最近涉及到一个超声波测距的项目，测距开始时需要触发指定个数的方波。此为背景。
在网上搜集了相关资料，有以下几个思路供参考：
1、最直观的

1. while(pulsecnt--)
   

2. {
   

3.     for(;;);//delay up to the frequency
   

4.     Pin_ON();
   

5.     for(;;);
   

6.     Pin_OFF();
   

7. }

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2、如果你的timer有RCR，那就好办了。看ST参考手册的解释：

If the repetition counter is used, the update event (UEV) is generated after upcounting is repeated for the number of times programmed in the repetition counter register (TIMx_RCR). Else the update event is generated at each counter overflow.

用此方法需要考虑到RCR只有8位，以及PWM的edge-aligned and center-aligned模式：

Each time the REP_CNT related downcounter reaches zero, an update event is generated
and it restarts counting from REP value. As REP_CNT is reloaded with REP value only at the
repetition update event U_RC, any write to the TIMx_RCR register is not taken in account until
the next repetition update event.
It means in PWM mode (REP+1) corresponds to:
– the number of PWM periods in edge-aligned mode
– the number of half PWM period in center-aligned mode.

3、DMA方式送数。
    将DMA与Tim的CCR绑定，通过控制DMA的数据数量，输出同等数量的波形，该方式有以下优缺点：
    优点：PWM的脉冲宽度控制较为灵活，可事先将脉冲宽度写入DMA的传输Buffer中；
    缺点：PWM的周期控制可能会不那么准确，因为DMA传输的触发到传输完成需要一定时间；

4、利用Timer之间的同步：

Configure Timer 1 in master mode so that it outputs a periodic trigger signal on each
update event UEV. If you write MMS=010 in the TIM1_CR2 register, a rising edge is
output on TRGO1 each time an update event is generated.
• To connect the TRGO1 output of Timer 1 to Timer 2, Timer 2 must be configured in
slave mode using ITR1 as internal trigger. You select this through the TS bits in the
TIM2_SMCR register (writing TS=000).
• Then the Timer2's slave mode controller should be configured in external clock mode 1
(write SMS=111 in the TIM2_SMCR register). This causes Timer 2 to be clocked by the
rising edge of the periodic Timer 1 trigger signal (which correspond to the timer 1
counter overflow).
• Finally both timers must be enabled by setting their respective CEN bits within their
respective TIMx_CR1 registers. Make sure to enable Timer2 before enabling Timer1.

  Tim1负责PWM输出，Tim2控制脉冲数。Tim1、Tim2设置好后，给Tim2的ARR设置为 PulseCnt-1 以及为Tim2的Update事件设置中断，在中断服务程序中关闭Tim以完成PulseCnt脉冲的发送。        
      Note:  1、Tim1设置为PWM mode 2，避免在结束输出时Tim1多输出一个脉冲；
                2、代码着重说明Tim的配置，若用此方法需要结合应用增添相应代码。
代码：

1. TIM1->CR2 |= TIM_CR2_MMS_1;
   

2. TIM2->SMCR |= TIM_SMCR_SMS_2 | TIM_SMCR_SMS_1 | TIM_SMCR_SMS_0;
   

3. TIM1->PSC = 47999;
   

4. TIM1->ARR = 999;
   

5. TIM2->ARR = PulseCnt-1;
   

6. TIM1->CR1 |= TIM_CR1_CEN;
   

7. TIM2->CR1 |= TIM_CR1_CEN;

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以上是个人通过学习相关资料所做的总结，如有不当之处，欢迎拍砖。

PS. 几个链接：
http://bbs.eeworld.com.cn/thread-252318-1-1.htmlhttp://www.amobbs.com/thread-5509627-1-1.html
http://www.stmcu.org/module/forum/thread-559739-1-1.html
PPS.几次想上传图片，以便更好说明问题，为什么总是失败，是我公司网络问题，还是Server的问题？PPPS.在这个位置贴的链接也会丢失，不知道是什么问题。了解更多