1. The clock came from
In the clock tree stm32f4 for example, as shown ARM general there are four clock sources (green circle)
- Internal low clock LSI (low speed inter)
- High-speed internal clock HSI (high speed inter)
- External speed clock LSE (Low speed external)
- External high-speed clock HSE (high speed external)
2. What devices require clock (red circle)
First, the kernel system requires a high frequency clock (FCLK), a number of high-speed peripherals such as DMA to mount on the AHB bus using high-speed peripherals HCLK, as well as low-speed peripheral portion of the mount a general APB peripherals used on the bus is PCLK.
Why do low speed at high school
First, to be able to stable and efficient operation of the program, the system must not be lower core clock, colleges and universities may have run only when the clock runs faster overall system. Why do not unified high-speed clock, the biggest reason is the power, with the improvement of the power consumption of the clock frequency will be doubled, so in order to be able to simultaneously stable and efficient operation, to ensure that power is not too high, clock distribution It requires very careful management.
3, how to configure the clock
In a typical process clock configuration is:
- Select the clock source (external / internal low-speed / high-speed clock source)
- After PLL (Phase Locked Loop) frequency multiplied to obtain a high frequency clock Pllclk
- After a series of data selection and divider achieve the configuration of the peripheral clock
