STM32 Clock Configuration with Registers – RCC Setup Example

This tutorial explains how to perform STM32 clock configuration using registers only, without HAL, SPL, or LL libraries. Many developers search for stm32 clock configuration, rcc register setup, and stm32 maximum system clock examples, so we’ll cover them step by step. We’ll configure the RCC registers, enable HSE, set PLL parameters, prescalers, and flash latency to achieve the maximum clock speed (180 MHz for STM32F4). By the end of this guide, you’ll understand how to manually program the STM32 RCC configuration to bring up the system clock from scratch.

Welcome to this new STM32 Register tutorial series, where we will program the STM32 microcontroller directly by working with its Registers, without using any libraries such as HAL, SPL, or LL.

This is the first and one of the most important tutorials in the series. Today, we will focus on learning how to configure and set up the clock.

Parts and software Required

I am going to use Keil IDE for the STM32 Register programming. It is a paid IDE but it offers a free version (MDK-Lite), which is limited to 32 KB of code size. This is enough for our register based programming.

I am also using STM32F4 series MCU for these projects but I will also explain the process for STM32F1 series MCUs. You can find the required parts at the links below.

• Keil MDK-ARM
• STM32F446 Development board
• STM32F401 Development board
• ST-Link V2 Programmer

Create the Project in Keil IDE

Let’s begin by creating a project in Keil and selecting the appropriate microcontroller.

Next, we need to include some essential files in the project. Select the CMSIS core and the Device Startup files, as shown below.

Then, add the main file to the project. Along with it, I have also included RccConfig.h and RccConfig.c. These files will contain the code for the clock setup and can be reused directly in our future projects.

Step-by-Step STM32 RCC Configuration (Register Level)

Since setting up the clock can be a complex process, for today’s tutorial we will use some information from CubeMX. Follow along carefully:

1. Open CubeMX.
2. Select your STM32 microcontroller.
3. Under RCC settings, choose the external crystal.
4. Set the HCLK to its maximum value and confirm your selection.
5. Keep CubeMX open, as we will refer to it shortly.
6. Next, create a new project in Keil and select the same microcontroller.

Below are the step-by-step instructions for setting up the clock:

	/*************>>>>>>> STEPS FOLLOWED <<<<<<<<************
	
	1. ENABLE HSE and wait for the HSE to become Ready
	2. Set the POWER ENABLE CLOCK and VOLTAGE REGULATOR
	3. Configure the FLASH PREFETCH and the LATENCY Related Settings
	4. Configure the PRESCALARS HCLK, PCLK1, PCLK2
	5. Configure the MAIN PLL
	6. Enable the PLL and wait for it to become ready
	7. Select the Clock Source and wait for it to be set
	
	********************************************************/

	/*************>>>>>>> STEPS FOLLOWED <<<<<<<<************
	
	1. ENABLE HSE and wait for the HSE to become Ready
	2. Set the POWER ENABLE CLOCK and VOLTAGE REGULATOR
	3. Configure the FLASH PREFETCH and the LATENCY Related Settings
	4. Configure the PRESCALARS HCLK, PCLK1, PCLK2
	5. Configure the MAIN PLL
	6. Enable the PLL and wait for it to become ready
	7. Select the Clock Source and wait for it to be set
	
	********************************************************/

Let’s go through each step one by one.

1. Enable HSE and wait for the HSE to become Ready

The HSE can be configured in the RCC_CR Register as you can see below

As you can see the 16th bit of this register can enable the HSE, and the 17th bit will set to 1, when the HSE is finally ready

RCC->CR |= 1<<16;  
while (!(RCC->CR & (1<<17)));

RCC->CR |= 1<<16;  
while (!(RCC->CR & (1<<17)));

above, I have set the bit 16 to HIGH (to enable the HSE), and then I am waiting for the 17th bit to set HIGH (HSE to become Ready)

2. Set the POWER ENABLE CLOCK and VOLTAGE REGULATOR

The Power Enable Clock can be set in the RCC_APB1ENR Register as shown in the picture below

In order to enable the Power, i need to set the bit 28 of the RCC_APB1ENR Register

RCC->APB1ENR |= 1<<28;

RCC->APB1ENR |= 1<<28;

The Voltage Regulator can be configured in the PWR_CR Register as shown below

In the RCC setup of the CubeMX, the voltage Regulator is selected for scale 1 by default. And i will also use the same for my application. Basically I need to write a 3 (11) in the 14th bit

PWR->CR |= 3<<14; 

PWR->CR |= 3<<14; 

3. Configure the FLASH PREFETCH and the LATENCY Related Settings

The settings are as shown in the RCC setup from the CubeMX below

To configure them, we need to look in the FLASH_ACR Register

As shown in the CubeMX picture above, the Instruction Cache, Prefetch Buffer, and the Data Cache all are enabled. Also the Flash Latency is 5WS

So i need to write a 1 in the 8th, 9th and the 10th positions. Also a 5 in the 0th position (for 5WS)

FLASH->ACR = (1<<8) | (1<<9)| (1<<10)| (5<<0);

FLASH->ACR = (1<<8) | (1<<9)| (1<<10)| (5<<0);

4. Configure the PRESCALARS HCLK, PCLK1, PCLK2

Here we will set the values for AHB Prescalar, APB1 Presclar, and APB2 Prescalar
The values are shown below (in RED color) in the Clock Setup from the CubeMX

To configure them, we will look into the RCC_CFGR Register

As shown in the CubeMX setup above

• The AHB Prescalar is 1, so the HPRE (bits 7:4) will be all 0s.
• APB1 Prescalar is 4, so the PPRE1 (bits 12:10) will be “101” i.e write 5 to the 10th position
• APB2 Prescalar is 2, so the PPRE2 (bits 15:13) will be “100” i.e write 4 to the 13th position

// AHB PR
RCC->CFGR &= ~(1<<4);
	
// APB1 PR
RCC->CFGR |= (5<<10);
	
// APB2 PR
RCC->CFGR |= (4<<13);

// AHB PR
RCC->CFGR &= ~(1<<4);
	
// APB1 PR
RCC->CFGR |= (5<<10);
	
// APB2 PR
RCC->CFGR |= (4<<13);

5. Configure the MAIN PLL

Here we will configure the PLL source (HSE or HSI), and also set the values for PLLM, PLLN and PLLP
The vales are shown below (in the BLUE color)

The configuration can be set in the RCC_PLLCFGR Register

We can directly push the required values to the PLLM and PLLN Bits, while the care must be taken when setting the PLLP value.

• The PLLP value, according to the CubeMX is 2, and to set it, I need to write 0s in the PLLP (Bits 17:16)
• PLLM and PLLN values are 4 and 180, according to the CubeMX, and I will directly set them in the PLLM (Bits 5:0) and PLLN (Bits 14:6)
• As I am using HSE crystal for the clock, I need to set the PLLSRC (Bit 22) to 1.

#define PLL_M 	4
#define PLL_N 	180
#define PLL_P 	0  // PLLP = 2

RCC->PLLCFGR = (PLL_M <<0) | (PLL_N << 6) | (PLL_P <<16) | (1<<22);

#define PLL_M 	4
#define PLL_N 	180
#define PLL_P 	0  // PLLP = 2

RCC->PLLCFGR = (PLL_M <<0) | (PLL_N << 6) | (PLL_P <<16) | (1<<22);

6. Enable the PLL and wait for it to become ready

The PLL can be enabled in the RCC_CR Register as shown below

To enable the PLL, we must write a 1 to the PLL ON (24th bit), and then we will wait for the PLL RDY (25th bit) to set, which will indicate that the PLL is ready.

RCC->CR |= (1<<24);
while (!(RCC->CR & (1<<25)));

RCC->CR |= (1<<24);
while (!(RCC->CR & (1<<25)));

7. Select the Clock Source and wait for it to be set

The clock source can be selected in the RCC_CFGR Register

• SW[1:0] is used to set the system clock. Since I am using the PLL_P as the system clock, I will write a 2 (1:0) to the SW Bits
• SWS[3:2] is used to monitor the status of the system clock. So here we will wait for these bits to indicate that the PLL_P has been set as the system clock (wait for the SWS bits to indicate 2 (1:0)).

RCC->CFGR |= (2<<0);
while (!(RCC->CFGR & (2<<2)));

RCC->CFGR |= (2<<0);
while (!(RCC->CFGR & (2<<2)));

That completes the setup process. The above setup will allow my controller to run at 180 MHz, as I did setup in the cubeMX

The main Function

We will test the clock configuration by blinking a LED. I am using Nucleo F401 and the LED on this board is connected to pin PA5. Below is the implementation of the main function.

int main (void)
{
	SysClockConfig ();
	GPIO_Config ();
	
	while (1)
	{
		GPIOA->BSRR |= (1<<5);  // Set the pin PA5
		delay (20000000); // some random Delay
		GPIOA->BSRR |= ((1<<5) <<16);  // Reset pin PA5
		delay (20000000); 
	}
}

int main (void)
{
	SysClockConfig ();
	GPIO_Config ();
	
	while (1)
	{
		GPIOA->BSRR |= (1<<5);  // Set the pin PA5
		delay (20000000); // some random Delay
		GPIOA->BSRR |= ((1<<5) <<16);  // Reset pin PA5
		delay (20000000); 
	}
}

The program starts with the main function. Inside it, the system clock is configured by calling SysClockConfig(), and the GPIO pins are set up by calling GPIO_Config(). This ensures the microcontroller is running at the desired speed and the required pin is ready for use.

We have already covered the system clock configuration in this tutorial and the GPIO configuration will be covered in the next one.

After the setup, the program enters an infinite while(1) loop. This loop runs continuously as long as the microcontroller is powered.

Inside the loop:

• GPIOA->BSRR |= (1<<5); sets pin PA5 to a HIGH state, which turns the connected LED ON.
• A delay is added so that the LED stays visible in the ON state for some time.
• GPIOA->BSRR |= ((1<<5)<<16); resets pin PA5 back to LOW, which turns the LED OFF.
• Another delay is added so the LED remains OFF for a while.

By repeating this process, the LED connected to PA5 will blink continuously, controlled entirely by register-level programming.

Common STM32 Clock Configuration Issues

Developers often face problems during manual RCC setup:

• Wrong PLL values → CPU won’t reach target frequency.
  
• Flash latency misconfigured → MCU hangs or resets.
  
• Not waiting for HSE/PLL ready flags → unstable system clock.
  
• Overclocking STM32 beyond datasheet → unexpected crashes.
  

Always cross-check with CubeMX clock tree and STM32 reference manual.

Result

The gif below shows the output of the above code.

As you can see, the LED on the board is blinking, which confirms that the clock has been successfully configured. However, we still need to verify whether the clock is running at the exact frequency we set. We will check this in another tutorial, where we will configure a Timer to generate precise delays in both microseconds and milliseconds.

With this setup, you now know how to configure the STM32 system clock using registers. We enabled the HSE, set up the PLL, configured prescalers and flash latency, and finally selected the PLL as the system clock. This configuration runs the STM32F4 at 180 MHz, the maximum frequency supported. Direct RCC register programming not only improves your understanding of microcontroller clocking but also gives you full control without relying on CubeMX or HAL. In upcoming tutorials, we’ll extend this to topics like clock security system (CSS) and low-power clock configurations for STM32.

STM32 Register based Tutorial Series

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STM32 GPIO Output Example Using Registers | BSRR, MODER, GPIOA Explained

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STM32 I2C Tutorial | Register-Based Configuration with Example Code

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STM32 SPI Tutorial Using Registers (Full-Duplex Master with ADXL345)

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