What does RCC mean on STM32?

 On STM32, RCC stands for Reset and Clock Control.

It’s a big central block inside the microcontroller that is responsible for:

1. Clocks (the “C” in RCC)
   RCC generates and distributes clocks to:

   • The CPU core

   • Buses (AHB, APB1, APB2, etc.)

   • Peripherals (GPIO, USART, SPI, I²C, TIM, ADC, etc.)

   Typical things RCC does:

   • Select clock sources: HSI (internal), HSE (external crystal), PLL, LSI, LSE, etc.

   • Set prescalers to divide clocks for different buses.

   • Choose the system clock (SYSCLK) and derived clocks like HCLK, PCLK1, PCLK2.

2. Resets (the “R” in RCC)
   RCC can reset:

   • The entire chip (system reset)

   • Individual peripherals (e.g. reset just USART1, SPI2, etc.)

   This is used to:

   • Bring peripherals to a known default state

   • Recover a misconfigured peripheral by toggling its reset bit.

3. Enabling / disabling peripheral clocks
   Before you can use most peripherals on STM32, you must enable their clock in RCC.
   Examples:

   // HAL example
   __HAL_RCC_GPIOA_CLK_ENABLE();
   __HAL_RCC_USART2_CLK_ENABLE();
   

   Or at register level (depends on family):

   RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;   // Enable GPIOA clock
   RCC->APB1ENR |= RCC_APB1ENR_USART2EN;  // Enable USART2 clock
   

   If you don’t enable the RCC clock:

   • The peripheral’s registers may appear “dead” or read as zero.

   • Your code may HardFault when trying to use it.

4. Where you see RCC in practice

• In CubeMX / CubeIDE, the “Clock Configuration” tab is basically configuring the RCC.

• In code, anything named like __HAL_RCC_XXX_CLK_ENABLE(), RCC_CFGR, RCC_CR, etc. is part of this block.

• When you see diagrams like “STM32 Clock Tree” in the datasheet – that’s all RCC-related.