本文主要介绍RT-thread中IIC总线设备驱动，涉及到的主要文件有：驱动框架文件（i2c_core.c，i2c_dev.c，i2c-bit-ops.c，i2c_dev.h，i2c.h）；底层硬件驱动文件（i2c_soft.c，i2c_soft.h）。这里的i2c_soft.c和i2c_soft.h是指利用MCU的GPIO口模拟IIC总线时序，而不是利用MCU的硬件IIC接口。应用IIC总线设备驱动时，需要在rtconfig.h中添加宏定义#define RT_USING_I2C。若使用GPIO口模拟IIC总线，则还需要添加宏定义#define RT_USING_I2C_BITOPS。
一、IIC总线设备驱动框架
先看i2c.h中定义的一些数据结构：
#define RT_I2C_WR 0x0000#define RT_I2C_RD (1u << 0)#define RT_I2C_ADDR_10BIT (1u << 2) /* this is a ten bit chip address */#define RT_I2C_NO_START (1u << 4)#define RT_I2C_IGNORE_NACK (1u << 5)#define RT_I2C_NO_READ_ACK (1u << 6) /* when I2C reading, we do not ACK */struct rt_i2c_msg{ rt_uint16_t addr; rt_uint16_t flags; rt_uint16_t len; rt_uint8_t *buf;};struct rt_i2c_bus_device;struct rt_i2c_bus_device_ops{ rt_size_t (*master_xfer)(struct rt_i2c_bus_device *bus, struct rt_i2c_msg msgs[], rt_uint32_t num); rt_size_t (*slave_xfer)(struct rt_i2c_bus_device *bus, struct rt_i2c_msg msgs[], rt_uint32_t num); rt_err_t (*i2c_bus_control)(struct rt_i2c_bus_device *bus, rt_uint32_t, rt_uint32_t);};/*for i2c bus driver*/struct rt_i2c_bus_device{ struct rt_device parent; const struct rt_i2c_bus_device_ops *ops; rt_uint16_t flags; rt_uint16_t addr; struct rt_mutex lock; rt_uint32_t timeout; rt_uint32_t retries; void *priv;}; i2c_dev.h中相关数据结构（struct rt_i2c_priv_data用于i2c_bus_device_control()函数中RT_I2C_DEV_CTRL_RW控制标志）：
#define RT_I2C_DEV_CTRL_10BIT 0x20#define RT_I2C_DEV_CTRL_ADDR 0x21#define RT_I2C_DEV_CTRL_TIMEOUT 0x22#define RT_I2C_DEV_CTRL_RW 0x23struct rt_i2c_priv_data{ struct rt_i2c_msg *msgs; rt_size_t number;}; i2c-bit-ops.h中主要定义了模拟IIC总线时序时需要的数据结构：
struct rt_i2c_bit_ops{ void *data; /* private data for lowlevel routines */ void (*set_sda)(void *data, rt_int32_t state); void (*set_scl)(void *data, rt_int32_t state); rt_int32_t (*get_sda)(void *data); rt_int32_t (*get_scl)(void *data); void (*udelay)(rt_uint32_t us); rt_uint32_t delay_us; /* scl and sda line delay */ rt_uint32_t timeout; /* in tick */}; 在i2c_dev.c主要实现IIC设备驱动统一接口函数：i2c_bus_device_read()，i2c_bus_device_write()，i2c_bus_device_control()以及rt_i2c_bus_device_device_init()。
rt_err_t rt_i2c_bus_device_device_init(struct rt_i2c_bus_device *bus, const char *name){ struct rt_device *device; RT_ASSERT(bus != RT_NULL); device = &bus->parent; device->user_data = bus; /* set device type */ device->type = RT_Device_Class_I2CBUS; /* initialize device interface */ device->init = RT_NULL; device->open = RT_NULL; device->close = RT_NULL; device->read = i2c_bus_device_read; device->write = i2c_bus_device_write; device->control = i2c_bus_device_control; /* register to device manager */ rt_device_register(device, name, RT_DEVICE_FLAG_RDWR); return RT_EOK;} i2c_core.c中实现IIC总线设备注册，以及使用IIC总线进行数据传输，如：rt_i2c_transfer()，rt_i2c_master_send()，rt_i2c_master_recv()。
rt_err_t rt_i2c_bus_device_register(struct rt_i2c_bus_device *bus, const char *bus_name){ rt_err_t res = RT_EOK; rt_mutex_init(&bus->lock, "i2c_bus_lock", RT_IPC_FLAG_FIFO); if (bus->timeout == 0) bus->timeout = RT_TICK_PER_SECOND; res = rt_i2c_bus_device_device_init(bus, bus_name); i2c_dbg("I2C bus [%s] registered\n", bus_name); return res;} i2c-bit-ops.c中主要实现了利用GPIO模拟IIC总线时序的相关接口函数，如：i2c_start()，i2c_restart()，i2c_stop()，i2c_waitack()，i2c_writeb()，i2c_readb()，i2c_send_bytes()，i2c_send_ack_or_nack()，i2c_recv_bytes()，i2c_send_address()，i2c_bit_send_address()等。并且实现了i2c_bit_xfer()：
static const struct rt_i2c_bus_device_ops i2c_bit_bus_ops ={ i2c_bit_xfer, RT_NULL, RT_NULL}; rt_err_t rt_i2c_bit_add_bus(struct rt_i2c_bus_device *bus, const char *bus_name){ bus->ops = &i2c_bit_bus_ops; return rt_i2c_bus_device_register(bus, bus_name);} 二、底层硬件驱动
本文采用的是模拟IIC，即用GPIO口模拟IIC时序。在i2c_soft.c中主要实现struct rt_i2c_bit_ops中的指针函数：
void stm32_set_sda(void *data, rt_int32_t state){ if(state == 1) GPIO_SetBits(I2C1_GPIO , I2C1_GPIO_SDA); //GPIOB->BSRRL = I2C1_GPIO_SDA else if(state == 0) GPIO_ResetBits(I2C1_GPIO , I2C1_GPIO_SDA); //GPIOB->BSRRH = I2C1_GPIO_SDA}void stm32_set_scl(void *data, rt_int32_t state){ if(state == 1) GPIO_SetBits(I2C1_GPIO , I2C1_GPIO_SCL); //GPIOB->BSRRL = I2C1_GPIO_SCL else if(state == 0) GPIO_ResetBits(I2C1_GPIO , I2C1_GPIO_SCL); //GPIOB->BSRRH = I2C1_GPIO_SCL}rt_int32_t stm32_get_sda(void *data){ return (rt_int32_t)GPIO_ReadInputDataBit(I2C1_GPIO , I2C1_GPIO_SDA);//return(GPIOB->IDR & I2C1_GPIO_SDA)}rt_int32_t stm32_get_scl(void *data){ return (rt_int32_t)GPIO_ReadInputDataBit(I2C1_GPIO , I2C1_GPIO_SCL);//return(GPIOB->IDR & I2C1_GPIO_SCL)}void stm32_udelay(rt_uint32_t us){ rt_uint32_t delta; /* ¼ÆËãusÑÓʱËùÐè¼ÆÊýÖµ£»sysTick->LOAD=21000, RT_TICK_PER_SECOND=1000 */ us = us * (SysTick->LOAD/(1000000/RT_TICK_PER_SECOND)); /* »ñÈ¡µ±Ç°àÖ઼ÆÊýÖµ */ delta = SysTick->VAL; /* ÑÓʱus */ while (delta - SysTick->VAL< us);}void stm32_mdelay(rt_uint32_t ms){ stm32_udelay(ms * 1000);}static const struct rt_i2c_bit_ops stm32_i2c_bit_ops ={ (void*)0xaa, //no use in set_sda,set_scl,get_sda,get_scl stm32_set_sda, stm32_set_scl, stm32_get_sda, stm32_get_scl, stm32_udelay, 20, 5}; 最后，实现IIC总线硬件初始化（包括RCC时钟配置和GPIO配置，最重要的是将stm32_i2c_bit_ops初始化为IIC总线设备结构体的priv变量，即stm32_i2c.priv = (void *)&stm32_i2c_bit_ops）：
int rt_hw_i2c_init(void){ static struct rt_i2c_bus_device stm32_i2c;//"static" add by me. It must be add "static", or it will be hard fault RCC_Configuration(); GPIO_Configuration(); rt_memset((void *)&stm32_i2c, 0, sizeof(struct rt_i2c_bus_device)); stm32_i2c.priv = (void *)&stm32_i2c_bit_ops; rt_i2c_bit_add_bus(&stm32_i2c, "i2c1"); return 0;}INIT_BOARD_EXPORT(rt_hw_i2c_init);//rt_hw_i2c_init will be called in rt_components_board_init() 三、IIC总线设备初始化
这里以cs43l22数字音频放大器为例：
static rt_err_t cs43l22_init(const char * i2c_bus_name){ i2c_bus = (struct rt_i2c_bus_device *)rt_device_find(i2c_bus_name); if(i2c_bus == RT_NULL) { rt_kprintf("\ni2c_bus %s for cs43l22 not found!\n", i2c_bus_name); return -RT_ENOSYS; } /* oflag has no meaning for spi device , so set to RT_NULL */ if(rt_device_open(&i2c_bus->parent, RT_NULL) != RT_EOK) { rt_kprintf("\ni2c_bus %s for cs43l22 opened failed!\n", i2c_bus_name); return -RT_EEMPTY; } EVAL_AUDIO_Init(OUTPUT_DEVICE_AUTO, volume, I2S_AudioFreq_48k); /* it must be at the back of EVAL_AUDIO_Init, which reset the cs43l22 */ uint8_t chip_id = Codec_ReadRegister(i2c_bus, 0x01); rt_kprintf("(chip_id of cs43l22 is 0x%02x)", chip_id); return 0;}int rt_cs43l22_init(void){ rt_sem_init(&sem_cs43l22, "cs43l22", 1, RT_IPC_FLAG_FIFO); cs43l22_init("i2c1"); return 0;}INIT_APP_EXPORT(rt_cs43l22_init); 注意事项：
1、在应用IIC总线设备驱动时，需要用到rt_device_read或rt_device_write，因此在初始化函数中需要调用rt_device_open将IIC总线设备打开。
2、下面利用rt_device_read和rt_device_write操作寄存器（每一次调用rt_device_read或rt_device_write都包括了i2c_start，i2c_bit_send_address，i2c_recv_bytes/i2c_send_bytes，i2c_stop这4个步骤）：
static uint32_t Codec_WriteRegister(struct rt_i2c_bus_device * i2c_bus, uint8_t RegisterAddr, uint8_t RegisterValue){ uint32_t result = 0; rt_uint16_t flags = 0x00; rt_uint16_t DevAddr = (rt_uint16_t)CODEC_ADDRESS >> 1; rt_off_t pos = (rt_off_t)((flags << 16) | DevAddr); rt_uint8_t buffer[2]; buffer[0] = RegisterAddr; buffer[1] = RegisterValue; rt_device_write(&i2c_bus->parent, pos, buffer, sizeof(buffer)); #ifdef VERIFY_WRITTENDATA /* Verify that the data has been correctly written */ result = (Codec_ReadRegister(i2c_bus, RegisterAddr) == RegisterValue)? 0:1; if(result == 0) rt_kprintf("\nthe reg 0x%02x verify passed\n",RegisterAddr); else rt_kprintf("\nthe reg 0x%02x verify failed\n",RegisterAddr);#endif /* VERIFY_WRITTENDATA */ /* Return the verifying value: 0 (Passed) or 1 (Failed) */ return result; } static uint8_t Codec_ReadRegister(struct rt_i2c_bus_device * i2c_bus, uint8_t RegisterAddr){ rt_uint16_t flags = 0x00; rt_uint16_t DevAddr = (rt_uint16_t)CODEC_ADDRESS >> 1; rt_off_t pos = (rt_off_t)((flags << 16) | DevAddr); rt_uint8_t buffer; buffer = RegisterAddr; rt_device_write(&i2c_bus->parent, pos, &buffer, 1); rt_device_read(&i2c_bus->parent, pos, &buffer, 1); /* Return the byte read from Codec */ return buffer;} 在上面两个函数中，有符号整型32位pos的高16位表示flags，低16位表示IIC器件地址。flags取值如i2c.h文件中宏定义所示。
这里说明一个问题：在rt_i2c_master_send和rt_i2c_master_recv函数中均有“msg.flags = flags & RT_I2C_ADDR_10BIT;”这一语句，该句用于标志IIC器件地址是否为10位地址，但是这条语句会将其他预置好的标志全部清除，如RT_I2C_NO_START，RT_I2C_IGNORE_NACK或RT_I2C_NO_READ_ACK。所以，在一般情况下，flags标志只能事先预置RT_I2C_ADDR_10BIT，若IIC器件地址为7位，则直接设置flags为0。
3、根据i2c_bit_send_address()函数中：
else { /* 7-bit addr */ addr1 = msg->addr << 1; if (flags & RT_I2C_RD) addr1 |= 1; ret = i2c_send_address(bus, addr1, retries); if ((ret != 1) && !ignore_nack) return -RT_EIO; } 可得，若IIC器件地址为7位，则pos低16位所表示的地址值DevAddr不包括读写标志位（最低位）。而cs43l22数据手册中的8位地址值包含了读写标志位，因此设置DevAddr为CODEC_ADDRESS >> 1。