按键中断处理函数

由于本系统包含的按键不多，使用频率不高，系统采用中断扫描工作方式，

当有键按下的时候会先进入相应的中断，然后再执行扫描判断是哪个按键被按

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下，即只有当有键按下时才执行扫描任务，这样可以提高CPU的效率，避免空

扫描-Key interrupt handler

Since the system contains few buttons, using the frequency is not high, the system uses interrupt scanning mode,

When a key is pressed when the corresponding interrupt will be the first to enter, and then perform a scan to determine which button is pressed

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, Ie only when a key is pressed when the scanning process, so you can improve CPU efficiency, to avoid air

Scanning