一个配置成reset的从模式，此例IC1作为触发，则IC1捕获触发时，CCR1锁存了此时计数器的值，下一次触发则是轮到与IC1相反极性的IC2触发，CCR2锁存计数器的值，并且复位计数器，因此CCR1和CCR2可以用于计算占空比。
这张图反映了IC1设置成reset的从模式，IC2作为触发源。