cortex-m3的时钟程序，内置两个按键，使用内部定时作为秒信号，走时很准确-cortex-m3 clock program, built two buttons, use internal timer as a second signal, very accurate travel time

ARM CORTEX A9 开发手册打包下载 部分为中文手册,包含《NEON 向量化编译器指南》《编译器用户指南》《编译器参考指南》《汇编程序指南》-ARM CORTEX A9 development manual packaged download， part of it are Chinese manual