在stackoverflow上查到了类似的问题，原因是数值溢出造成的
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《Arduino Arithmetic error negative result》
问题：
I am trying to times 52 by 1000 and i am getting a negative result
int getNewSum = 52 * 1000;
but the following code is ouputting a negative result: -13536
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解决方法1：将int更换成更大的data type
int:
16 bit: from -32,768 to 32,767
32 bit: from -2,147,483,648 to 2,147,483,647
unsigned int: from 0 to 65,535
long: from 2,147,483,648 to 2,147,483,647
unsigned long: from 0 to 4,294,967,295
解决方法2：化简计算方法
比如我要实现一个倒计时的程序，以前的计算方法是这样的：
int countDownData[2]={25};//minute,second
val = (countDownData[0]*60000 + countDownData[1]*1000)/intervalSpeed
换成下面这样后，结果虽然一样，但通过分开计算，先算除法，计算过程中产生的数值就会小很多
val = countDownData[0]*(60000/intervalSpeed) + countDownData[1]*(1000/intervalSpeed);
参考文档：
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《数值溢出（arithmetic overflow）问题与解决方案》
https://blog.csdn.net/lanchunhui/article/details/52425939--------------------------------
《Arduino Arithmetic error negative result》
https://stackoverflow.com/questions/20594628/arduino-arithmetic-error-negative-result
