Clock Initialization on STM32F411
=================================

Overview of Clock Initialization
-----------------------------------

Clock initialization is essential in STM32F411 microcontrollers to set up and optimize the system's operational speed and power efficiency. 
The system clock determines the timing for various peripherals and core functionalities, such as timers, ADCs, and communication modules (e.g., UART, SPI, I2C). 
By configuring the clock settings, we ensure the microcontroller operates at a frequency suitable for the application's requirements while balancing power consumption.


Key Concepts in STM32 Clock System
-------------------------------------

1. **High-Speed External (HSE) Oscillator:**
   Uses an external crystal (e.g., 25 MHz) for accurate timing, often chosen for applications requiring precise clocking.

2. **High-Speed Internal (HSI) Oscillator:**
   A less accurate internal oscillator, generally used in low-power applications where precise timing is not critical. Frequency - 16MHz.

3. **Phase-Locked Loop (PLL):**
   A frequency multiplier that takes a base clock input (e.g., HSE or HSI) and scales it to achieve higher frequencies for system operations.

The STM32F411 typically uses the HSE as a primary source for the PLL, allowing it to generate a system clock frequency up to 100 MHz or higher.


Phase-Locked Loop (PLL) Configuration
----------------------------------------

The PLL on STM32F411 is a central component in achieving higher clock frequencies by multiplying the HSE input. Configuring the PLL involves selecting and tuning several parameters, including PLL multipliers and dividers, to achieve a desired output frequency. Here’s how to configure the PLL for a 100 MHz system clock from a 25 MHz input clock.


Example Configuration for 100 MHz Clock
-------------------------------------------

Given a 25 MHz external clock (HSE), we can achieve a 100 MHz system clock by adjusting the PLL parameters as follows:

1. PLL Source Selection: Choose HSE (25 MHz) as the PLL source.

2. PLL Multipliers and Dividers:

   - **PLLM:** Set to 25 to divide the HSE down to 1 MHz.

   - **PLLN:** Set to 200 to scale 1 MHz up to 200 MHz.

   - **PLLP:** Set to 2, dividing 200 MHz to yield the desired 100 MHz system clock.

This configuration allows the microcontroller to operate at an optimal 100 MHz frequency.


Summary
----------

Clock initialization on STM32F411 involves configuring the PLL and setting the system clock source. By using the HSE as a base and carefully adjusting PLL parameters, we achieve an accurate and stable system clock frequency. 
This configuration allows the STM32F411 to operate efficiently at 100 MHz, balancing performance and power requirements for demanding applications.