📚 Interacting with a Port and Pin on the STM32G0B1RE

🎯 Learning Objectives

By the end of this lesson, you will:

• Understand how to enable GPIO clocks
• Configure pins as input or output
• Write to and read from pins using registers
• Blink LEDs and respond to a button press — all in bare metal programming

---

1️⃣ Project Setup in Segger Embedded Studio

• Create a new project for an STM32G0B1RE target.
• Add the CMSIS device header for STM32G0 (stm32g0b1xx.h) — this gives you access to the register definitions.

---

2️⃣ Enable Peripheral Clocks

• Before using GPIO, you must enable its clock in the RCC (Reset and Clock Control).

//Enable power interface clock (RM 5.4.15)
RCC->APBENR1 |= RCC_APBENR1_PWREN;

//Peripherals clock enable (RM 5.4.13)
RCC->IOPENR |= RCC_IOPENR_GPIOAEN; // Port A
RCC->IOPENR |= RCC_IOPENR_GPIOBEN; // Port B
RCC->IOPENR |= RCC_IOPENR_GPIOCEN; // Port C

📌 Why? If the clock to a peripheral is not enabled, its registers won’t respond — it’s like trying to talk to a device that’s powered off

---

3️⃣ Configure Pin Modes

Each GPIO pin has a MODER register field:

• 00 = Input
• 01 = General purpose output
• 10 = Alternate function
• 11 = Analog

Example: Set PB3 as output:

GPIOB->MODER &= ~GPIO_MODER_MODE3_Msk; // Clear mode bits
GPIOB->MODER |= GPIO_MODER_MODE3_0;    // Set to output (01)

Do the same for:

• PC6 → Output
• PB5 → Output
• PC13 → Input (just clear bits, no set)

---

4️⃣ Writing to Output Pins

The ODR (Output Data Register) controls pin states:

To SET

GPIOB->ODR |= GPIO_ODR_OD3; // Set PB3 high
GPIOC->ODR |= GPIO_ODR_OD6; // Set PC6 high

To TOGGLE

GPIOB->ODR ^= GPIO_ODR_OD3; // Flip PB3

To CLEAR

GPIOC->ODR &= ~GPIO_ODR_OD6; // CLear PC6

---

5️⃣ Reading from Input Pins

The IDR (Input Data Register) holds the current pin state:

if (GPIOB->IDR & GPIO_IDR_ID4) 
{
    // PB4 is high
} 
else 
{
    // PB4 is low
}

---

✅ Key Takeaways

• Always enable the peripheral clock before using GPIO.
• Configure pin mode via MODER.
• Use ODR to write, IDR to read.
• Bare-metal programming means direct register acces.

---