集成电路总线 (I2C)

[English]

简介

I2C 是一种串行同步半双工通信协议，总线上可以同时挂载多个主机和从机。I2C 使用两条双向开漏线：串行数据线 (SDA) 和串行时钟线 (SCL)，通过电阻上拉。

ESP32 有 2 个 I2C 控制器（也被称为端口），负责处理 I2C 总线上的通信。

单个 I2C 控制器既可以是主机也可以是从机。

通常，I2C 从机设备具有 7 位地址或 10 位地址。ESP32 支持 I2C 标准模式 (Sm) 和快速模式 (Fm)，可分别达到 100 kHz 和 400 kHz。

警告

主机模式的 SCL 时钟频率不应大于 400 kHz。

备注

SCL 的频率受上拉电阻及导线电容的影响。因此，强烈建议选择适当的上拉电阻，确保频率准确。推荐的上拉电阻值通常在 1 kΩ 到 10 kΩ 之间。

请注意，SCL 的频率越高，上拉电阻应该越小（但不能小于 1 kΩ）。较大的电阻会降低电流，增加时钟切换时间并降低频率。通常推荐 2 kΩ 到 5 kΩ 左右的电阻，也可根据电流需求进行一定调整。

I2C 时钟配置

i2c_clock_source_t::I2C_CLK_SRC_DEFAULT：默认的 I2C 时钟源。
i2c_clock_source_t::I2C_CLK_SRC_APB：以 APB 时钟作为 I2C 时钟源。

I2C 文件结构

需要包含在 I2C 应用程序中的公共头文件

i2c.h：遗留 I2C API 的头文件（用于使用旧驱动程序的应用）。
i2c_master.h：提供标准通信模式下特定 API 的头文件（用于使用主机模式的新驱动程序的应用）。
i2c_slave.h：提供标准通信模式下特定 API 的头文件（用于使从机模式的新驱动程序的应用）。

备注

旧驱动程序与新驱动程序无法共存。包含 i2c.h 头文件可使用旧驱动程序，或包含 i2c_master.h 和 i2c_slave.h 来使用新驱动程序。请注意，现已弃用旧驱动程序，之后将移除。

上述头文件中包含的公共头文件

i2c_types_legacy.h：仅在旧驱动程序中使用的旧公共类型。
i2c_types.h：提供公共类型的头文件。

功能概述

I2C 驱动程序提供以下服务：

资源分配 - 包括如何使用正确的配置来分配 I2C 总线，以及如何在完成工作后回收资源。
I2C 主机控制器 - 包括 I2C 主机控制器的行为，介绍了数据发送、数据接收和数据的双向传输。
I2C 从机控制器 - 包括 I2C 从机控制器的行为，涉及数据发送和数据接收。
电源管理 - 描述了不同时钟源对功耗的影响。
IRAM 安全 - 描述了如何在 cache 被禁用时正常运行 I2C 中断。
线程安全 - 列出了驱动程序中线程安全的 API。
Kconfig 选项 - 列出了支持的 Kconfig 选项，这些选项可以对驱动程序产生不同影响。

资源分配

若系统支持 I2C 主机总线和 I2C 从机总线，则由驱动程序中的 i2c_bus_handle_t 来表示。资源池管理可用的端口，并在有请求时分配空闲端口。

安装 I2C 主机总线和设备

I2C 主机总线是基于总线-设备模型设计的，因此需要分别使用 i2c_master_bus_config_t 和 i2c_device_config_t 来分配 I2C 主机总线实例和 I2C 设备实例。

I2C 主机总线需要 i2c_master_bus_config_t 指定的配置：

i2c_master_bus_config_t::i2c_port 设置控制器使用的 I2C 端口。
i2c_master_bus_config_t::sda_io_num 设置串行数据总线 (SDA) 的 GPIO 编号。
i2c_master_bus_config_t::scl_io_num 设置串行时钟总线 (SCL) 的 GPIO 编号。
i2c_master_bus_config_t::clk_source 选择 I2C 总线的时钟源。可用时钟列表见 i2c_clock_source_t。有关不同时钟源对功耗的影响，请参阅电源管理部分。
i2c_master_bus_config_t::glitch_ignore_cnt 设置主机总线的毛刺周期。如果线上的毛刺周期小于设置的值（通常设为 7），则可以被滤除。
i2c_master_bus_config_t::intr_priority 设置中断的优先级。如果设置为 0，则驱动程序将使用低或中优先级的中断（优先级可设为 1、2 或 3 中的一个），若未设置，则将使用 i2c_master_bus_config_t::intr_priority 指示的优先级。请使用数字形式（1、2、3），不要用位掩码形式（(1<<1)、(1<<2)、(1<<3)）。
i2c_master_bus_config_t::trans_queue_depth 设置内部传输队列的深度，但仅在异步传输中有效。
i2c_master_bus_config_t::enable_internal_pullup 启用内部上拉电阻。注意：该设置无法在高速频率下拉高总线，此时建议使用合适的外部上拉电阻。
i2c_master_bus_config_t::allow_pd 配置驱动程序是否允许系统在睡眠模式下关闭外设电源。在进入睡眠之前，系统将备份 I2C 寄存器上下文，当系统退出睡眠模式时，这些上下文将被恢复。关闭外设可以节省更多功耗，但代价是消耗更多内存来保存寄存器上下文。你需要在功耗和内存消耗之间做权衡。此配置选项依赖于特定的硬件功能，如果在不支持的芯片上启用它，你将看到类似 not able to power down in light sleep 的错误消息。

如果在 i2c_master_bus_config_t 中指定了配置，则可调用 i2c_new_master_bus() 来分配和初始化 I2C 主机总线。如果函数运行正确，则将返回一个 I2C 总线句柄。若没有可用的 I2C 端口，此函数将返回 ESP_ERR_NOT_FOUND 错误。

I2C 主机设备需要 i2c_device_config_t 指定的配置：

i2c_device_config_t::dev_addr_length 配置从机设备的地址位长度，可从枚举 I2C_ADDR_BIT_LEN_7 或 I2C_ADDR_BIT_LEN_10 （如果支持）中进行选择。
i2c_device_config_t::device_address 设置 I2C 设备原始地址，请直接将设备地址解析到此成员。例如，若设备地址为 0x28，则将 0x28 解析到 i2c_device_config_t::device_address，不要带写入或读取位。
i2c_device_config_t::scl_speed_hz 设置此设备的 SCL 线频率。
i2c_device_config_t::scl_wait_us 设置 SCL 等待时间（以微秒为单位）。通常此值较大，因为从机延伸时间会很长（甚至可能延伸到 12 ms）。设置为 0 表示使用默认的寄存器值。

一旦填充好 i2c_device_config_t 结构体的必要参数，就可调用 i2c_master_bus_add_device() 来分配 I2C 设备实例，并将设备挂载到主机总线上。如果函数运行正确，则将返回一个 I2C 设备句柄。若未正确初始化 I2C 总线，此函数将返回 ESP_ERR_INVALID_ARG 错误。

#include "driver/i2c_master.h"

i2c_master_bus_config_t i2c_mst_config = {
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = TEST_I2C_PORT,
    .scl_io_num = I2C_MASTER_SCL_IO,
    .sda_io_num = I2C_MASTER_SDA_IO,
    .glitch_ignore_cnt = 7,
    .flags.enable_internal_pullup = true,
};

i2c_master_bus_handle_t bus_handle;
ESP_ERROR_CHECK(i2c_new_master_bus(&i2c_mst_config, &bus_handle));

i2c_device_config_t dev_cfg = {
    .dev_addr_length = I2C_ADDR_BIT_LEN_7,
    .device_address = 0x58,
    .scl_speed_hz = 100000,
};

i2c_master_dev_handle_t dev_handle;
ESP_ERROR_CHECK(i2c_master_bus_add_device(bus_handle, &dev_cfg, &dev_handle));

通过端口获取 I2C 主控句柄

当在某个模块（例如音频模块）中已经初始化了 I2C 主控句柄，但在另一个模块（例如视频模块）中不方便获取该句柄。使用辅助函数 i2c_master_get_bus_handle() 可通过端口获取已初始化的句柄。但请确保句柄已经提前初始化，否则可能会报错。

// 源文件 1
#include "driver/i2c_master.h"
i2c_master_bus_handle_t bus_handle;
i2c_master_bus_config_t i2c_mst_config = {
    ... // 与其他相同
};
ESP_ERROR_CHECK(i2c_new_master_bus(&i2c_mst_config, &bus_handle));

// 源文件 2
#include "driver/i2c_master.h"
i2c_master_bus_handle_t handle;
ESP_ERROR_CHECK(i2c_master_get_bus_handle(0, &handle));

卸载 I2C 主机总线和设备

如果不再需要之前安装的 I2C 总线或设备，建议调用 i2c_master_bus_rm_device() 或 i2c_del_master_bus() 来回收资源，以释放底层硬件。

安装 I2C 从机设备

I2C 从机设备需要 i2c_slave_config_t 指定的配置：

i2c_slave_config_t::i2c_port 设置控制器使用的 I2C 端口。
i2c_slave_config_t::sda_io_num 设置串行数据总线 (SDA) 的 GPIO 编号。
i2c_slave_config_t::scl_io_num 设置串行时钟总线 (SCL) 的 GPIO 编号。
i2c_slave_config_t::clk_source 选择 I2C 总线的时钟源。可用时钟列表见 i2c_clock_source_t。有关不同时钟源对功耗的影响，请参阅电源管理。
i2c_slave_config_t::send_buf_depth 设置发送 buffer 的长度。
i2c_slave_config_t::slave_addr 设置从机地址。
i2c_master_bus_config_t::intr_priority 设置中断的优先级。如果设置为 0 ，则驱动程序将使用低或中优先级的中断（优先级可设为 1、2 或 3 中的一个），若未设置，则将使用 i2c_master_bus_config_t::intr_priority 指示的优先级。请使用数字形式（1、2、3），不要用位掩码形式（(1<<1)、(1<<2)、(1<<3)）。请注意，中断优先级一旦设置完成，在调用 i2c_del_master_bus() 之前都无法更改。
i2c_slave_config_t::addr_bit_len。如果需要从机设备具有 10 位地址，则将该成员变量设为 I2C_ADDR_BIT_LEN_10。

一旦填充好 i2c_slave_config_t 结构体的必要参数，就可调用 i2c_new_slave_device() 来分配和初始化 I2C 主机总线。如果函数运行正确，则将返回一个 I2C 总线句柄。若没有可用的 I2C 端口，此函数将返回 ESP_ERR_NOT_FOUND 错误。

i2c_slave_config_t i2c_slv_config = {
    .addr_bit_len = I2C_ADDR_BIT_LEN_7,
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = TEST_I2C_PORT,
    .send_buf_depth = 256,
    .scl_io_num = I2C_SLAVE_SCL_IO,
    .sda_io_num = I2C_SLAVE_SDA_IO,
    .slave_addr = 0x58,
};

i2c_slave_dev_handle_t slave_handle;
ESP_ERROR_CHECK(i2c_new_slave_device(&i2c_slv_config, &slave_handle));

卸载 I2C 从机设备

如果不再需要之前安装的 I2C 总线，建议调用 i2c_del_slave_device() 来回收资源，以释放底层硬件。

I2C 主机控制器

通过调用 i2c_new_master_bus() 安装好 I2C 主机控制器驱动程序后，ESP32 就可以与其他 I2C 设备进行通信了。I2C API 允许标准事务，如下图所示：

I2C 主机写入

在成功安装 I2C 主机总线之后，可以通过调用 i2c_master_transmit() 来向从机设备写入数据。下图解释了该函数的原理。

简单来说，驱动程序用一系列命令填充了一个命令链，并将该命令链传递给 I2C 控制器执行。

将数据写入从机设备的简单示例：

#define DATA_LENGTH 100
i2c_master_bus_config_t i2c_mst_config = {
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = I2C_PORT_NUM_0,
    .scl_io_num = I2C_MASTER_SCL_IO,
    .sda_io_num = I2C_MASTER_SDA_IO,
    .glitch_ignore_cnt = 7,
};
i2c_master_bus_handle_t bus_handle;

ESP_ERROR_CHECK(i2c_new_master_bus(&i2c_mst_config, &bus_handle));

i2c_device_config_t dev_cfg = {
    .dev_addr_length = I2C_ADDR_BIT_LEN_7,
    .device_address = 0x58,
    .scl_speed_hz = 100000,
};

i2c_master_dev_handle_t dev_handle;
ESP_ERROR_CHECK(i2c_master_bus_add_device(bus_handle, &dev_cfg, &dev_handle));

ESP_ERROR_CHECK(i2c_master_transmit(dev_handle, data_wr, DATA_LENGTH, -1));

I2C 主机写入操作还支持在单次传输事务中传输多个 buffer。如下所示：

uint8_t control_phase_byte = 0;
size_t control_phase_size = 0;
if (/*condition*/) {
    control_phase_byte = 1;
    control_phase_size = 1;
}

uint8_t *cmd_buffer = NULL;
size_t cmd_buffer_size = 0;
if (/*condition*/) {
    uint8_t cmds[4] = {BYTESHIFT(lcd_cmd, 3), BYTESHIFT(lcd_cmd, 2), BYTESHIFT(lcd_cmd, 1), BYTESHIFT(lcd_cmd, 0)};
    cmd_buffer = cmds;
    cmd_buffer_size = 4;
}

uint8_t *lcd_buffer = NULL;
size_t lcd_buffer_size = 0;
if (buffer) {
    lcd_buffer = (uint8_t*)buffer;
    lcd_buffer_size = buffer_size;
}

i2c_master_transmit_multi_buffer_info_t lcd_i2c_buffer[3] = {
    {.write_buffer = &control_phase_byte, .buffer_size = control_phase_size},
    {.write_buffer = cmd_buffer, .buffer_size = cmd_buffer_size},
    {.write_buffer = lcd_buffer, .buffer_size = lcd_buffer_size},
};

i2c_master_multi_buffer_transmit(handle, lcd_i2c_buffer, sizeof(lcd_i2c_buffer) / sizeof(i2c_master_transmit_multi_buffer_info_t), -1);

I2C 主机读取

在成功安装 I2C 主机总线后，可以通过调用 i2c_master_receive() 从从机设备读取数据。下图解释了该函数的原理。

从从机设备读取数据的简单示例：

#define DATA_LENGTH 100
i2c_master_bus_config_t i2c_mst_config = {
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = I2C_PORT_NUM_0,
    .scl_io_num = I2C_MASTER_SCL_IO,
    .sda_io_num = I2C_MASTER_SDA_IO,
    .glitch_ignore_cnt = 7,
};
i2c_master_bus_handle_t bus_handle;

ESP_ERROR_CHECK(i2c_new_master_bus(&i2c_mst_config, &bus_handle));

i2c_device_config_t dev_cfg = {
    .dev_addr_length = I2C_ADDR_BIT_LEN_7,
    .device_address = 0x58,
    .scl_speed_hz = 100000,
};

i2c_master_dev_handle_t dev_handle;
ESP_ERROR_CHECK(i2c_master_bus_add_device(bus_handle, &dev_cfg, &dev_handle));

i2c_master_receive(dev_handle, data_rd, DATA_LENGTH, -1);

I2C 主机写入后读取

从一些 I2C 设备中读取数据之前需要进行写入配置，可通过 i2c_master_transmit_receive() 接口进行配置。下图解释了该函数的原理。

请注意，在写入操作和读取操作之间没有插入 STOP 条件位，因此该功能适用于从 I2C 设备读取寄存器。以下是向从机设备写入数据并从从机设备读取数据的简单示例：

i2c_device_config_t dev_cfg = {
    .dev_addr_length = I2C_ADDR_BIT_LEN_7,
    .device_address = 0x58,
    .scl_speed_hz = 100000,
};

i2c_master_dev_handle_t dev_handle;
ESP_ERROR_CHECK(i2c_master_bus_add_device(I2C_PORT_NUM_0, &dev_cfg, &dev_handle));
uint8_t buf[20] = {0x20};
uint8_t buffer[2];
ESP_ERROR_CHECK(i2c_master_transmit_receive(dev_handle, buf, sizeof(buf), buffer, 2, -1));

I2C 主机探测

I2C 驱动程序可以使用 i2c_master_probe() 来检测设备是否已经连接到 I2C 总线上。如果该函数返回 ESP_OK，则表示该设备已经被检测到。

重要

在调用该函数时，必须将上拉电阻连接到 SCL 和 SDA 管脚。如果在正确解析 xfer_timeout_ms 时收到 ESP_ERR_TIMEOUT，则应检查上拉电阻。若暂无合适的电阻，也可将 flags.enable_internal_pullup 设为 true。

探测 I2C 设备的简单示例：

i2c_master_bus_config_t i2c_mst_config_1 = {
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = TEST_I2C_PORT,
    .scl_io_num = I2C_MASTER_SCL_IO,
    .sda_io_num = I2C_MASTER_SDA_IO,
    .glitch_ignore_cnt = 7,
    .flags.enable_internal_pullup = true,
};
i2c_master_bus_handle_t bus_handle;

ESP_ERROR_CHECK(i2c_new_master_bus(&i2c_mst_config_1, &bus_handle));
ESP_ERROR_CHECK(i2c_master_probe(bus_handle, 0x22, -1));
ESP_ERROR_CHECK(i2c_del_master_bus(bus_handle));

I2C 从机控制器

通过调用 i2c_new_slave_device() 安装好 I2C 从机驱动程序后，ESP32 就可以作为从机与其他 I2C 主机进行通信了。

I2C 从机写入

I2C 从机的发送 buffer 可作为 FIFO 来存储要发送的数据。在主机请求这些数据前，它们会一直排队。可通过调用 i2c_slave_transmit() 来传输数据。

将数据写入 FIFO 的简单示例：

uint8_t *data_wr = (uint8_t *) malloc(DATA_LENGTH);

i2c_slave_config_t i2c_slv_config = {
    .addr_bit_len = I2C_ADDR_BIT_LEN_7,   // 7 位地址
    .clk_source = I2C_CLK_SRC_DEFAULT,    // 设置时钟源
    .i2c_port = 0,                        // 设置 I2C 端口编号
    .send_buf_depth = 256,                // 设置 TX buffer 长度
    .scl_io_num = 2,                      // SCL 管脚编号
    .sda_io_num = 1,                      // SDA 管脚编号
    .slave_addr = 0x58,                   // 从机地址
};

i2c_bus_handle_t i2c_bus_handle;
ESP_ERROR_CHECK(i2c_new_slave_device(&i2c_slv_config, &i2c_bus_handle));
for (int i = 0; i < DATA_LENGTH; i++) {
    data_wr[i] = i;
}

ESP_ERROR_CHECK(i2c_slave_transmit(i2c_bus_handle, data_wr, DATA_LENGTH, 10000));

I2C 从机读取

每当主机将数据写入从机，从机都会自动将数据存储在接收 buffer 中，从而使从机应用程序能自由调用 i2c_slave_receive()。i2c_slave_receive() 为非阻塞接口，因此要想知道接收是否完成，需注册回调函数 i2c_slave_register_event_callbacks()。

static IRAM_ATTR bool i2c_slave_rx_done_callback(i2c_slave_dev_handle_t channel, const i2c_slave_rx_done_event_data_t *edata, void *user_data)
{
    BaseType_t high_task_wakeup = pdFALSE;
    QueueHandle_t receive_queue = (QueueHandle_t)user_data;
    xQueueSendFromISR(receive_queue, edata, &high_task_wakeup);
    return high_task_wakeup == pdTRUE;
}

uint8_t *data_rd = (uint8_t *) malloc(DATA_LENGTH);
uint32_t size_rd = 0;

i2c_slave_config_t i2c_slv_config = {
    .addr_bit_len = I2C_ADDR_BIT_LEN_7,
    .clk_source = I2C_CLK_SRC_DEFAULT,
    .i2c_port = TEST_I2C_PORT,
    .send_buf_depth = 256,
    .scl_io_num = I2C_SLAVE_SCL_IO,
    .sda_io_num = I2C_SLAVE_SDA_IO,
    .slave_addr = 0x58,
};

i2c_slave_dev_handle_t slave_handle;
ESP_ERROR_CHECK(i2c_new_slave_device(&i2c_slv_config, &slave_handle));

s_receive_queue = xQueueCreate(1, sizeof(i2c_slave_rx_done_event_data_t));
i2c_slave_event_callbacks_t cbs = {
    .on_recv_done = i2c_slave_rx_done_callback,
};
ESP_ERROR_CHECK(i2c_slave_register_event_callbacks(slave_handle, &cbs, s_receive_queue));

i2c_slave_rx_done_event_data_t rx_data;
ESP_ERROR_CHECK(i2c_slave_receive(slave_handle, data_rd, DATA_LENGTH));
xQueueReceive(s_receive_queue, &rx_data, pdMS_TO_TICKS(10000));
// 接收完成。

注册事件回调函数

I2C 主机回调

当 I2C 主机总线触发中断时，将生成特定事件并通知 CPU。如果需要在发生这些事件时调用某些函数，可通过 i2c_master_register_event_callbacks() 将这些函数挂接到中断服务程序 (ISR) 上。由于注册的回调函数是在中断上下文中被调用的，所以应确保这些函数不会阻塞（例如，确保仅从函数内部调用带有 ISR 后缀的 FreeRTOS API）。回调函数需要返回一个布尔值，告诉 ISR 是否唤醒了高优先级任务。

I2C 主机事件回调函数列表见 i2c_master_event_callbacks_t。

虽然 I2C 是一种同步通信协议，但也支持通过注册上述回调函数来实现异步行为，此时 I2C API 将成为非阻塞接口。但请注意，在同一总线上只有一个设备可以采用异步操作。

重要

I2C 主机异步传输仍然是一个实验性功能（问题在于当异步传输量过大时，会导致内存异常。）

i2c_master_event_callbacks_t::on_recv_done 可设置用于主机“传输完成”事件的回调函数。该函数原型在 i2c_master_callback_t 中声明。

I2C 从机回调

当 I2C 从机总线触发中断时，将生成特定事件并通知 CPU。如果需要在发生这些事件时调用某些函数，可通过 i2c_slave_register_event_callbacks() 将这些函数挂接到中断服务程序 (ISR) 上。由于注册的回调函数是在中断上下文中被调用的，所以应确保这些函数不会导致延迟（例如，确保仅从函数中调用带有 ISR 后缀的 FreeRTOS API）。回调函数需要返回一个布尔值，告诉调用者是否唤醒了高优先级任务。

I2C 从机事件回调函数列表见 i2c_slave_event_callbacks_t。

i2c_slave_event_callbacks_t::on_recv_done 可设置用于“接收完成”事件的回调函数。该函数原型在 i2c_slave_received_callback_t 中声明。

电源管理

启用电源管理（即打开 CONFIG_PM_ENABLE），系统会在进入 Light-sleep 模式前调整或暂停 I2C FIFO 的时钟源，这可能会导致 I2C 信号改变，传输或接收到无效数据。

但驱动程序可以通过获取 ESP_PM_APB_FREQ_MAX 类型的电源管理锁来防止系统改变 APB 频率。每当用户创建一个以 I2C_CLK_SRC_APB 为时钟源的 I2C 总线，驱动程序将在开始 I2C 操作时获取电源管理锁，并在结束 I2C 操作时自动释放锁。

IRAM 安全

默认情况下，若 cache 因写入或擦除 flash 等原因而被禁用时，将推迟 I2C 中断。此时事件回调函数将无法按时执行，会影响实时应用的系统响应。

Kconfig 选项 CONFIG_I2C_ISR_IRAM_SAFE 能够做到以下几点：

即使 cache 被禁用，I2C 中断依旧正常运行。
将 ISR 使用的所有函数放入 IRAM 中。
将驱动程序对象放入 DRAM 中（以防它被意外映射到 PSRAM 中）。

启用以上选项，即使 cache 被禁用，I2C 中断依旧正常运行，但会增加 IRAM 的消耗。

线程安全

工厂函数 i2c_new_master_bus() 和 i2c_new_slave_device() 由驱动程序保证其线程安全，不需要额外的锁保护，也可从不同的 RTOS 任务中调用这些函数。应避免从多个任务中调用其他非线程安全的公共 I2C API，若确实需要调用，请添加额外的锁。

Kconfig 选项

CONFIG_I2C_ISR_IRAM_SAFE 将在 cache 被禁用时控制默认的 ISR 处理程序正常工作，详情请参阅 IRAM 安全。
CONFIG_I2C_ENABLE_DEBUG_LOG 可启用调试日志，但会增加固件二进制文件大小。

应用示例

peripherals/i2c/i2c_eeprom 演示了如何使用 I2C 主机模式从连接的 EEPROM 读取和写入数据。
peripherals/i2c/i2c_tools 演示了如何使用 I2C 工具开发 I2C 相关的应用程序，提供了用于配置 I2C 总线、扫描设备、读取、设置和检查寄存器的命令行工具。

API 参考

Header File

components/esp_driver_i2c/include/driver/i2c_master.h
This header file can be included with:
#include "driver/i2c_master.h"
This header file is a part of the API provided by the esp_driver_i2c component. To declare that your component depends on esp_driver_i2c, add the following to your CMakeLists.txt:
REQUIRES esp_driver_i2c
or
PRIV_REQUIRES esp_driver_i2c

Functions

esp_err_t i2c_new_master_bus(const i2c_master_bus_config_t *bus_config, i2c_master_bus_handle_t *ret_bus_handle)

Allocate an I2C master bus.

参数

bus_config -- [in] I2C master bus configuration.
ret_bus_handle -- [out] I2C bus handle

返回

ESP_OK: I2C master bus initialized successfully.
ESP_ERR_INVALID_ARG: I2C bus initialization failed because of invalid argument.
ESP_ERR_NO_MEM: Create I2C bus failed because of out of memory.
ESP_ERR_NOT_FOUND: No more free bus.

esp_err_t i2c_master_bus_add_device(i2c_master_bus_handle_t bus_handle, const i2c_device_config_t *dev_config, i2c_master_dev_handle_t *ret_handle)

Add I2C master BUS device.

参数

bus_handle -- [in] I2C bus handle.
dev_config -- [in] device config.
ret_handle -- [out] device handle.

返回

ESP_OK: Create I2C master device successfully.
ESP_ERR_INVALID_ARG: I2C bus initialization failed because of invalid argument.
ESP_ERR_NO_MEM: Create I2C bus failed because of out of memory.

esp_err_t i2c_del_master_bus(i2c_master_bus_handle_t bus_handle)

Deinitialize the I2C master bus and delete the handle.

参数

bus_handle -- [in] I2C bus handle.

返回

ESP_OK: Delete I2C bus success, otherwise, failed.
Otherwise: Some module delete failed.

esp_err_t i2c_master_bus_rm_device(i2c_master_dev_handle_t handle)

I2C master bus delete device.

参数

handle -- i2c device handle

返回

ESP_OK: If device is successfully deleted.

esp_err_t i2c_master_transmit(i2c_master_dev_handle_t i2c_dev, const uint8_t *write_buffer, size_t write_size, int xfer_timeout_ms)

Perform a write transaction on the I2C bus. The transaction will be undergoing until it finishes or it reaches the timeout provided.

备注

If a callback was registered with i2c_master_register_event_callbacks, the transaction will be asynchronous, and thus, this function will return directly, without blocking. You will get finish information from callback. Besides, data buffer should always be completely prepared when callback is registered, otherwise, the data will get corrupt.

参数

i2c_dev -- [in] I2C master device handle that created by i2c_master_bus_add_device.
write_buffer -- [in] Data bytes to send on the I2C bus.
write_size -- [in] Size, in bytes, of the write buffer.
xfer_timeout_ms -- [in] Wait timeout, in ms. Note: -1 means wait forever.

返回

ESP_OK: I2C master transmit success
ESP_ERR_INVALID_ARG: I2C master transmit parameter invalid.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the bus is busy or hardware crash.

esp_err_t i2c_master_multi_buffer_transmit(i2c_master_dev_handle_t i2c_dev, i2c_master_transmit_multi_buffer_info_t *buffer_info_array, size_t array_size, int xfer_timeout_ms)

Transmit multiple buffers of data over an I2C bus.

This function transmits multiple buffers of data over an I2C bus using the specified I2C master device handle. It takes in an array of buffer information structures along with the size of the array and a transfer timeout value in milliseconds.

参数

i2c_dev -- I2C master device handle that created by i2c_master_bus_add_device.
buffer_info_array -- Pointer to buffer information array.
array_size -- size of buffer information array.
xfer_timeout_ms -- Wait timeout, in ms. Note: -1 means wait forever.

返回

ESP_OK: I2C master transmit success
ESP_ERR_INVALID_ARG: I2C master transmit parameter invalid.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the bus is busy or hardware crash.

esp_err_t i2c_master_transmit_receive(i2c_master_dev_handle_t i2c_dev, const uint8_t *write_buffer, size_t write_size, uint8_t *read_buffer, size_t read_size, int xfer_timeout_ms)

Perform a write-read transaction on the I2C bus. The transaction will be undergoing until it finishes or it reaches the timeout provided.

备注

If a callback was registered with i2c_master_register_event_callbacks, the transaction will be asynchronous, and thus, this function will return directly, without blocking. You will get finish information from callback. Besides, data buffer should always be completely prepared when callback is registered, otherwise, the data will get corrupt.

参数

i2c_dev -- [in] I2C master device handle that created by i2c_master_bus_add_device.
write_buffer -- [in] Data bytes to send on the I2C bus.
write_size -- [in] Size, in bytes, of the write buffer.
read_buffer -- [out] Data bytes received from i2c bus.
read_size -- [in] Size, in bytes, of the read buffer.
xfer_timeout_ms -- [in] Wait timeout, in ms. Note: -1 means wait forever.

返回

ESP_OK: I2C master transmit-receive success
ESP_ERR_INVALID_ARG: I2C master transmit parameter invalid.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the bus is busy or hardware crash.

esp_err_t i2c_master_receive(i2c_master_dev_handle_t i2c_dev, uint8_t *read_buffer, size_t read_size, int xfer_timeout_ms)

Perform a read transaction on the I2C bus. The transaction will be undergoing until it finishes or it reaches the timeout provided.

备注

If a callback was registered with i2c_master_register_event_callbacks, the transaction will be asynchronous, and thus, this function will return directly, without blocking. You will get finish information from callback. Besides, data buffer should always be completely prepared when callback is registered, otherwise, the data will get corrupt.

参数

i2c_dev -- [in] I2C master device handle that created by i2c_master_bus_add_device.
read_buffer -- [out] Data bytes received from i2c bus.
read_size -- [in] Size, in bytes, of the read buffer.
xfer_timeout_ms -- [in] Wait timeout, in ms. Note: -1 means wait forever.

返回

ESP_OK: I2C master receive success
ESP_ERR_INVALID_ARG: I2C master receive parameter invalid.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the bus is busy or hardware crash.

esp_err_t i2c_master_probe(i2c_master_bus_handle_t bus_handle, uint16_t address, int xfer_timeout_ms)

Probe I2C address, if address is correct and ACK is received, this function will return ESP_OK.

Attention: Pull-ups must be connected to the SCL and SDA pins when this function is called. If you get ESP_ERR_TIMEOUT whilexfer_timeout_mswas parsed correctly, you should check the pull-up resistors. If you do not have proper resistors nearby. flags.enable_internal_pullup` is also acceptable.

备注

The principle of this function is to sent device address with a write command. If the device on your I2C bus, there would be an ACK signal and function returns ESP_OK. If the device is not on your I2C bus, there would be a NACK signal and function returns ESP_ERR_NOT_FOUND. ESP_ERR_TIMEOUT is not an expected failure, which indicated that the i2c probe not works properly, usually caused by pull-up resistors not be connected properly. Suggestion check data on SDA/SCL line to see whether there is ACK/NACK signal is on line when i2c probe function fails.

备注

There are lots of I2C devices all over the world, we assume that not all I2C device support the behavior like device_address+nack/ack. So, if the on line data is strange and no ack/nack got respond. Please check the device datasheet.

参数

bus_handle -- [in] I2C master device handle that created by i2c_master_bus_add_device.
address -- [in] I2C device address that you want to probe.
xfer_timeout_ms -- [in] Wait timeout, in ms. Note: -1 means wait forever (Not recommended in this function).

返回

ESP_OK: I2C device probe successfully
ESP_ERR_NOT_FOUND: I2C probe failed, doesn't find the device with specific address you gave.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the bus is busy or hardware crash.

esp_err_t i2c_master_register_event_callbacks(i2c_master_dev_handle_t i2c_dev, const i2c_master_event_callbacks_t *cbs, void *user_data)

Register I2C transaction callbacks for a master device.

备注

User can deregister a previously registered callback by calling this function and setting the callback member in the cbs structure to NULL.

备注

When CONFIG_I2C_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well. The user_data should also reside in SRAM.

备注

If the callback is used for helping asynchronous transaction. On the same bus, only one device can be used for performing asynchronous operation.

参数

i2c_dev -- [in] I2C master device handle that created by i2c_master_bus_add_device.
cbs -- [in] Group of callback functions
user_data -- [in] User data, which will be passed to callback functions directly

返回

ESP_OK: Set I2C transaction callbacks successfully
ESP_ERR_INVALID_ARG: Set I2C transaction callbacks failed because of invalid argument
ESP_FAIL: Set I2C transaction callbacks failed because of other error

esp_err_t i2c_master_bus_reset(i2c_master_bus_handle_t bus_handle)

Reset the I2C master bus.

参数

bus_handle -- I2C bus handle.

返回

ESP_OK: Reset succeed.
ESP_ERR_INVALID_ARG: I2C master bus handle is not initialized.
Otherwise: Reset failed.

esp_err_t i2c_master_bus_wait_all_done(i2c_master_bus_handle_t bus_handle, int timeout_ms)

Wait for all pending I2C transactions done.

参数

bus_handle -- [in] I2C bus handle
timeout_ms -- [in] Wait timeout, in ms. Specially, -1 means to wait forever.

返回

ESP_OK: Flush transactions successfully
ESP_ERR_INVALID_ARG: Flush transactions failed because of invalid argument
ESP_ERR_TIMEOUT: Flush transactions failed because of timeout
ESP_FAIL: Flush transactions failed because of other error

esp_err_t i2c_master_get_bus_handle(i2c_port_num_t port_num, i2c_master_bus_handle_t *ret_handle)

Retrieves the I2C master bus handle for a specified I2C port number.

This function retrieves the I2C master bus handle for the given I2C port number. Please make sure the handle has already been initialized, and this function would simply returns the existing handle. Note that the returned handle still can't be used concurrently

参数

port_num -- I2C port number for which the handle is to be retrieved.
ret_handle -- Pointer to a variable where the retrieved handle will be stored.

返回

ESP_OK: Success. The handle is retrieved successfully.
ESP_ERR_INVALID_ARG: Invalid argument, such as invalid port number
ESP_ERR_INVALID_STATE: Invalid state, such as the I2C port is not initialized.

Structures

struct i2c_master_bus_config_t

I2C master bus specific configurations.

Public Members

i2c_port_num_t i2c_port: I2C port number, -1 for auto selecting, (not include LP I2C instance)

gpio_num_t sda_io_num: GPIO number of I2C SDA signal, pulled-up internally

gpio_num_t scl_io_num: GPIO number of I2C SCL signal, pulled-up internally

i2c_clock_source_t clk_source: Clock source of I2C master bus

uint8_t glitch_ignore_cnt: If the glitch period on the line is less than this value, it can be filtered out, typically value is 7 (unit: I2C module clock cycle)

int intr_priority: I2C interrupt priority, if set to 0, driver will select the default priority (1,2,3).

size_t trans_queue_depth: Depth of internal transfer queue, increase this value can support more transfers pending in the background, only valid in asynchronous transaction. (Typically max_device_num * per_transaction)

uint32_t enable_internal_pullup: Enable internal pullups. Note: This is not strong enough to pullup buses under high-speed frequency. Recommend proper external pull-up if possible

uint32_t allow_pd: If set, the driver will backup/restore the I2C registers before/after entering/exist sleep mode. By this approach, the system can power off I2C's power domain. This can save power, but at the expense of more RAM being consumed

struct i2c_master_bus_config_t::[anonymous] flags: I2C master config flags

struct i2c_device_config_t

I2C device configuration.

Public Members

i2c_addr_bit_len_t dev_addr_length: Select the address length of the slave device.

uint16_t device_address: I2C device raw address. (The 7/10 bit address without read/write bit)

uint32_t scl_speed_hz: I2C SCL line frequency.

uint32_t scl_wait_us: Timeout value. (unit: us). Please note this value should not be so small that it can handle stretch/disturbance properly. If 0 is set, that means use the default reg value

uint32_t disable_ack_check: Disable ACK check. If this is set false, that means ack check is enabled, the transaction will be stopped and API returns error when nack is detected.

struct i2c_device_config_t::[anonymous] flags: I2C device config flags

struct i2c_master_transmit_multi_buffer_info_t

I2C master transmit buffer information structure.

Public Members

uint8_t *write_buffer: Pointer to buffer to be written.

size_t buffer_size: Size of data to be written.

struct i2c_master_event_callbacks_t

Group of I2C master callbacks, can be used to get status during transaction or doing other small things. But take care potential concurrency issues.

备注

The callbacks are all running under ISR context

备注

When CONFIG_I2C_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well.

Public Members

i2c_master_callback_t on_trans_done: I2C master transaction finish callback

Header File

components/esp_driver_i2c/include/driver/i2c_slave.h
This header file can be included with:
#include "driver/i2c_slave.h"
This header file is a part of the API provided by the esp_driver_i2c component. To declare that your component depends on esp_driver_i2c, add the following to your CMakeLists.txt:
REQUIRES esp_driver_i2c
or
PRIV_REQUIRES esp_driver_i2c

Functions

esp_err_t i2c_new_slave_device(const i2c_slave_config_t *slave_config, i2c_slave_dev_handle_t *ret_handle)

Initialize an I2C slave device.

参数

slave_config -- [in] I2C slave device configurations
ret_handle -- [out] Return a generic I2C device handle

返回

ESP_OK: I2C slave device initialized successfully
ESP_ERR_INVALID_ARG: I2C device initialization failed because of invalid argument.
ESP_ERR_NO_MEM: Create I2C device failed because of out of memory.

esp_err_t i2c_del_slave_device(i2c_slave_dev_handle_t i2c_slave)

Deinitialize the I2C slave device.

参数

i2c_slave -- [in] I2C slave device handle that created by i2c_new_slave_device.

返回

ESP_OK: Delete I2C device successfully.
ESP_ERR_INVALID_ARG: I2C device initialization failed because of invalid argument.

esp_err_t i2c_slave_receive(i2c_slave_dev_handle_t i2c_slave, uint8_t *data, size_t buffer_size)

Read bytes from I2C internal buffer. Start a job to receive I2C data.

备注

This function is non-blocking, it initiates a new receive job and then returns. User should check the received data from the on_recv_done callback that registered by i2c_slave_register_event_callbacks().

参数

i2c_slave -- [in] I2C slave device handle that created by i2c_new_slave_device.
data -- [out] Buffer to store data from I2C fifo. Should be valid until on_recv_done is triggered.
buffer_size -- [in] Buffer size of data that provided by users.

返回

ESP_OK: I2C slave receive success.
ESP_ERR_INVALID_ARG: I2C slave receive parameter invalid.
ESP_ERR_NOT_SUPPORTED: This function should be work in fifo mode, but I2C_SLAVE_NONFIFO mode is configured

esp_err_t i2c_slave_transmit(i2c_slave_dev_handle_t i2c_slave, const uint8_t *data, int size, int xfer_timeout_ms)

Write bytes to internal ringbuffer of the I2C slave data. When the TX fifo empty, the ISR will fill the hardware FIFO with the internal ringbuffer's data.

备注

If you connect this slave device to some master device, the data transaction direction is from slave device to master device.

参数

i2c_slave -- [in] I2C slave device handle that created by i2c_new_slave_device.
data -- [in] Buffer to write to slave fifo, can pickup by master. Can be freed after this function returns. Equal or larger than size.
size -- [in] In bytes, of data buffer.
xfer_timeout_ms -- [in] Wait timeout, in ms. Note: -1 means wait forever.

返回

ESP_OK: I2C slave transmit success.
ESP_ERR_INVALID_ARG: I2C slave transmit parameter invalid.
ESP_ERR_TIMEOUT: Operation timeout(larger than xfer_timeout_ms) because the device is busy or hardware crash.
ESP_ERR_NOT_SUPPORTED: This function should be work in fifo mode, but I2C_SLAVE_NONFIFO mode is configured

esp_err_t i2c_slave_register_event_callbacks(i2c_slave_dev_handle_t i2c_slave, const i2c_slave_event_callbacks_t *cbs, void *user_data)

Set I2C slave event callbacks for I2C slave channel.

备注

User can deregister a previously registered callback by calling this function and setting the callback member in the cbs structure to NULL.

备注

When CONFIG_I2C_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well. The user_data should also reside in SRAM.

参数

i2c_slave -- [in] I2C slave device handle that created by i2c_new_slave_device.
cbs -- [in] Group of callback functions
user_data -- [in] User data, which will be passed to callback functions directly

返回

ESP_OK: Set I2C transaction callbacks successfully
ESP_ERR_INVALID_ARG: Set I2C transaction callbacks failed because of invalid argument
ESP_FAIL: Set I2C transaction callbacks failed because of other error

Structures

struct i2c_slave_config_t

I2C slave specific configurations.

Public Members

i2c_port_num_t i2c_port: I2C port number, -1 for auto selecting

gpio_num_t sda_io_num: SDA IO number used by I2C bus

gpio_num_t scl_io_num: SCL IO number used by I2C bus

i2c_clock_source_t clk_source: Clock source of I2C bus.

uint32_t send_buf_depth: Depth of internal transfer ringbuffer, increase this value can support more transfers pending in the background

uint16_t slave_addr: I2C slave address

i2c_addr_bit_len_t addr_bit_len: I2C slave address in bit length

int intr_priority: I2C interrupt priority, if set to 0, driver will select the default priority (1,2,3).

uint32_t allow_pd: If set, the driver will backup/restore the I2C registers before/after entering/exist sleep mode. By this approach, the system can power off I2C's power domain. This can save power, but at the expense of more RAM being consumed

struct i2c_slave_config_t::[anonymous] flags: I2C slave config flags

struct i2c_slave_event_callbacks_t

Group of I2C slave callbacks (e.g. get i2c slave stretch cause). But take care of potential concurrency issues.

备注

The callbacks are all running under ISR context

备注

When CONFIG_I2C_ISR_IRAM_SAFE is enabled, the callback itself and functions called by it should be placed in IRAM. The variables used in the function should be in the SRAM as well.

Public Members

i2c_slave_received_callback_t on_recv_done: I2C slave receive done callback

Header File

components/esp_driver_i2c/include/driver/i2c_types.h
This header file can be included with:
#include "driver/i2c_types.h"
This header file is a part of the API provided by the esp_driver_i2c component. To declare that your component depends on esp_driver_i2c, add the following to your CMakeLists.txt:
REQUIRES esp_driver_i2c
or
PRIV_REQUIRES esp_driver_i2c

Structures

struct i2c_master_event_data_t

Data type used in I2C event callback.

Public Members

i2c_master_event_t event: The I2C hardware event that I2C callback is called.

struct i2c_slave_rx_done_event_data_t

Event structure used in I2C slave.

Public Members

uint8_t *buffer: Pointer for buffer received in callback.

Type Definitions

typedef int i2c_port_num_t: I2C port number.

typedef struct i2c_master_bus_t *i2c_master_bus_handle_t: Type of I2C master bus handle.

typedef struct i2c_master_dev_t *i2c_master_dev_handle_t: Type of I2C master bus device handle.

typedef struct i2c_slave_dev_t *i2c_slave_dev_handle_t: Type of I2C slave device handle.

typedef bool (*i2c_master_callback_t)(i2c_master_dev_handle_t i2c_dev, const i2c_master_event_data_t *evt_data, void *arg)

An callback for I2C transaction.

Param i2c_dev: [in] Handle for I2C device.
Param evt_data: [out] I2C capture event data, fed by driver
Param arg: [in] User data, set in i2c_master_register_event_callbacks()
Return: Whether a high priority task has been waken up by this function

typedef bool (*i2c_slave_received_callback_t)(i2c_slave_dev_handle_t i2c_slave, const i2c_slave_rx_done_event_data_t *evt_data, void *arg)

Callback signature for I2C slave.

Param i2c_slave: [in] Handle for I2C slave.
Param evt_data: [out] I2C capture event data, fed by driver
Param arg: [in] User data, set in i2c_slave_register_event_callbacks()
Return: Whether a high priority task has been waken up by this function

Enumerations

enum i2c_master_status_t

Enumeration for I2C fsm status.

Values:

enumerator I2C_STATUS_READ: read status for current master command

enumerator I2C_STATUS_WRITE: write status for current master command

enumerator I2C_STATUS_START: Start status for current master command

enumerator I2C_STATUS_STOP: stop status for current master command

enumerator I2C_STATUS_IDLE: idle status for current master command

enumerator I2C_STATUS_ACK_ERROR: ack error status for current master command

enumerator I2C_STATUS_DONE: I2C command done

enumerator I2C_STATUS_TIMEOUT: I2C bus status error, and operation timeout

enum i2c_master_event_t

Enumeration for I2C event.

Values:

enumerator I2C_EVENT_ALIVE: i2c bus in alive status.

enumerator I2C_EVENT_DONE: i2c bus transaction done

enumerator I2C_EVENT_NACK: i2c bus nack

enumerator I2C_EVENT_TIMEOUT: i2c bus timeout

Header File

components/hal/include/hal/i2c_types.h
This header file can be included with:
#include "hal/i2c_types.h"

Structures

struct i2c_hal_clk_config_t

Data structure for calculating I2C bus timing.

Public Members

uint16_t clkm_div: I2C core clock divider

uint16_t scl_low: I2C scl low period

uint16_t scl_high: I2C scl high period

uint16_t scl_wait_high: I2C scl wait_high period

uint16_t sda_hold: I2C scl low period

uint16_t sda_sample: I2C sda sample time

uint16_t setup: I2C start and stop condition setup period

uint16_t hold: I2C start and stop condition hold period

uint16_t tout: I2C bus timeout period

Type Definitions

typedef soc_periph_i2c_clk_src_t i2c_clock_source_t: I2C group clock source.

Enumerations

enum i2c_port_t

I2C port number, can be I2C_NUM_0 ~ (I2C_NUM_MAX-1).

Values:

enumerator I2C_NUM_0: I2C port 0

enumerator I2C_NUM_1: I2C port 1

enumerator I2C_NUM_MAX: I2C port max

enum i2c_addr_bit_len_t

Enumeration for I2C device address bit length.

Values:

enumerator I2C_ADDR_BIT_LEN_7: i2c address bit length 7

enum i2c_mode_t

Values:

enumerator I2C_MODE_SLAVE: I2C slave mode

enumerator I2C_MODE_MASTER: I2C master mode

enumerator I2C_MODE_MAX

enum i2c_rw_t

Values:

enumerator I2C_MASTER_WRITE: I2C write data

enumerator I2C_MASTER_READ: I2C read data

enum i2c_trans_mode_t

Values:

enumerator I2C_DATA_MODE_MSB_FIRST: I2C data msb first

enumerator I2C_DATA_MODE_LSB_FIRST: I2C data lsb first

enumerator I2C_DATA_MODE_MAX

enum i2c_addr_mode_t

Values:

enumerator I2C_ADDR_BIT_7: I2C 7bit address for slave mode

enumerator I2C_ADDR_BIT_10: I2C 10bit address for slave mode

enumerator I2C_ADDR_BIT_MAX

enum i2c_ack_type_t

Values:

enumerator I2C_MASTER_ACK: I2C ack for each byte read

enumerator I2C_MASTER_NACK: I2C nack for each byte read

enumerator I2C_MASTER_LAST_NACK: I2C nack for the last byte

enumerator I2C_MASTER_ACK_MAX

enum i2c_slave_stretch_cause_t

Enum for I2C slave stretch causes.

Values:

enumerator I2C_SLAVE_STRETCH_CAUSE_ADDRESS_MATCH: Stretching SCL low when the slave is read by the master and the address just matched

enumerator I2C_SLAVE_STRETCH_CAUSE_TX_EMPTY: Stretching SCL low when TX FIFO is empty in slave mode

enumerator I2C_SLAVE_STRETCH_CAUSE_RX_FULL: Stretching SCL low when RX FIFO is full in slave mode

enumerator I2C_SLAVE_STRETCH_CAUSE_SENDING_ACK: Stretching SCL low when slave sending ACK

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