STM32嵌入式学习 01：GPIO 输出与 LED 闪烁

1. 本课目标

理解 main() 中 STM32 程序的基本执行顺序。
理解"打开外设时钟 -> 配置 GPIO -> 应用配置 -> 主循环"的固定结构。
使用 PA0 输出高、低电平控制 LED。
区分推挽输出、开漏输出。
理解 GPIO_Pin_0 等宏的位掩码含义。
区分整体写入 GPIO_Write() 和局部修改 GPIO_SetBits() / GPIO_ResetBits()。

2. 知识主线

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时钟
RCC_APB2PeriphClockCmd
GPIOA 属于 APB2
外设默认需要开启时钟
配置
GPIO_InitTypeDef
GPIO_Mode_Out_PP
GPIO_Pin_0
GPIO_Speed_50MHz
GPIO_Init
输出
GPIO_SetBits
GPIO_ResetBits
GPIO_WriteBit
GPIO_Write
程序结构
main
1
状态 -> 延时 -> 状态
位操作
GPIO_Pin_0 = 0x0001
GPIO_Pin_1 = 0x0002
| 选择多个引脚
工程边界
STM32F103C8
Keil ARMCC 5.06
标准外设库
硬件现象需单独验证

3. 最小 LED 程序

c 复制代码

#include "stm32f10x.h"
#include "Delay.h"

int main(void)
{
    // GPIOA 在使用前必须打开 APB2 外设时钟
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

    // 用结构体集中保存 GPIO 配置
    GPIO_InitTypeDef GPIO_InitStructure;
    GPIO_InitStructure.GPIO_Mode = GPIO_Mode_Out_PP; // 推挽输出
    GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;         // PA0
    GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;

    // 将配置写入 GPIOA 的相关寄存器
    GPIO_Init(GPIOA, &GPIO_InitStructure);

    while (1)
    {
        GPIO_ResetBits(GPIOA, GPIO_Pin_0); // PA0 输出低电平
        Delay_ms(500);

        GPIO_SetBits(GPIOA, GPIO_Pin_0);   // PA0 输出高电平
        Delay_ms(500);
    }
}

4. 程序执行流程

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打开 GPIOA 时钟
填写 GPIO_InitTypeDef
GPIO_Init 应用配置
PA0 输出低电平
Delay_ms 500
PA0 输出高电平
Delay_ms 500

一次完整循环的理论等待时间约为：

text 复制代码

500 ms + 500 ms = 1000 ms

实际 LED 的亮灭相位取决于开发板 LED 的电路接法。低电平可能点亮 LED，也可能熄灭 LED，不能只根据 API 名称推断实际亮灭。

5. 四个关键步骤

5.1 打开时钟

c 复制代码

RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

这是对 RCC 时钟控制器的调用。GPIOA 挂在 APB2 总线上，使用前需要开启对应时钟。漏掉这句时，GPIO 配置通常不会按预期工作。

5.2 配置结构体

c 复制代码

GPIO_InitTypeDef GPIO_InitStructure;

它是一张配置表。结构体字段描述 GPIO 的模式、引脚和速度，GPIO_Init() 再根据这些字段配置底层寄存器。

5.3 推挽输出

c 复制代码

GPIO_Mode_Out_PP

推挽输出可以主动输出高电平，也可以主动输出低电平，适合 LED 和普通数字控制信号。

开漏输出 GPIO_Mode_Out_OD 只能主动拉低；释放时需要外部上拉电阻形成高电平，常见于 I2C 和共享信号线。不能因为"都是输出模式"就把两者当作等价配置。

5.4 主循环

c 复制代码

while (1)

单片机程序通常不会像桌面程序一样执行完后退出，而是在无限循环中持续执行控制逻辑。

6. 位掩码

源码中的宏本质上是不同的二进制位：

c 复制代码

#define GPIO_Pin_0 ((uint16_t)0x0001)
#define GPIO_Pin_1 ((uint16_t)0x0002)
#define GPIO_Pin_2 ((uint16_t)0x0004)

因此可以用按位或选择多个引脚：

c 复制代码

GPIO_Pin_0 | GPIO_Pin_1 | GPIO_Pin_2

这表示同时选择 PA0、PA1、PA2。引脚选择使用 |，不要把"位集合"误写成普通加法。

7. 整体写入与局部修改

标准库实现中：

c 复制代码

GPIO_Write(GPIOA, value);       // GPIOA->ODR = value，整体覆盖
GPIO_SetBits(GPIOA, pins);      // GPIOA->BSRR = pins，指定位置 1
GPIO_ResetBits(GPIOA, pins);    // GPIOA->BRR = pins，指定位置 0

选择原则

API	影响范围	适用场景
`GPIO_SetBits`	指定引脚置 1	单独打开 LED
`GPIO_ResetBits`	指定引脚置 0	单独关闭 LED
`GPIO_WriteBit`	一个指定引脚	根据条件设置单个引脚
`GPIO_Write`	整个 GPIO 端口	独占端口的流水灯或整体刷新

如果 GPIOA 的其他引脚连接了别的功能，直接 GPIO_Write() 可能误改它们；此时优先用 SetBits、ResetBits 或先明确维护掩码。

8. 延时函数与前提

Delay.c 使用 Cortex-M3 的 SysTick：

c 复制代码

SysTick->LOAD = 72 * xus;
SysTick->VAL = 0x00;
SysTick->CTRL = 0x00000005;
while (!(SysTick->CTRL & 0x00010000)) {}

其中 72 * xus 假设系统时钟为 72 MHz。若系统主频改变而延时计算没有同步修改，Delay_ms(500) 不一定仍然是 500 ms。这是本工程的时钟假设，不是所有 STM32 工程的通用常数。

该延时是阻塞式的：等待期间 CPU 主要在等待计数结束，暂时不能高效处理其他任务。复杂项目通常改用定时器中断、SysTick 节拍或 RTOS 调度。

9. 今日练习

练习 A：修改速度

把两处 Delay_ms(500) 改成 Delay_ms(100)，预测完整循环约为 200 ms。

练习 B：双引脚交替

c 复制代码

GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0 | GPIO_Pin_1;
GPIO_Init(GPIOA, &GPIO_InitStructure);

while (1)
{
    GPIO_SetBits(GPIOA, GPIO_Pin_0);
    GPIO_ResetBits(GPIOA, GPIO_Pin_1);
    Delay_ms(500);

    GPIO_ResetBits(GPIOA, GPIO_Pin_0);
    GPIO_SetBits(GPIOA, GPIO_Pin_1);
    Delay_ms(500);
}

练习 C：判断影响范围

回答：如果 PA1 当前为高电平，执行下面代码后 PA1 是否一定变低？

c 复制代码

GPIO_Write(GPIOA, GPIO_Pin_0);

答案：是。因为 GPIO_Write() 整体写入 ODR，未设置的位会变为 0。

10. 事实、假设与验证边界

已知事实：工程目标设备为 STM32F103C8，使用 Keil ARMCC 5.06 和 STM32 标准外设库；源码配置了 GPIOA 的 PA0 推挽输出。
合理推断：若 LED 电路连接到 PA0 且供电、下载和接线正确，PA0 电平会周期性变化。
尚未确认：本次没有收到编译输出、下载结果或实际 LED 现象，因此不能把硬件闪烁记为已验证事实。
重要约束：LED 高低电平有效逻辑取决于开发板原理图；延时准确性依赖系统时钟为 72 MHz 的假设。

11. 下一课预告

下一章进入 GPIO 输入：按键读取。

知识链将变为：

text 复制代码

读取输入 -> 判断按键状态 -> 修改 LED 输出

这会引入输入模式、上拉/下拉、电平有效逻辑和按键消抖。