timer settings

http://www.51hei.com/bbs/dpj-40951-1.html

http://blog.csdn.net/minyuanxiani/article/details/21465779

https://wenku.baidu.com/view/64a62e9cd4d8d15abe234ec4.html //72000,200

STM32学习总结-定时器PWM输入捕获测量脉冲的最小和最大频率计算公式

http://blog.sina.com.cn/s/blog_13e68b0390102xkzl.html