bare-metal-patterns

bare-metal-patterns

Knowledge Base

Production bare-metal C patterns for ARM Cortex-M. All code uses arm-none-eabi-gcc, C11.

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Pattern 1: Register Bit Manipulation Macros

Consistent, readable register access without HAL dependency.

/* Generic bit manipulation — safe for any 32-bit register */
#define REG_SET_BIT(reg, bit)      ((reg) |=  (1U << (bit)))
#define REG_CLR_BIT(reg, bit)      ((reg) &= ~(1U << (bit)))
#define REG_TST_BIT(reg, bit)      (((reg) >> (bit)) & 1U)

/* Set a multi-bit field: mask off old value, OR in new value */
#define REG_SET_FIELD(reg, mask, shift, val) \
    ((reg) = ((reg) & ~(mask)) | (((val) << (shift)) & (mask)))

/* Example: set USART1 baud rate divisor in BRR register */
/* BRR = fCK / baud (oversampling by 16) */
#define USART_BRR_SET(uart, fck, baud) \
    ((uart)->BRR = (uint32_t)((fck) / (baud)))

USART_BRR_SET(USART1, 84000000UL, 115200UL);  /* STM32F4 APB2 @ 84MHz */

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Pattern 2: GPIO Configuration (Register Level)

Full GPIO setup for STM32F4 without HAL:

/* PA5 = LED (push-pull output), PA0 = button (input, pull-up) */
void gpio_init(void)
{
    /* 1. Enable GPIOA clock */
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
    (void)RCC->AHB1ENR;            /* Bus latency flush: read back after write */

    /* 2. PA5: General purpose output, push-pull, high speed */
    GPIOA->MODER   = (GPIOA->MODER & ~(3U << 10)) | (1U << 10);  /* MODER5 = 01 */
    GPIOA->OTYPER &= ~(1U << 5);                                  /* Push-pull */
    GPIOA->OSPEEDR|=  (3U << 10);                                 /* Very high speed */
    GPIOA->PUPDR   = (GPIOA->PUPDR & ~(3U << 10));                /* No pull */

    /* 3. PA0: Input, pull-up */
    GPIOA->MODER   = (GPIOA->MODER & ~(3U << 0));   /* MODER0 = 00: input */
    GPIOA->PUPDR   = (GPIOA->PUPDR & ~(3U << 0)) | (1U << 0);  /* Pull-up */
}

static inline void led_on(void)  { GPIOA->BSRR = (1U << 5);       }
static inline void led_off(void) { GPIOA->BSRR = (1U << (5+16));  }
static inline void led_toggle(void) { GPIOA->ODR ^= (1U << 5);    }
static inline int  btn_read(void) { return (int)((GPIOA->IDR >> 0) & 1U); }