📚 Getting Started with stm32g0b1xx.h — Masks & Structs

🎯 Learning Objectives

By the end of this lesson, students will:

• Understand the purpose of the stm32g0b1xx.h.h header file
• Learn how peripheral registers are accessed using data structures
• Use bit masks to manipulate register values
• Write simple code to control GPIO using register-level programming

---

1️⃣ What Is stm32g0b1xx.h?

This header file is part of the CMSIS (Cortex Microcontroller Software Interface Standard) and provides:

• Definitions for peripheral base addresses
• Bit masks for register fields
• Memory-mapped structs for hardware registers

Example snippet from the file:

#define GPIOA_BASE (0x50000000UL)
#define GPIOA ((GPIO_TypeDef *) GPIOA_BASE)

2️⃣ Understanding Data Structures and Registers

Peripheral registers are grouped into structs. For example, GPIO_TypeDef might look like:

typedef struct {
  volatile uint32_t MODER;   // GPIO port mode register
  volatile uint32_t OTYPER;  // GPIO port output type register
  volatile uint32_t OSPEEDR; // GPIO port output speed register
  volatile uint32_t PUPDR;   // GPIO port pull-up/pull-down register
  volatile uint32_t IDR;     // GPIO port input data register
  volatile uint32_t ODR;     // GPIO port output data register
} GPIO_TypeDef;

Let’s say we want to modify the MODER register for GPIOA

• Each port, in this case GPIOA, has 16-PINs (GPIA15..GPIOA00)
• To configure the MODER register, we need 2-bits per Port PIN as there are 4 possible configurations:
• Then the MODER register has 16x2 = 32bits (b31..b00) to configure it according to the following table:
• 
• The 32-bits of the MODER register as distributed as follows:
• 
• 
• Let’s say we want to configure GPIOA0 as an output, that means we need to write 10b on b1 and b0 of the MODER register.

GPIOA->MODER = 0x00000001; // Set PA0 as output

• 0x00000001 = 00000000 000000000 000000000 000000001 in binary
• If we wanted to configure GPIOA10 as an output, that means we need to write 10b on b21 and b00 of the MODER register.

GPIOA->MODER = 0x00200000; // Set PA0 as output

• 0x00000001 = 00000000 000100000 000000000 000000000 in binary

3️⃣ Using Bit Masks

• The vendor provide us with convinient masks to avoid having to write raw numbers in hex or binary
• Extract from the header file (STM32G031xx.h)

#define GPIO_MODER_MODE0_Msk (0x3U << 0)  // Mask for PA0 mode bits
#define GPIO_MODER_MODE0_Output (0x1U << 0)

GPIOA->MODER &= ~GPIO_MODER_MODE0_Msk;       // Clear PA0 mode bits
GPIOA->MODER |= GPIO_MODER_MODE0_Output;     // Set PA0 as output

4️⃣ Sumary - Write Into the Registers using Masks

• Clear the BITS using the *xxx_Msk
• Write the proper BITS using the corresponding mask for the BITS

5️⃣ Challenge for Students

• Configure GPIOA12 as output using the BIT masks from the library
• Configure GPIOA15 as analog mode using the BIT masks from the library
• Verify it is working by using the debugger