07. GPIO

Goal

NUCLEO-F439ZI의 LD1(PB0) 과 B1(PC13) 을 직접 제어한다.

이번 장에서 확인할 핵심:

PB0  → Output → LED
PC13 → Input  → Push Button

사용 register:

RCC->AHB1ENR
GPIOB->MODER
GPIOB->ODR
GPIOB->BSRR
GPIOC->MODER
GPIOC->PUPDR
GPIOC->IDR

1. GPIO 핵심

GPIO pin은 MODER에서 동작 모드를 결정한다.

00 = Input
01 = Output
10 = Alternate Function
11 = Analog

Pin 하나당 2bit를 사용한다.

Pin n
→ MODER[(2n+1):(2n)]

따라서:

PB0  → MODER[1:0]
PC13 → MODER[27:26]

2. Board Mapping

NUCLEO-F439ZI:

LD1 Green LED
→ PB0

B1 User Button
→ PC13

이번 board에서 B1은:

Released → 0
Pressed  → 1

즉 active-HIGH로 사용한다.

3. GPIO Data Registers

IDR

GPIOC->IDR

Input pin의 실제 logic level을 읽는다.

PC13:

GPIOC->IDR & (1U << 13)

ODR

GPIOB->ODR

Output state를 저장한다.

PB0 toggle:

GPIOB->ODR ^= (1U << 0);

BSRR

특정 output pin을 한 번의 write로 set/reset할 수 있다.

BSRR[15:0]
→ SET

BSRR[31:16]
→ RESET

PB0:

GPIOB->BSRR = (1U << 0);   /* SET   */
GPIOB->BSRR = (1U << 16);  /* RESET */

ODR |= ...처럼 Read-Modify-Write를 하지 않아도 되므로 단순 set/reset에는 BSRR이 유리하다.

4. Practice Code

이번 장부터 GPIO 코드를 driver 형태로 분리한다.

Inc/
└── gpio.h

Src/
├── gpio.c
└── main.c

`gpio.h`

#ifndef GPIO_H__
#define GPIO_H__

#include <stdbool.h>
#include "stm32f4xx.h"

/*
 * LED control functions
 *
 * LD1 on NUCLEO-F439ZI is connected to PB0.
 */
void led_init(void);
void led_on(void);
void led_off(void);
void led_toggle(void);

/*
 * Push button functions
 *
 * B1 on NUCLEO-F439ZI is connected to PC13.
 * In this project:
 *
 * Released = LOW
 * Pressed  = HIGH
 */
void button_init(void);
bool get_btn_state(void);

#endif

5. `gpio.c`

#include "gpio.h"

/*
 * ============================================================
 * Pin Definitions
 * ============================================================
 */

/*
 * LD1 is connected to PB0.
 *
 * BSRR lower 16 bits are used to SET GPIO pins.
 *
 * bit0 = PB0 set
 */
#define LED_SET      (1U << 0)

/*
 * BSRR upper 16 bits are used to RESET GPIO pins.
 *
 * PB0 reset:
 *
 * pin number 0 + 16 = bit16
 */
#define LED_RESET    (1U << 16)

/*
 * ODR bit0 corresponds to PB0.
 *
 * Used when toggling the LED.
 */
#define LED_PIN      (1U << 0)

/*
 * B1 user button is connected to PC13.
 *
 * IDR bit13 represents PC13 input state.
 */
#define BTN_PIN      (1U << 13)


/*
 * ============================================================
 * LED Initialization
 * ============================================================
 */

void led_init(void)
{
    /*
     * Step 1
     * Enable GPIOB peripheral clock.
     *
     * GPIO peripheral registers cannot be used properly
     * before its clock is enabled.
     *
     * RCC_AHB1ENR_GPIOBEN = GPIOB clock enable bit.
     */
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOBEN;

    /*
     * Step 2
     * Configure PB0 as General Purpose Output.
     *
     * GPIO MODER uses 2 bits for each pin.
     *
     * PB0:
     * MODER[1:0]
     *
     * 00 = Input
     * 01 = Output
     * 10 = Alternate Function
     * 11 = Analog
     */

    /*
     * Clear PB0 mode field first.
     *
     * 0x3 = binary 11
     *
     * ~(0x3 << 0)
     * clears MODER[1:0].
     */
    GPIOB->MODER &= ~(0x3UL << (0 * 2));

    /*
     * Set MODER[1:0] = 01.
     *
     * PB0 becomes General Purpose Output.
     */
    GPIOB->MODER |= (0x1UL << (0 * 2));
}


/*
 * ============================================================
 * LED ON
 * ============================================================
 */

void led_on(void)
{
    /*
     * Set PB0 using BSRR.
     *
     * BSRR lower 16 bits:
     * writing 1 sets the corresponding GPIO output.
     *
     * PB0 → bit0
     *
     * GPIOB->BSRR = 0x00000001
     *
     * Result:
     * PB0 = HIGH
     * LD1 = ON
     */
    GPIOB->BSRR = LED_SET;
}


/*
 * ============================================================
 * LED OFF
 * ============================================================
 */

void led_off(void)
{
    /*
     * Reset PB0 using BSRR.
     *
     * BSRR upper 16 bits:
     * writing 1 resets the corresponding GPIO output.
     *
     * PB0 reset → bit16
     *
     * GPIOB->BSRR = 0x00010000
     *
     * Result:
     * PB0 = LOW
     * LD1 = OFF
     */
    GPIOB->BSRR = LED_RESET;
}


/*
 * ============================================================
 * LED Toggle
 * ============================================================
 */

void led_toggle(void)
{
    /*
     * XOR the current ODR bit0.
     *
     * 0 XOR 1 = 1
     * 1 XOR 1 = 0
     *
     * Therefore:
     *
     * LOW  → HIGH
     * HIGH → LOW
     *
     * Note:
     * ODR toggle requires Read-Modify-Write.
     * BSRR is preferred when only explicit SET/RESET is needed.
     */
    GPIOB->ODR ^= LED_PIN;
}


/*
 * ============================================================
 * Button Initialization
 * ============================================================
 */

void button_init(void)
{
    /*
     * Step 1
     * Enable GPIOC peripheral clock.
     *
     * B1 is connected to PC13.
     */
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOCEN;

    /*
     * Step 2
     * Configure PC13 as input.
     *
     * Pin 13 uses:
     *
     * MODER[27:26]
     *
     * Input mode = 00
     *
     * Clear both bits.
     */
    GPIOC->MODER &= ~(0x3UL << (13 * 2));

    /*
     * Step 3
     * Disable internal pull-up / pull-down.
     *
     * PUPDR also uses 2 bits per pin.
     *
     * PC13:
     * PUPDR[27:26]
     *
     * 00 = No Pull
     */
    GPIOC->PUPDR &= ~(0x3UL << (13 * 2));
}


/*
 * ============================================================
 * Read Button State
 * ============================================================
 */

bool get_btn_state(void)
{
    /*
     * Read PC13 from GPIOC Input Data Register.
     *
     * BTN_PIN = (1U << 13)
     *
     * If bit13 is HIGH:
     *     GPIOC->IDR & BTN_PIN != 0
     *
     *     return true
     *
     * If bit13 is LOW:
     *     GPIOC->IDR & BTN_PIN == 0
     *
     *     return false
     *
     * NUCLEO-F439ZI B1:
     *
     * Released → LOW
     * Pressed  → HIGH
     */
    return ((GPIOC->IDR & BTN_PIN) != 0U);
}

6. `main.c`

#include <stdbool.h>
#include "gpio.h"

int main(void)
{
    /*
     * Variable used to store the current
     * PC13 button input state.
     */
    bool btn_state;

    /*
     * Initialize PB0 as output.
     *
     * PB0 → LD1
     */
    led_init();

    /*
     * Initialize PC13 as input.
     *
     * PC13 → B1 user button
     */
    button_init();

    while (1)
    {
        /*
         * Read current PC13 input level.
         *
         * false → button released
         * true  → button pressed
         */
        btn_state = get_btn_state();

        /*
         * B1 on NUCLEO-F439ZI is active-HIGH.
         *
         * Pressed:
         * PC13 = HIGH
         * → turn LED on
         *
         * Released:
         * PC13 = LOW
         * → turn LED off
         */
        if (btn_state)
        {
            led_on();
        }
        else
        {
            led_off();
        }
    }
}

7. Code Flow

전체 software flow:

main()
 │
 ├─ led_init()
 │    │
 │    ├─ GPIOB Clock ON
 │    └─ PB0 Output
 │
 ├─ button_init()
 │    │
 │    ├─ GPIOC Clock ON
 │    ├─ PC13 Input
 │    └─ No Pull
 │
 └─ while(1)
      │
      ├─ Read GPIOC->IDR[13]
      │
      ├─ Pressed?
      │
      ├─ YES → BSRR Set PB0
      │
      └─ NO  → BSRR Reset PB0

Hardware flow:

B1
 ↓
PC13
 ↓
GPIOC IDR[13]
 ↓
CPU
 ↓
if()
 ↓
GPIOB BSRR
 ↓
PB0
 ↓
LD1

8. Why Use BSRR?

다음 코드는:

GPIOB->ODR |= (1U << 0);

CPU 입장에서는 보통:

Read ODR
   ↓
Modify bit0
   ↓
Write ODR

가 된다.

이것을:

Read-Modify-Write

라고 한다.

반면:

GPIOB->BSRR = (1U << 0);

은 원하는 bit를 직접 set하는 write command다.

Write BSRR
   ↓
PB0 SET

따라서 다른 GPIO output bit에 영향을 주지 않고 특정 pin만 변경하기에 적합하다.

9. Polling

현재 main loop:

while (1)
{
    btn_state = get_btn_state();
}

는 CPU가 계속 button을 확인한다.

Read
 ↓
Read
 ↓
Read
 ↓
Read
 ↓
...

이를 Polling이라고 한다.

장점:

Simple
Easy to debug

단점:

CPU continuously checks the peripheral

Ch14에서는 같은 button을 EXTI Interrupt 방식으로 변경한다.

10. Button Bounce

Mechanical switch는 실제로 한 번 눌렀을 때 signal이 완벽하게 한 번만 바뀌지 않을 수 있다.

Ideal

LOW ──────────┐
              └──── HIGH

실제 switch에서는:

LOW ───┐ ┌─┐ ┌──── HIGH
       └─┘ └─┘

처럼 짧게 여러 번 변할 수 있다.

이를:

Switch Bounce

라고 한다.

이번 실습은 단순히:

현재 button level
→ LED level

을 연결하기 때문에 큰 문제가 없지만,

Button Counter
LED Toggle Once
Interrupt

같은 동작에서는 debounce 처리가 필요하다.

11. Board Porting

원서에서 사용한 board와 현재 board의 LED pin이 다르다.

NUCLEO-F411
LD2 → PA5

NUCLEO-F439ZI
LD1 → PB0

따라서 변경:

GPIOA
→ GPIOB

bit5
→ bit0

Button은 PC13을 사용하지만 polarity를 board 회로에 맞게 확인해야 한다.

이번 project에서는:

B1 Press
→ PC13 HIGH

이다.

12. Verification

Build

make clean
make

확인:

Build PASS

Board

Expected:

B1 Released
→ LD1 OFF

B1 Pressed
→ LD1 ON

Debugger

다음 register를 확인한다.

RCC->AHB1ENR
GPIOB->MODER
GPIOB->ODR
GPIOC->MODER
GPIOC->IDR

Button released:

GPIOC->IDR bit13 = 0

Button pressed:

GPIOC->IDR bit13 = 1

13. Recommended Debug Test

Breakpoint를:

btn_state = get_btn_state();

다음 줄에 걸고 B1을 눌러본다.

Watch:

btn_state
GPIOC->IDR
GPIOB->ODR

확인:

Released

btn_state = false
IDR[13]  = 0
ODR[0]   = 0

Pressed

btn_state = true
IDR[13]  = 1
ODR[0]   = 1

이렇게 나오면:

Physical Button
→ Input Register
→ Software
→ Output Register
→ Physical LED

전체 흐름을 확인한 것이다.

14. Screenshots to Add

images/ch07_build.png
images/ch07_button_released.png
images/ch07_button_pressed.png
images/ch07_idr_debug.png

가장 좋은 구성:

Button Released / LED OFF
+
Button Pressed / LED ON
+
Debugger IDR[13]

15. Key Points

PC13
 ↓
IDR
 ↓
Software
 ↓
BSRR
 ↓
PB0
 ↓
LED

GPIO를 사용하기 전에 RCC clock을 enable한다.
MODER는 GPIO pin 하나당 2bit를 사용한다.
PB0의 mode field는 MODER[1:0]이다.
PC13의 mode field는 MODER[27:26]이다.
Input 상태는 IDR에서 읽는다.
Output state는 ODR로 확인할 수 있다.
특정 output의 set/reset에는 BSRR을 사용할 수 있다.
NUCLEO-F439ZI의 B1은 이번 실습에서 active-HIGH로 사용한다.
현재 방식은 polling이며 이후 EXTI interrupt로 확장한다.

Chapter 07 - Hongjayeong-02/Basic_SoC_implementation GitHub Wiki

07. GPIO

Goal

1. GPIO 핵심

2. Board Mapping

3. GPIO Data Registers

IDR

ODR

BSRR

4. Practice Code

`gpio.h`

5. `gpio.c`

6. `main.c`

7. Code Flow

8. Why Use BSRR?

9. Polling

10. Button Bounce

11. Board Porting

12. Verification

Build

Board

Debugger

13. Recommended Debug Test

14. Screenshots to Add

15. Key Points

⚠️ GitHub.com Fallback ⚠️

Chapter 07 - Hongjayeong-02/Basic_SoC_implementation GitHub Wiki

07. GPIO

Goal

1. GPIO 핵심

2. Board Mapping

3. GPIO Data Registers

IDR

ODR

BSRR

4. Practice Code

gpio.h

5. gpio.c

6. main.c

7. Code Flow

8. Why Use BSRR?

9. Polling

10. Button Bounce

11. Board Porting

12. Verification

Build

Board

Debugger

13. Recommended Debug Test

14. Screenshots to Add

15. Key Points

⚠️ **GitHub.com Fallback** ⚠️

`gpio.h`

5. `gpio.c`

6. `main.c`

⚠️ GitHub.com Fallback ⚠️