Raspberry Pi是什么？
引用维基百科的一句话：
The Raspberry Pi is a credit card sized single-board computer developed in the UK by the Raspberry Pi Foundation with the intention of stimulating the teaching of basic computer science in schools.
简单地说，它就是一个基于ARM CPU的、信用卡那么大的迷你计算机。
In short, Pi is an ARM-based mini computer which has a credit card size.
在阅读本文之前，请确保你已经阅读过我的另外几篇关于Pi的文章，因为本文与它们有或多或少的关系。
Before reading this article, please make sure that you've read some other articles of mine on Pi, because this article is more or less based on them.
【1】实验目的 / Experiment purpose
在树莓派上编写一个C程序，通过其GPIO口控制步进电机的转动方向以及速度。
Write a C program to control the direction & speed of rotation of a stepper motor through the GPIO on Raspberry Pi.