This is the fourth video tutorial on the tutorial series on stm32f4discovery board. In this tutorial we will understand about clock frequency. We will learn what is meant by clock frequency. How clock frequency is related to speed of microcontroller. What are crystal oscillators. How program execution speed depends on clock frequency. We will then see what is the current speed of our stm32f4discovery board. How we can change that speed. Then finally we will modify the code written in previous tutorial and learn how to write code for selecting a particular clock frequency.