既成ARM系统中创建“第二实时钟”

Creation of a second RTC in existing ARM system

主流ARM芯片内都附有WatchDog硬件逻辑。在不改变既有系统的硬件结构前提下,系统正常运行中可以“借用”该硬件逻辑构建自己的精密实时钟(“第二RTC”),使之能很方便地实现微秒级计时;这样做,等于把系统实时钟的可使用时间范围向下扩展2,3个数量级(高主频ARM情形下会更短)。这一尝试的原始驱动来自一项加密算法移植探索试验,实现了有关函数,经实验验证有效。着重对这种“借用”思路进行分析,对具体实施的细节作了介绍。

All mainstream ARM CPUs have WatchDog hardware logic inside. This hardware logic can be borrowed to build a chronometric RTC （ the second RTC） for itself without changing hardware architecture of the existing system. Then calculating on microsecond level can be carried out conveniently. In this way, time range used by system RTC can be extended. This idea comes from an experiment of transplanting crypt algorithm, and has been validated in the experiment. This idea of borrowing is introduced and analyzed in the article.