I2C总线初探

I2C（Inter-Integrated Circuit，内置集成电路）最初为飞利浦所开发，为2线接口。这2根线为时钟线和双向数据线，分别被称为SCL（Serial CLock，串行时钟）和SDA（Serial DAta，串行数据）。由于I2C总线仅需要一对总线，因此在电路板上占用的空间更少，带来的问题是带宽较窄。即使I2C支持双向数据交换，由于仅有一根数据线，故通信是半双工的。

由于I2C被大量的微控制器所支持，在当前的市场上可找到大量便宜的I2C设备。

I2C主机：master，一般来说是主控制器，也可以理解为CPU

I2C从机：slave，一般来说是外设

I2C 总线上有3 种类型的信号，分别是：开始信号，结束信号和应答信号。这些信号都是由SDA和SCL上的电平变化来表示的。

开始信号（S）：当SCL为高电平时，SDA由高电平向低电平跳变，表示开始传输数据。

结束信号（P）：当SCL为高电平时，SDA由低电平向高电平跳变，表示结束传输数据。

相应信号（ACK）：从机接收到8位数据后，在第9个时钟周期，拉低SDA电平，表示已经接收到数据。

当总线空闲时，SDA 和SCL 都处于高电平，主机检测到总线空闲就可以向从机发送数据。主机首先发送开始信号S，接着发出8位数据（包括前7位的从机地址和1 为的方向位），然后等待从机发回确认信号ACK.

当第8位为0时，表示向从机传输数据，主机收到确认信号后就可以连续的向从机写入8 位数据；当第8 位为1时，表示向从读取数据，这时主机就可以接收来自从机的一系列数据。最后当总个数据传输过程完成后，由主机发送结束信号P，表示本次的数据传输完成。

说到I2C总线，就不得不提一下它的一个好基友——SMBus。

SMBus (System Management Bus,系统管理总线) 是1995年由Intel提出的。它大部分基于I2C总线规范，和I2C一样，SMBus不需要增加额外的引脚。

当然I2C和SMBus还是有很多差异的，比如以运作频率，I2C此方面相当宽裕，最低频可至0Hz（直流状态，等于时间暂停），高可至100kHz（Standard Mode）、400kHz（Fast Mode）、乃至3.4MHz（High Speed Mode），相对的SMBus就很局限，最慢不慢于10kHz，最快不快于100kHz。很明显的，I2C与SMBus的交集运作频率即是10kHz〜100kHz间。其它还有不少差异，这里不做细论。

I2C设备挂载到I2C总线下，是需要一个确定的地址的。一般来说I2C都使用7位地址，协议也支持10位地址，但大多数的设备仅响应7位地址，因此在总线上最多有127个设备，0号"设备"作为群呼地址。

I2C设备，第一个字节(为slave address)由7位地址和一位R/W读写位组成的，这字节是个器件地址。
首先，你要知道：常用IIC接口通用器件的器件地址是由种类型号，及寻址码组成的，共7位。
如格式如下：
D7 D6 D5 D4 D3 D2 D1 D0
器件类型由：D7-D4 共4位决定的。这是由半导公司生产时就已固定此类型的了，也就是说这4位已是固定的。
用户自定义地址码：D3-D1共3位。这是由用户自己设置的，通常的作法如EEPROM这些器件是由外部IC的3个引脚所组合电平决定的（用常用的名字如A0,A1,A2）。这也就是寻址码。
所以为什么同一IIC总线上同一型号的IC只能最多共挂8片同种类芯片的原因了。
最低一位：就是R/W位。

 

就I2C的整体结构而言，I2C可以分为核心层、总线层、设备层。

简单来说：

核心层提供统一的接口。

总线层是用来驱动I2C总线的。

设备层这是用来驱动I2C设备的。

 

一般来说的I2C设备只要关注设备层的内容即可。

在kernel-4.9/include/linux/i2c.h中定义了几个关键的结构。

表示设备的i2c_clinet：

/**
 * struct i2c_client - represent an I2C slave device
 * @flags: I2C_CLIENT_TEN indicates the device uses a ten bit chip address;
 *	I2C_CLIENT_PEC indicates it uses SMBus Packet Error Checking
 * @addr: Address used on the I2C bus connected to the parent adapter.
 * @name: Indicates the type of the device, usually a chip name that's
 *	generic enough to hide second-sourcing and compatible revisions.
 * @adapter: manages the bus segment hosting this I2C device
 * @dev: Driver model device node for the slave.
 * @irq: indicates the IRQ generated by this device (if any)
 * @detected: member of an i2c_driver.clients list or i2c-core's
 *	userspace_devices list
 * @slave_cb: Callback when I2C slave mode of an adapter is used. The adapter
 *	calls it to pass on slave events to the slave driver.
 *
 * An i2c_client identifies a single device (i.e. chip) connected to an
 * i2c bus. The behaviour exposed to Linux is defined by the driver
 * managing the device.
 */
struct i2c_client {
	unsigned short flags;		/* div., see below		*/
	unsigned short addr;		/* chip address - NOTE: 7bit	*/
					/* addresses are stored in the	*/
					/* _LOWER_ 7 bits		*/
	char name[I2C_NAME_SIZE];
	struct i2c_adapter *adapter;	/* the adapter we sit on	*/
	struct device dev;		/* the device structure		*/
	int irq;			/* irq issued by device		*/
	struct list_head detected;
#if IS_ENABLED(CONFIG_I2C_SLAVE)
	i2c_slave_cb_t slave_cb;	/* callback for slave mode	*/
#endif
};

代表驱动的i2c_driver：

/**
 * struct i2c_driver - represent an I2C device driver
 * @class: What kind of i2c device we instantiate (for detect)
 * @attach_adapter: Callback for bus addition (deprecated)
 * @probe: Callback for device binding
 * @remove: Callback for device unbinding
 * @shutdown: Callback for device shutdown
 * @alert: Alert callback, for example for the SMBus alert protocol
 * @command: Callback for bus-wide signaling (optional)
 * @driver: Device driver model driver
 * @id_table: List of I2C devices supported by this driver
 * @detect: Callback for device detection
 * @address_list: The I2C addresses to probe (for detect)
 * @clients: List of detected clients we created (for i2c-core use only)
 *
 * The driver.owner field should be set to the module owner of this driver.
 * The driver.name field should be set to the name of this driver.
 *
 * For automatic device detection, both @detect and @address_list must
 * be defined. @class should also be set, otherwise only devices forced
 * with module parameters will be created. The detect function must
 * fill at least the name field of the i2c_board_info structure it is
 * handed upon successful detection, and possibly also the flags field.
 *
 * If @detect is missing, the driver will still work fine for enumerated
 * devices. Detected devices simply won't be supported. This is expected
 * for the many I2C/SMBus devices which can't be detected reliably, and
 * the ones which can always be enumerated in practice.
 *
 * The i2c_client structure which is handed to the @detect callback is
 * not a real i2c_client. It is initialized just enough so that you can
 * call i2c_smbus_read_byte_data and friends on it. Don't do anything
 * else with it. In particular, calling dev_dbg and friends on it is
 * not allowed.
 */
struct i2c_driver {
	unsigned int class;

	/* Notifies the driver that a new bus has appeared. You should avoid
	 * using this, it will be removed in a near future.
	 */
	int (*attach_adapter)(struct i2c_adapter *) __deprecated;

	/* Standard driver model interfaces */
	int (*probe)(struct i2c_client *, const struct i2c_device_id *);
	int (*remove)(struct i2c_client *);

	/* driver model interfaces that don't relate to enumeration  */
	void (*shutdown)(struct i2c_client *);

	/* Alert callback, for example for the SMBus alert protocol.
	 * The format and meaning of the data value depends on the protocol.
	 * For the SMBus alert protocol, there is a single bit of data passed
	 * as the alert response's low bit ("event flag").
	 * For the SMBus Host Notify protocol, the data corresponds to the
	 * 16-bit payload data reported by the slave device acting as master.
	 */
	void (*alert)(struct i2c_client *, enum i2c_alert_protocol protocol,
		      unsigned int data);

	/* a ioctl like command that can be used to perform specific functions
	 * with the device.
	 */
	int (*command)(struct i2c_client *client, unsigned int cmd, void *arg);

	struct device_driver driver;
	const struct i2c_device_id *id_table;

	/* Device detection callback for automatic device creation */
	int (*detect)(struct i2c_client *, struct i2c_board_info *);
	const unsigned short *address_list;
	struct list_head clients;
};

总线的基本工作方式是将设备和驱动都注册到总线上，然后总线负责它们的匹配工作。

这里已经有了设备和驱动，下面就看一下总线了，它的结构体是i2c_adapter，其中成员algo是传输数据的算法：

/*
 * i2c_adapter is the structure used to identify a physical i2c bus along
 * with the access algorithms necessary to access it.
 */
struct i2c_adapter {
	struct module *owner;
	unsigned int class;		  /* classes to allow probing for */
	const struct i2c_algorithm *algo; /* the algorithm to access the bus */
	void *algo_data;

	/* data fields that are valid for all devices	*/
	const struct i2c_lock_operations *lock_ops;
	struct rt_mutex bus_lock;
	struct rt_mutex mux_lock;

	int timeout;			/* in jiffies */
	int retries;
	struct device dev;		/* the adapter device */

	int nr;
	char name[48];
	struct completion dev_released;

	struct mutex userspace_clients_lock;
	struct list_head userspace_clients;

	struct i2c_bus_recovery_info *bus_recovery_info;
	const struct i2c_adapter_quirks *quirks;
};


/**
 * struct i2c_algorithm - represent I2C transfer method
 * @master_xfer: Issue a set of i2c transactions to the given I2C adapter
 *   defined by the msgs array, with num messages available to transfer via
 *   the adapter specified by adap.
 * @smbus_xfer: Issue smbus transactions to the given I2C adapter. If this
 *   is not present, then the bus layer will try and convert the SMBus calls
 *   into I2C transfers instead.
 * @functionality: Return the flags that this algorithm/adapter pair supports
 *   from the I2C_FUNC_* flags.
 * @reg_slave: Register given client to I2C slave mode of this adapter
 * @unreg_slave: Unregister given client from I2C slave mode of this adapter
 *
 * The following structs are for those who like to implement new bus drivers:
 * i2c_algorithm is the interface to a class of hardware solutions which can
 * be addressed using the same bus algorithms - i.e. bit-banging or the PCF8584
 * to name two of the most common.
 *
 * The return codes from the @master_xfer field should indicate the type of
 * error code that occurred during the transfer, as documented in the kernel
 * Documentation file Documentation/i2c/fault-codes.
 */
struct i2c_algorithm {
	/* If an adapter algorithm can't do I2C-level access, set master_xfer
	   to NULL. If an adapter algorithm can do SMBus access, set
	   smbus_xfer. If set to NULL, the SMBus protocol is simulated
	   using common I2C messages */
	/* master_xfer should return the number of messages successfully
	   processed, or a negative value on error */
	int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,
			   int num);
	int (*smbus_xfer) (struct i2c_adapter *adap, u16 addr,
			   unsigned short flags, char read_write,
			   u8 command, int size, union i2c_smbus_data *data);

	/* To determine what the adapter supports */
	u32 (*functionality) (struct i2c_adapter *);

#if IS_ENABLED(CONFIG_I2C_SLAVE)
	int (*reg_slave)(struct i2c_client *client);
	int (*unreg_slave)(struct i2c_client *client);
#endif
};

添加驱动到总线i2c_add_driver：

/* use a define to avoid include chaining to get THIS_MODULE */
#define i2c_add_driver(driver) \
	i2c_register_driver(THIS_MODULE, driver)

添加设备到总线i2c_register_board_info，这里有个宏I2C_BOARD_INFO可以帮忙快速赋值：

/**
 * i2c_register_board_info - statically declare I2C devices
 * @busnum: identifies the bus to which these devices belong
 * @info: vector of i2c device descriptors
 * @len: how many descriptors in the vector; may be zero to reserve
 *	the specified bus number.
 *
 * Systems using the Linux I2C driver stack can declare tables of board info
 * while they initialize.  This should be done in board-specific init code
 * near arch_initcall() time, or equivalent, before any I2C adapter driver is
 * registered.  For example, mainboard init code could define several devices,
 * as could the init code for each daughtercard in a board stack.
 *
 * The I2C devices will be created later, after the adapter for the relevant
 * bus has been registered.  After that moment, standard driver model tools
 * are used to bind "new style" I2C drivers to the devices.  The bus number
 * for any device declared using this routine is not available for dynamic
 * allocation.
 *
 * The board info passed can safely be __initdata, but be careful of embedded
 * pointers (for platform_data, functions, etc) since that won't be copied.
 */
int i2c_register_board_info(int busnum, struct i2c_board_info const *info, unsigned len)


/**
 * I2C_BOARD_INFO - macro used to list an i2c device and its address
 * @dev_type: identifies the device type
 * @dev_addr: the device's address on the bus.
 *
 * This macro initializes essential fields of a struct i2c_board_info,
 * declaring what has been provided on a particular board.  Optional
 * fields (such as associated irq, or device-specific platform_data)
 * are provided using conventional syntax.
 */
#define I2C_BOARD_INFO(dev_type, dev_addr) \
	.type = dev_type, .addr = (dev_addr)