(periph_clk + baudrate / 2U) / baudrate

APB1 clock = 16 MHz
baud       = 115200
BRR        ≈ 139

while (!(USART3->SR & SR_TXE)) { }
USART3->DR = data;

GPIOD clock enable
      │
      ▼
PD8 / PD9 → Alternate Function
      │
      ▼
AF7 → USART3
      │
      ▼
USART3 clock enable
      │
      ▼
BRR = 115200 bps 설정
      │
      ▼
TE + UE enable
      │
      ▼
TXE polling
      │
      ▼
DR write
      │
      ▼
ST-LINK VCP → PC terminal

Ch10_uart/
├── Inc/gpio.h
├── Src/gpio.c
├── Inc/systick.h
├── Src/systick.c
├── Inc/uart.h
├── Src/uart.c
├── Src/main.c

#ifndef GPIO_H
#define GPIO_H
#include <stdbool.h>
/* ---- LED (PB0, LD1 녹색) ---- */
void led_init(void);
void led_on(void);
void led_off(void);
/* ---- Push Button (PC13, B1) ---- */
void button_init(void);
bool get_btn_state(void);
void led_toggle(void);   /* Ch08에서 추가 */
#endif /* GPIO_H */

#include "stm32f4xx.h"
#include "gpio.h"
#define LED_PIN    0U   /* PB0 = LD1 (녹색) /
#define BTN_PIN    13U  / PC13 = B1 */
void led_init(void)
{
RCC->AHB1ENR |= RCC_AHB1ENR_GPIOBEN;
GPIOB->MODER &= ~(0x3UL << (LED_PIN * 2));
GPIOB->MODER |=  (0x1UL << (LED_PIN * 2));

}
void led_on(void)
{
GPIOB->BSRR = (1UL << LED_PIN);        /* Set */
}
void led_off(void)
{
GPIOB->BSRR = (1UL << (LED_PIN + 16)); /* Reset */
}
void button_init(void)
{
RCC->AHB1ENR |= RCC_AHB1ENR_GPIOCEN;
GPIOC-&gt;MODER &amp;= ~(0x3UL &lt;&lt; (BTN_PIN * 2));  /* Input */
GPIOC-&gt;PUPDR &amp;= ~(0x3UL &lt;&lt; (BTN_PIN * 2));  /* No pull (보드가 자체 처리) */

}
bool get_btn_state(void)
{
return (GPIOC->IDR & (1UL << BTN_PIN)) != 0U;  /* active-HIGH */
}
void led_toggle(void)
{
GPIOB->ODR ^= (1UL << 0);  /* PB0 = LD1 */
}

#ifndef SYSTICK_H_
#define SYSTICK_H_
#include <stdint.h>
#include "stm32f4xx.h"
void systick_msec_delay(uint32_t delay);
#endif /* SYSTICK_H_ */

#include "systick.h"
#define CTRL_ENABLE     (1UL << 0)
#define CTRL_CLKSRC     (1UL << 2)
#define CTRL_COUNTFLAG  (1UL << 16)
/* 코어클럭 16MHz(기본 HSI) 기준 1ms당 클럭 사이클 수 */
#define ONE_MSEC_LOAD   16000UL
void systick_msec_delay(uint32_t delay)
{
SysTick->LOAD = ONE_MSEC_LOAD - 1UL;
SysTick->VAL  = 0UL;
SysTick->CTRL = CTRL_CLKSRC;
SysTick->CTRL |= CTRL_ENABLE;
for (uint32_t i = 0; i &lt; delay; i++)
{
    while ((SysTick-&gt;CTRL &amp; CTRL_COUNTFLAG) == 0UL) { }
}

SysTick-&gt;CTRL = 0UL;

}

#ifndef UART_H
#define UART_H
#include "stm32f4xx.h"
void uart_init(void);
void uart_write_string(const char *s);
int __io_putchar(int ch);
#endif

#include <stdint.h>
#include "uart.h"
#define GPIODEN    (1U << 3)
#define USART3EN   (1U << 18)
#define DBG_UART_BAUDRATE 115200U
#define SYS_FREQ           16000000U
#define APB1_CLK           SYS_FREQ
#define CR1_TE   (1U << 3)
#define CR1_UE   (1U << 13)
#define SR_TXE   (1U << 7)
static void uart_write(int ch);
static uint16_t compute_uart_bd(uint32_t periph_clk, uint32_t baudrate);
static void uart_set_baudrate(uint32_t periph_clk, uint32_t baudrate);
int __io_putchar(int ch)
{
uart_write(ch);
return ch;
}
void uart_init(void)
{
RCC->AHB1ENR |= GPIODEN;
GPIOD-&gt;MODER &amp;= ~(0x3UL &lt;&lt; (8 * 2));
GPIOD-&gt;MODER |=  (0x2UL &lt;&lt; (8 * 2));
GPIOD-&gt;MODER &amp;= ~(0x3UL &lt;&lt; (9 * 2));
GPIOD-&gt;MODER |=  (0x2UL &lt;&lt; (9 * 2));

GPIOD-&gt;AFR[1] &amp;= ~(0xFUL &lt;&lt; 0);
GPIOD-&gt;AFR[1] |=  (0x7UL &lt;&lt; 0);   /* PD8 -&gt; AF7 (USART3_TX) */
GPIOD-&gt;AFR[1] &amp;= ~(0xFUL &lt;&lt; 4);
GPIOD-&gt;AFR[1] |=  (0x7UL &lt;&lt; 4);   /* PD9 -&gt; AF7 (USART3_RX) */

RCC-&gt;APB1ENR |= USART3EN;

uart_set_baudrate(APB1_CLK, DBG_UART_BAUDRATE);

USART3-&gt;CR1 = CR1_TE;
USART3-&gt;CR1 |= CR1_UE;

}
static void uart_write(int ch)
{
while (!(USART3->SR & SR_TXE)) { }
USART3->DR = (ch & 0xFF);
}
void uart_write_string(const char *s)
{
while (*s) uart_write((int)*s++);
}
static uint16_t compute_uart_bd(uint32_t periph_clk, uint32_t baudrate)
{
return (uint16_t)((periph_clk + (baudrate / 2U)) / baudrate);
}
static void uart_set_baudrate(uint32_t periph_clk, uint32_t baudrate)
{
USART3->BRR = compute_uart_bd(periph_clk, baudrate);
}

#include "gpio.h"
#include "systick.h"
#include "uart.h"
int main(void)
{
led_init();
uart_init();
while (1)
{
    uart_write_string("Hello from NUCLEO-F439ZI! (USART3, 115200bps)\r\n");
    led_toggle();
    systick_msec_delay(1000);
}

}

text = 1168 bytes
data = 0 bytes
bss  = 2560 bytes
warning 없음