4×4按键， 常用的读出键值的方法有“查表法”和“反转法”。查表法的程序最为简短，但是稍多占用一点存储空间；反转法的程序执行速度最快，只是要求接口是双向的。-4 × 4 keys, keys used to read out the method of " look-up table"　and " reversal."　Look-up table of the most short program, but take a little bit more storage space　reversal of the fastest program execution, but requires a two-way interface.