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

This STM32 GPIO tutorial explains how to configure GPIO output using STM32 registers. We’ll demonstrate a complete GPIO output example using MODER, OTYPER, OSPEEDR, PUPDR, and BSRR registers. This guide is ideal for understanding how to manipulate the GPIOA port and use BSRR register to toggle pins on STM32F4 microcontrollers. Whether you’re exploring stm32 gpio example projects or learning about gpio registers and stm32 registers, this post covers it all.

In this tutorial, we will learn how to configure a GPIO pin as output on an STM32 microcontroller using direct register programming instead of HAL or STM32CubeMX.

Here’s what you’ll learn step-by-step:

• How to enable the GPIO peripheral clock
• How to set a GPIO pin as output mode
• How to choose between Push-Pull and Open-Drain output types
• How to set the output speed for the pin
• How to configure pull-up or pull-down resistors
• How to write data to the pin using ODR and BSRR registers
• How to toggle an LED using register-level GPIO control

This tutorial gives you a deep understanding of how STM32 GPIO works at the register level and is perfect for those wanting to go beyond HAL and CubeMX for more control and optimization.

You should also take a look at the following:

• How to Configure STM32 using Registers
• STM32 GPIO Input Configuration using Registers

Why should you Configure the GPIO as output

When we configure a GPIO pin as output on an STM32 microcontroller, we can use it to control other devices.

For example, we can:

• Turn an LED ON or OFF
• Control a buzzer or a relay
• Send signals to other circuits
• Switch motors or sensors using transistors
• Create simple timing signals

By setting up the GPIO output correctly, we make sure the pin works safely and reliably. It’s one of the first things you learn when starting with STM32, and it’s used in almost every embedded project.

Configure the GPIO in Output Mode

Let’s start with the simplest, and the most used one i.e the output mode. I will use Pin PA5 as the output pin. Following are the steps required to configure the pin as the output Pin.

/************** STEPS TO FOLLOW *****************
1. Enable GPIOA clock
2. Set the PIN PA5 as output
3. Configure the output mode i.e state, speed, and pull
************************************************/

/************** STEPS TO FOLLOW *****************
1. Enable GPIOA clock
2. Set the PIN PA5 as output
3. Configure the output mode i.e state, speed, and pull
************************************************/

1. Enable GPIOA Clock

The GPIO clock can be enabled in the RCC_AHB1ENR Register

As you can see above, the 0th bit of RCC_AHB1ENR Register enables the clock for the GPIOA. That’s why we need to write a 1 in the 0th position

RCC->AHB1ENR |= (1<<0);  // Enable the GPIOA clock

RCC->AHB1ENR |= (1<<0);  // Enable the GPIOA clock

2. Set the PIN PA5 as output

To configure the pin as output, we will modify the GPIOx_MODER Register. This register is responsible for configuring different modes for the GPIO and in this case, we will configure it as the output mode

Since I am using pin PA5, I need to modify the pins 10 and 11. This basically works like, if the PIN is ‘y‘, the we need to configure the bits ‘2y’ and ‘2y+1’
Also, in order to set the pin as output, we need to set the bits (11:10) as 0:1. This means that we need to write a ‘1’ in the 10th position

GPIOA->MODER |= (1<<10);  // pin PA5(bits 11:10) as Output (01)

GPIOA->MODER |= (1<<10);  // pin PA5(bits 11:10) as Output (01)

3. Configure the output mode i.e Output type, speed, and pull

The output type can be configured in the GPIOx_OTYPER Register.

In the simplest words, the difference between the Push-Pull and open Drain can be understood as follows
If you choose open drain, then the output pin can either be LOW (Gnd) or it can be floating, while if you choose Push-Pull, the the output pin can be HIGH or LOW

Since I want to blink the LED on pin PA5, i will choose the push-pull type output, and to do so, I need to write a ‘0‘ in the 5th position

GPIOA->OTYPER &= ~(1<<5);  // bit 5=0 --> Output push pull

GPIOA->OTYPER &= ~(1<<5);  // bit 5=0 --> Output push pull

Next, The output speed can be configured in the GPIOx_OSPEEDR Register

Here you can select different Speeds for the Output Pin, as per your requirement. GPIO speed controls the rate at which a signal can change between low/high values (the “rise time” and “fall time”).
I want the speed not too high and not too low, so I will choose the Fast speed. To do so, I need to write 1:0 to bits (11:10)

GPIOA->OSPEEDR |= (1<<11);  // Pin PA5 (bits 11:10) as Fast Speed (1:0)

GPIOA->OSPEEDR |= (1<<11);  // Pin PA5 (bits 11:10) as Fast Speed (1:0)

Now the final step is to configure the pull-up/pull-down (GPIOx_PUPDR) register.

Here we can Pull-up the pin to Vcc or Pull-down to ground, as per the requirement. Since I am using the pin to blink LED, i will not use the Pull-up or Pull-down. I will set the bits (11:10) as 0

GPIOA->PUPDR &= ~((1<<10) | (1<<11));  // Pin PA5 (bits 11:10) are 0:0 --> no pull up or pulldown

GPIOA->PUPDR &= ~((1<<10) | (1<<11));  // Pin PA5 (bits 11:10) are 0:0 --> no pull up or pulldown

This completes the configuration for the output mode. Now we will see how to set or reset the pin so that we can toggle the LED attached to it

How to Set/Reset the Pin

After setting a GPIO pin as output, we can use it to turn things ON or OFF. In STM32, this is done by writing to special registers. We can use the BSRR register to quickly set or reset the pin, or the ODR register to control the pin value directly.
This helps us control devices like LEDs, buzzers, or relays from our code.

Control Output Pin Using BSRR

To set or reset an individual pin, we need to modify the GPIOx_BSRR Register

BSRR is a 32 bit Register. The lower 16 bits (bit 0 – bit 15) are responsible to set a bit, and the higher 16 bits (bit 16 – bit 31) are responsible to reset a bit

As I have connected the LED to PA5, in order to set it (turn the LED ON), I will write a 1 to bit 5

GPIOA->BSRR |= (1<<5); // Set the Pin PA5

GPIOA->BSRR |= (1<<5); // Set the Pin PA5

And to reset the pin PA5 (turn the LED OFF), I will write a 1 to the bit 21

GPIOA->BSRR |= (1<<21); // Reset the Pin PA5

GPIOA->BSRR |= (1<<21); // Reset the Pin PA5

Control Output Pin Using ODR

Output Data Register (ODR) can also be used to Set/Reset an individual Pin or the entire Port.

In order to Set a pin, we can directly write a ‘1’ to the respective bit, and in order to Reset it, write a ‘0’ in the respective bit

GPIOA->ODR |= 1<<5; // Set the Pin PA5

GPIOA->ODR &= ~(1<<5); // Reset the Pin PA5

GPIOA->ODR |= 1<<5; // Set the Pin PA5

GPIOA->ODR &= ~(1<<5); // Reset the Pin PA5

To set the entire PORT HIGH, we can write 0xFFF to the ODR

GPIOA->ODR = 0xFFF;  // Set the PORTA HIGH

GPIOA->ODR = 0x00;  // Reset the PORTA

GPIOA->ODR = 0xFFF;  // Set the PORTA HIGH

GPIOA->ODR = 0x00;  // Reset the PORTA

This will toggle all the Pins of PORTA (PA0 – PA15).

STM32 GPIOA Register-Level LED Blinking Example

Now let’s combine our understanding of MODER, BSRR register, and other GPIO registers to toggle PA5 (connected to an LED). This GPIOA example uses register-level control for precise and low-level pin manipulation.

void Delay (uint32_t time)
{
	while (time--);  
}

int main (void)
{
	SysClockConfig ();
	GPIOConfig ();
		
	while (1)
	{
		GPIOA->BSRR |= (1<<5); // Set the Pin PA5
//		GPIOA->ODR = 1<<5;
		Delay (10000000);  // random delay
		GPIOA->BSRR |= (1<<5) <<16;  // Clear the Pin PA5
//		GPIOA->ODR &= ~(1<<5);
		Delay (10000000);  // random delay
	}
	
}

void Delay (uint32_t time)
{
	while (time--);  
}

int main (void)
{
	SysClockConfig ();
	GPIOConfig ();
		
	while (1)
	{
		GPIOA->BSRR |= (1<<5); // Set the Pin PA5
//		GPIOA->ODR = 1<<5;
		Delay (10000000);  // random delay
		GPIOA->BSRR |= (1<<5) <<16;  // Clear the Pin PA5
//		GPIOA->ODR &= ~(1<<5);
		Delay (10000000);  // random delay
	}
	
}

This code turns an LED connected to PA5 ON and OFF with a delay in between.

• GPIOA->BSRR |= (1<<5); sets (turns ON) the PA5 pin
• GPIOA->BSRR |= (1<<5)<<16; resets (turns OFF) the PA5 pin
• The Delay() function creates a simple wait loop
• This ON/OFF cycle repeats forever in the while(1) loop

This STM32 GPIO output example shows how to configure GPIOA using registers. We used the BSRR register to toggle pins efficiently and explained how each GPIO register like MODER, OTYPER, and OSPEEDR works. If you’re looking for more stm32 gpio register or bsrr register examples, check out the related tutorials.

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