Chapter 10 10. UART — USART3

Goal

NUCLEO-F439ZI의 USART3를 register level에서 설정하고, ST-LINK Virtual COM Port를 통해 PC terminal로 문자열을 전송한다.

이번 실습:

STM32F439ZI
    │
    │ USART3
    │
    └── PD8 (TX)
          ↓
       ST-LINK
          ↓
       USB VCP
          ↓
         PC
          ↓
"Hello from NUCLEO-F439ZI!"

설정:

USART  : USART3
TX     : PD8
RX     : PD9
AF     : AF7
Baud   : 115200 bps
Clock  : 16 MHz
Format : 115200-8-N-1

1. Board Mapping

원서의 NUCLEO-F411과 현재 사용하는 NUCLEO-F439ZI는 Virtual COM Port 연결이 다르다.

따라서 원서 코드를 그대로 사용하면 compile은 되더라도 VCP에 출력이 나타나지 않을 수 있다.

이번 board에서는:

USART3 TX → PD8
USART3 RX → PD9

를 사용한다.

2. UART Registers

이번 코드에서 직접 사용하는 register:

RCC->AHB1ENR
RCC->APB1ENR
GPIOD->MODER
GPIOD->AFR[1]
USART3->BRR
USART3->CR1
USART3->SR
USART3->DR

USART 주요 bit:

CR1 bit3   TE
→ Transmitter Enable
CR1 bit13  UE
→ USART Enable
SR bit7    TXE
→ Transmit Data Register Empty

3. Baud Rate

Peripheral clock:

APB1 Clock = 16 MHz

Target:

Baud Rate = 115200

기존 실습 코드:

BRR = (periph_clk + baudrate / 2) / baudrate;

따라서:

16,000,000 + 57,600
───────────────────
115,200
≈ 139

즉:

USART3->BRR ≈ 139

를 사용한다.

baudrate / 2를 먼저 더하는 것은 정수 나눗셈에서 반올림 효과를 주기 위한 것이다.

4. Practice Code

Project:

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

gpio.c와 systick.c는 앞 chapter에서 만든 driver를 재사용한다.

5. `uart.h`

#ifndef UART_H
#define UART_H
#include "stm32f4xx.h"
/*

Initialize USART3.


Board:

NUCLEO-F439ZI




Pin:

PD8 → USART3_TX



PD9 → USART3_RX




Alternate Function:

AF7




Baud:

115200 bps



*/
void uart_init(void);
/*

Send a null-terminated string through USART3.


Example:


uart_write_string("Hello\r\n");
*/
void uart_write_string(const char *s);

/*

printf redirection hook.


This function is useful when the project is built
in an environment where newlib/newlib-nano is linked.


The standalone bare-metal Make build does not depend
on printf, so uart_write_string() is used directly.
*/
int __io_putchar(int ch);

#endif

6. `uart.c`

#include <stdint.h>
#include "uart.h"
/*

============================================================
RCC Clock Enable Bits
============================================================
*/

/*

RCC AHB1ENR bit3:


Enable GPIOD peripheral clock.


PD8 / PD9 cannot be configured until
the GPIOD clock is enabled.
*/
#define GPIODEN         (1U << 3)

/*

RCC APB1ENR bit18:


Enable USART3 peripheral clock.
*/
#define USART3EN        (1U << 18)

/*

============================================================
UART Clock / Baud Configuration
============================================================
*/

/*

Target communication speed.
*/
#define DBG_UART_BAUDRATE   115200U

/*

This Bare-Metal project does not configure the PLL.


System clock:



HSI = 16 MHz



*/
#define SYS_FREQ            16000000U
/*

USART3 is connected to APB1.


In this project APB1 clock is also 16 MHz.
*/
#define APB1_CLK            SYS_FREQ

/*

============================================================
USART Register Bits
============================================================
*/

/*

USART CR1 bit3:


TE = Transmitter Enable


0 → transmitter disabled
1 → transmitter enabled
*/
#define CR1_TE              (1U << 3)

/*

USART CR1 bit13:


UE = USART Enable


0 → USART disabled
1 → USART enabled
*/
#define CR1_UE              (1U << 13)

/*

USART SR bit7:


TXE = Transmit Data Register Empty


0 → DR is not ready for new data
1 → next byte may be written to DR
*/
#define SR_TXE              (1U << 7)

/*

============================================================
Private Function Prototypes
============================================================
*/

/*

Send one byte.


This is kept static because it is used only
inside uart.c.
*/
static void uart_write(int ch);

/*

Calculate BRR value from peripheral clock
and target baud rate.
*/
static uint16_t compute_uart_bd(uint32_t periph_clk,
uint32_t baudrate);

/*

Program the calculated baud rate into USART3 BRR.
*/
static void uart_set_baudrate(uint32_t periph_clk,
uint32_t baudrate);

/*

============================================================
printf Redirection
============================================================
*/

int __io_putchar(int ch)
{
/*
* Redirect one character to USART3.
*/
uart_write(ch);
/*
 * Standard putchar-style function returns
 * the transmitted character.
 */
return ch;

}
/*

============================================================
USART3 Initialization
============================================================
*/

void uart_init(void)
{
/*
* --------------------------------------------------------
* Step 1. Enable GPIOD Clock
* --------------------------------------------------------
*
* USART3 VCP pins:
*
*     PD8 → TX
*     PD9 → RX
*
* Before configuring these pins,
* GPIOD must receive an AHB1 clock.
*/
RCC->AHB1ENR |= GPIODEN;
/*
 * --------------------------------------------------------
 * Step 2. Configure PD8 as Alternate Function
 * --------------------------------------------------------
 *
 * GPIO MODER uses 2 bits per pin.
 *
 * PD8:
 *
 *     MODER<span>[17:16]</span>
 *
 * Mode:
 *
 *     00 → Input
 *     01 → Output
 *     10 → Alternate Function
 *     11 → Analog
 *
 * First clear the field.
 */
GPIOD-&gt;MODER &amp;= ~(0x3UL &lt;&lt; (8 * 2));

/*
 * Then write 10b.
 *
 * PD8 becomes Alternate Function mode.
 */
GPIOD-&gt;MODER |=  (0x2UL &lt;&lt; (8 * 2));


/*
 * --------------------------------------------------------
 * Step 3. Configure PD9 as Alternate Function
 * --------------------------------------------------------
 *
 * PD9:
 *
 *     MODER<span>[19:18]</span>
 */
GPIOD-&gt;MODER &amp;= ~(0x3UL &lt;&lt; (9 * 2));
GPIOD-&gt;MODER |=  (0x2UL &lt;&lt; (9 * 2));


/*
 * --------------------------------------------------------
 * Step 4. Select AF7 for PD8
 * --------------------------------------------------------
 *
 * AFR register:
 *
 *     AFR<span>[0]</span> → Pin 0 ~ 7
 *     AFR<span>[1]</span> → Pin 8 ~ 15
 *
 * PD8 is therefore controlled by AFR<span>[1]</span>.
 *
 * Each pin uses 4 bits.
 *
 * PD8 position:
 *
 *     (8 - 8) × 4
 *     = bit <span>[3:0]</span>
 *
 * Clear existing AF field first.
 */
GPIOD-&gt;AFR<span>[1]</span> &amp;= ~(0xFUL &lt;&lt; ((8 - 8) * 4));

/*
 * AF7 = 0111b
 *
 * PD8 → USART3_TX
 */
GPIOD-&gt;AFR<span>[1]</span> |=  (0x7UL &lt;&lt; ((8 - 8) * 4));


/*
 * --------------------------------------------------------
 * Step 5. Select AF7 for PD9
 * --------------------------------------------------------
 *
 * PD9 position:
 *
 *     (9 - 8) × 4
 *     = bit <span>[7:4]</span>
 */
GPIOD-&gt;AFR<span>[1]</span> &amp;= ~(0xFUL &lt;&lt; ((9 - 8) * 4));

/*
 * PD9 → AF7 → USART3_RX
 */
GPIOD-&gt;AFR<span>[1]</span> |=  (0x7UL &lt;&lt; ((9 - 8) * 4));


/*
 * --------------------------------------------------------
 * Step 6. Enable USART3 Clock
 * --------------------------------------------------------
 *
 * USART3 belongs to APB1.
 *
 * RCC APB1ENR bit18:
 *
 *     USART3EN = 1
 */
RCC-&gt;APB1ENR |= USART3EN;


/*
 * --------------------------------------------------------
 * Step 7. Configure Baud Rate
 * --------------------------------------------------------
 *
 * Peripheral clock:
 *
 *     16 MHz
 *
 * Baud:
 *
 *     115200 bps
 *
 * Calculated BRR:
 *
 *     approximately 139
 */
uart_set_baudrate(APB1_CLK,
                  DBG_UART_BAUDRATE);


/*
 * --------------------------------------------------------
 * Step 8. Enable Transmitter
 * --------------------------------------------------------
 *
 * This chapter transmits only.
 *
 * TE = 1
 *
 * Note:
 * PD9 is configured for USART3_RX,
 * but RE(receiver enable) is not set here.
 *
 * Therefore actual communication in this exercise
 * is board → PC TX only.
 */
USART3-&gt;CR1 = CR1_TE;


/*
 * --------------------------------------------------------
 * Step 9. Enable USART3
 * --------------------------------------------------------
 *
 * UE = 1
 *
 * USART3 is now operational.
 */
USART3-&gt;CR1 |= CR1_UE;

}
/*

============================================================
Send One Character
============================================================
*/

static void uart_write(int ch)
{
/*
* --------------------------------------------------------
* Step 1. Wait Until DR Can Accept New Data
* --------------------------------------------------------
*
* TXE:
*
*     0 → transmit data register is not ready
*     1 → transmit data register is empty
*
* CPU continuously checks TXE.
*
* This is polling / busy waiting.
/
while (!(USART3->SR & SR_TXE))
{
/
* Wait here until TXE becomes 1.
*/
}
/*
 * --------------------------------------------------------
 * Step 2. Write Data
 * --------------------------------------------------------
 *
 * Only the lower 8 bits are transmitted.
 */
USART3-&gt;DR = (ch &amp; 0xFF);

}
/*

============================================================
Send String
============================================================
*/

void uart_write_string(const char s)
{
/
* C string:
*
*     H e l l o \0
*
* Continue until null terminator '\0'.
*/
while (s)
{
/
* Send current character.
*/
uart_write((int)*s);
    /*
     * Move pointer to next character.
     */
    s++;
}

}
/*

============================================================
Baud Rate Calculation
============================================================
*/

static uint16_t compute_uart_bd(uint32_t periph_clk,
uint32_t baudrate)
{
/*
* Integer arithmetic would normally truncate
* the fractional part.
*
* Adding baudrate / 2 before division gives
* a rounding effect.
*
* Example:
*
* periph_clk = 16,000,000
* baudrate   = 115,200
*
* BRR =
*
* (16,000,000 + 57,600)
* --------------------
*       115,200
*
* ≈ 139
*/
return (uint16_t)
((periph_clk + (baudrate / 2U))
/ baudrate);
}
/*

============================================================
Program Baud Rate Register
============================================================
*/

static void uart_set_baudrate(uint32_t periph_clk,
uint32_t baudrate)
{
/*
* Store the calculated divider into BRR.
*/
USART3->BRR =
compute_uart_bd(periph_clk, baudrate);
}

7. `main.c`

#include "gpio.h"
#include "systick.h"
#include "uart.h"
int main(void)
{
/*
* --------------------------------------------------------
* Step 1. Initialize LD1
* --------------------------------------------------------
*
* NUCLEO-F439ZI:
*
*     LD1 → PB0
*/
led_init();
/*
 * --------------------------------------------------------
 * Step 2. Initialize USART3
 * --------------------------------------------------------
 *
 * Configuration:
 *
 *     USART3
 *     PD8  → TX
 *     PD9  → RX pin mapping
 *     AF7
 *     115200 bps
 */
uart_init();


while (1)
{
    /*
     * ----------------------------------------------------
     * Step 3. Send String to PC
     * ----------------------------------------------------
     *
     * uart_write_string() sends the string
     * one character at a time.
     *
     * '\r\n':
     *
     *     \r → Carriage Return
     *     \n → Line Feed
     *
     * Terminal output moves to the next line.
     */
    uart_write_string(
        "Hello from NUCLEO-F439ZI! "
        "(USART3, 115200bps)\r\n"
    );


    /*
     * ----------------------------------------------------
     * Step 4. Toggle LD1
     * ----------------------------------------------------
     *
     * Provides a visual indication that
     * the main loop is running.
     */
    led_toggle();


    /*
     * ----------------------------------------------------
     * Step 5. Wait 1 Second
     * ----------------------------------------------------
     *
     * Ch08 SysTick driver is reused.
     */
    systick_msec_delay(1000);
}

}

8. UART Initialization Flow

전체 초기화 순서:

uart_init()
│
├── GPIOD Clock Enable
│
├── PD8 → Alternate Function
│
├── PD9 → Alternate Function
│
├── PD8 → AF7
│
├── PD9 → AF7
│
├── USART3 Clock Enable
│
├── BRR = 139
│
├── TE = 1
│
└── UE = 1

이후:

USART3 Ready

상태가 된다.

9. Why Alternate Function?

GPIO pin 하나는 GPIO 기능만 수행하는 것이 아니다.

예를 들어 PD8은 설정에 따라 여러 peripheral function으로 연결될 수 있다.

PD8
│
├── GPIO
├── USART
├── Timer
└── Other Peripheral

MODER=10은:

이 pin을 GPIO output으로 직접 쓰지 않고
Peripheral에게 넘기겠다

는 의미다.

그 후:

GPIOD->AFR[1] = ...

에서 어떤 peripheral function을 연결할지 선택한다.

이번 설정:

PD8
↓
Alternate Function Mode
↓
AF7
↓
USART3_TX

10. Why `AFR[1]`?

Alternate Function register는 두 개다.

AFR[0]
→ Pin 0 ~ 7
AFR[1]
→ Pin 8 ~ 15

PD8과 PD9는 모두 pin number가 8 이상이다.

따라서:

GPIOD->AFR[1]

을 사용한다.

PD8 field:

AFR[1][3:0]

PD9 field:

AFR[1][7:4]

그래서 코드가:

((8 - 8) * 4)
((9 - 8) * 4)

형태가 된다.

11. Character Transmission

문자 'A'를 보낸다고 가정한다.

ASCII:

'A'
0x41

호출:

uart_write('A');

흐름:

CPU
↓
TXE 확인
↓
TXE = 1
↓
USART3->DR = 0x41
↓
USART Hardware
↓
Serial Frame 생성
↓
PD8 TX
↓
ST-LINK VCP
↓
PC Terminal
↓
A

12. TXE Polling

핵심:

while (!(USART3->SR & SR_TXE))
{
}

CPU가 계속:

TXE?
↓
0 → 기다림
↓
0 → 기다림
↓
1 → 다음 data write

를 수행한다.

여기서 TXE의 정확한 의미는:

Transmit Data Register Empty

이다.

즉:

다음 byte를 DR에 쓸 수 있다

는 뜻이다.

TXE와 **전체 serial frame 전송 완료를 의미하는 TC**는 구분해야 한다.

이번 문자열 연속 송신에서는:

TXE

를 이용하면 다음 character를 연속해서 공급할 수 있다.

13. String Transmission

uart_write_string("ABC");

memory:

'A' 'B' 'C' '\0'
↑
s

첫 번째 loop:

*s = 'A'
→ uart_write('A')
→ s++

두 번째:

*s = 'B'
→ uart_write('B')
→ s++

세 번째:

*s = 'C'
→ uart_write('C')
→ s++

마지막:

*s = '\0'

이면:

while (*s)

조건이 false가 되어 종료된다.

14. Why No `printf()`?

현재 bare-metal Make build는 표준 library 의존성을 최소화한다.

따라서 단순 출력에는:

uart_write_string()

을 직접 사용한다.

장점:

작은 코드
명확한 실행 흐름
Library dependency 최소화
UART hardware 동작 직접 확인

__io_putchar()는 이후 newlib/newlib-nano가 연결된 환경에서 printf() redirection에 활용할 수 있도록 남겨둔 함수다.

15. TX Only in This Practice

PD9까지 AF7로 설정하지만 현재 CR1에는:

TE

만 enable한다.

즉:

Transmitter Enable
= YES
Receiver Enable
= NO

이다.

따라서 현재 기능:

Board
↓
PC

단방향이다.

RX까지 사용하려면 이후:

CR1.RE
SR.RXNE
DR Read

등이 추가로 필요하다.

16. Runtime Flow

main()
│
├── led_init()
│
├── uart_init()
│
└── while(1)
│
├── uart_write_string()
│      │
│      ├── TXE wait
│      ├── DR write
│      ├── TXE wait
│      ├── DR write
│      └── ...
│
├── LED toggle
│
├── SysTick 1000 ms
│
└── Repeat

17. Hardware Data Flow

CPU
│
│ write
▼
USART3->DR
│
▼
USART3 Transmitter
│
▼
PD8 / AF7
│
▼
ST-LINK VCP
│
▼
USB
│
▼
PC Terminal

18. Build Result

기존 실습 결과:

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

경고 없이 build되었고, 실제 NUCLEO-F439ZI에 download/run하여 VCP 115200-8-N-1에서 1초마다 문자열 출력과 LD1 toggle을 확인했다.

19. Verification

Build

make clean
make

Expected:

Build PASS
text = 1168
data = 0
bss  = 2560

Debugger — GPIO

확인:

GPIOD->MODER
GPIOD->AFR[1]

Expected:

PD8 MODER = 10
PD9 MODER = 10
PD8 AF = 7
PD9 AF = 7

Debugger — USART

확인:

USART3->BRR
USART3->CR1
USART3->SR

Expected:

BRR ≈ 139
CR1.TE = 1
CR1.UE = 1

TX 동작 중:

SR.TXE

변화를 확인할 수 있다.

PC Terminal

Terminal configuration:

Baud Rate : 115200
Data Bits : 8
Parity    : None
Stop Bits : 1
Flow Ctrl : None

즉:

115200-8-N-1

Expected output:

Hello from NUCLEO-F439ZI! (USART3, 115200bps)
Hello from NUCLEO-F439ZI! (USART3, 115200bps)
Hello from NUCLEO-F439ZI! (USART3, 115200bps)
...

약 1초 간격으로 출력된다.

20. If Nothing Appears

가장 먼저 확인할 부분:

Board
→ F439ZI 맞는가?
USART
→ USART3인가?
TX
→ PD8인가?
AF
→ AF7인가?
AFR
→ AFR[1]인가?
Clock
→ APB1 USART3EN bit18인가?
Baud
→ 115200인가?
Terminal
→ 올바른 ST-LINK COM Port인가?

특히 원서의:

USART2 / PA2

를 그대로 사용하면 F439ZI VCP에서는 출력이 보이지 않는 것이 핵심 porting issue다.

21. Screenshots to Add

images/ch10_build.png
images/ch10_gpio_af.png
images/ch10_usart_registers.png
images/ch10_terminal.png
images/ch10_board_led.gif

가장 중요한 증거:

USART3 Register
+
115200 Terminal Output
+
LD1 Toggle

22. Key Points

GPIOD Clock
↓
PD8 / PD9 AF Mode
↓
AF7
↓
USART3 Clock
↓
BRR = 139
↓
TE + UE
↓
TXE Polling
↓
DR Write
↓
PD8
↓
PC

NUCLEO-F439ZI의 ST-LINK VCP는 USART3를 사용한다.
USART3 TX는 PD8, RX는 PD9다.
PD8/PD9는 AF7로 설정한다.
Pin 8~15는 AFR[1]을 사용한다.
USART3는 APB1 peripheral이다.
이번 project clock은 16MHz다.
115200bps에서 BRR은 약 139다.
CR1.TE는 transmitter를 enable한다.
CR1.UE는 USART peripheral 자체를 enable한다.
SR.TXE는 다음 data를 DR에 쓸 수 있음을 나타낸다.
문자열은 한 character씩 TXE를 polling하며 전송한다.
이번 실습은 TX-only communication이다.
실제 board에서 VCP 115200-8-N-1 출력까지 검증했다.

Bare-Metal ARM

Home

Chapter 10

10. UART — USART3

Goal

1. Board Mapping

2. UART Registers

3. Baud Rate

4. Practice Code

5. uart.h

6. uart.c

7. main.c

8. UART Initialization Flow

9. Why Alternate Function?

10. Why AFR[1]?

AFR[1] → Pin 8 ~ 15

11. Character Transmission

12. TXE Polling

13. String Transmission

14. Why No printf()?

15. TX Only in This Practice

16. Runtime Flow

17. Hardware Data Flow

18. Build Result

19. Verification

Build

Debugger — GPIO

Debugger — USART

PC Terminal

20. If Nothing Appears

21. Screenshots to Add

22. Key Points

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

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Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

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Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

Uh oh!

5. `uart.h`

6. `uart.c`

7. `main.c`

10. Why `AFR[1]`?

14. Why No `printf()`?