I2C学习

一、I2C总线介绍
1.1 I2C电气特性

I2C（Inter－Integrated Circuit）总线是由PHILIPS公司开发的两线式串行总线，用于连接微控制器及其外围设备。

I2C总线只有两根双向信号线。

SDA: Serial Data Line-数据线

SCL :Serial Clock-时钟线

1.2 总线寻址

I2C总线协议规定：从设备采用7位的地址。D7～D1：从设备地址。
D0位：数据传送方向位，为“0”时表示主设备向从设备写数据，为“1”时表示主机由从设备读数据。
主设备发送地址时，总线上的每个从设备都将这7位地址码与自己的地址进行比较，如果相同，则认为自己正被主设备寻址，根据R/W位将自己确定为发送器或接收器。

从设备的地址由固定部分和用户自定义部分组成。

1. 固定部分：D7-D4 共4位决定的。这是由从设备的生产厂商生产时就已确定的值。

2. 用户自定义部分： D3-D1 共3位。这3位通常对应设备的3个引脚(A0~A2)。把3个引脚接到不同的电平上，就可以形成一个3位的数值。

硬件设置A0,A1,A2拉低，I2C就可以按照000来寻找了。

二、I2C总线时序
2.1 空闲状态
I2C总线总线的SDA和SCL两条信号线同时处于高电平时，规定为总线的空闲状态。

2.2 起始状态
在时钟线SCL保持高电平期间，数据线SDA上的电平被拉低（即负跳变），定义为I2C总线总线的启动信号，它标志着一次数据传输的开始。

2.3 结束状态
在时钟线SCL保持高电平时，数据线SDA被释放，使得SDA返回高电平（即正跳变），称为I2C总线的停止信号。

2.4 数据位传送
I2C总线上的所有数据（地址和数据）都是以8位一个字节为单位传送的。

2.5 应答位
发送器每发送一个字节，就在时钟脉冲第9位释放数据线，由接收器反馈一个应答信号。 应答信号为低电平时，定为有效应答位ACK,表示接收器已经成功地接收了该字节；应答信号为高电平时，定为非应答位（NACK），表示接收器没有成功接收该字节。

三、LINUX-I2C子系统
3.1 I2C子系统架构
①I2C设备驱动：用户自写驱动、通用驱动+用户层驱动
②I2C总线驱动：I2C-ADAPTER;adapter.algo
③I2C核心：①和②联系作用，包含注册、注销的方法


3.2 I2C总线驱动

描述一个I2C控制器

struct i2c_adapter {
    struct module *owner;
    unsigned int id;
    unsigned int class;         /* classes to allow probing for */
    const struct i2c_algorithm *algo; /* the algorithm to access the bus */
    void *algo_data;

    /* --- administration stuff. */
    int (*client_register)(struct i2c_client *) __deprecated;
    int (*client_unregister)(struct i2c_client *) __deprecated;

    /* data fields that are valid for all devices    */
    u8 level;             /* nesting level for lockdep */
    struct mutex bus_lock;
    struct mutex clist_lock;

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

    int nr;
    struct list_head clients;    /* DEPRECATED */
    char name[48];
    struct completion dev_released;
};

i2c_algorithm:

struct i2c_algorithm {

    int (*master_xfer)(struct i2c_adapter *adap, struct i2c_msg *msgs,                                     //描述数据传输的一些方法
             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);

    u32 (*functionality) (struct i2c_adapter *);
};

i2c-s3c2410.c>>i2c_adap_s3c_init()>>s3c2410_i2c_driver>>s3c24xx_i2c_probe:

static int s3c24xx_i2c_probe(struct platform_device *pdev)
{
    struct s3c24xx_i2c *i2c;
    struct s3c2410_platform_i2c *pdata;
    struct resource *res;
    int ret;

    ............................

    /* initialise the i2c controller */

    ret = s3c24xx_i2c_init(i2c);                                          //初始化i2c_init
    if (ret != 0)
        goto err_iomap;

    /* find the IRQ for this unit (note, this relies on the init call to
     * ensure no current IRQs pending
     */

    i2c->irq = ret = platform_get_irq(pdev, 0);
    if (ret <= 0) {
        dev_err(&pdev->dev, "cannot find IRQ\n");
        goto err_iomap;
    }

    ret = request_irq(i2c->irq, s3c24xx_i2c_irq, IRQF_DISABLED,
              dev_name(&pdev->dev), i2c);

    if (ret != 0) {
        dev_err(&pdev->dev, "cannot claim IRQ %d\n", i2c->irq);
        goto err_iomap;
    }

    ret = s3c24xx_i2c_register_cpufreq(i2c);
    if (ret < 0) {
        dev_err(&pdev->dev, "failed to register cpufreq notifier\n");
        goto err_irq;
    }

    /* Note, previous versions of the driver used i2c_add_adapter()
     * to add the bus at any number. We now pass the bus number via
     * the platform data, so if unset it will now default to always
     * being bus 0.
     */

    i2c->adap.nr = pdata->bus_num;

    ret = i2c_add_numbered_adapter(&i2c->adap);                              //注册i2c控制器（CPU内部），类似函数还有i2c_add_adpater（CPU外接控制器）。由I2C核心提供
    if (ret < 0) {
        dev_err(&pdev->dev, "failed to add bus to i2c core\n");
        goto err_cpufreq;
    }

    platform_set_drvdata(pdev, i2c);

    dev_info(&pdev->dev, "%s: S3C I2C adapter\n", dev_name(&i2c->adap.dev));
    return 0;

 err_cpufreq:
    s3c24xx_i2c_deregister_cpufreq(i2c);

 err_irq:
    free_irq(i2c->irq, i2c);

 err_iomap:
    iounmap(i2c->regs);

 err_ioarea:
    release_resource(i2c->ioarea);
    kfree(i2c->ioarea);

 err_clk:
    clk_disable(i2c->clk);
    clk_put(i2c->clk);

 err_noclk:
    kfree(i2c);
    return ret;
}

s3c24xx_i2c_init:

static int s3c24xx_i2c_init(struct s3c24xx_i2c *i2c)
{
    unsigned long iicon = S3C2410_IICCON_IRQEN | S3C2410_IICCON_ACKEN;                            //赋值IICCON寄存器
    struct s3c2410_platform_i2c *pdata;
    unsigned int freq;

    /* get the plafrom data */

    pdata = i2c->dev->platform_data;

    /* inititalise the gpio */                                                                    //初始化GPIO引脚

    if (pdata->cfg_gpio)
        pdata->cfg_gpio(to_platform_device(i2c->dev));

    /* write slave address */

    writeb(pdata->slave_addr, i2c->regs + S3C2410_IICADD);                                          //写入从设备地址

    dev_info(i2c->dev, "slave address 0x%02x\n", pdata->slave_addr);

    writel(iicon, i2c->regs + S3C2410_IICCON);

    /* we need to work out the divisors for the clock... */

    if (s3c24xx_i2c_clockrate(i2c, &freq) != 0) {
        writel(0, i2c->regs + S3C2410_IICCON);
        dev_err(i2c->dev, "cannot meet bus frequency required\n");                                 //设置时钟频率
        return -EINVAL;
    }

    /* todo - check that the i2c lines aren't being dragged anywhere */

    dev_info(i2c->dev, "bus frequency set to %d KHz\n", freq);
    dev_dbg(i2c->dev, "S3C2410_IICCON=0x%02lx\n", iicon);

    /* check for s3c2440 i2c controller */

    if (s3c24xx_i2c_is2440(i2c))
        writel(0x0, i2c->regs + S3C2440_IICLC);

    return 0;
}

i2c_algorithm：

static const struct i2c_algorithm s3c24xx_i2c_algorithm = {
    .master_xfer        = s3c24xx_i2c_xfer,
    .functionality        = s3c24xx_i2c_func,
};

s3c24xx_i2c_xfer：

static int s3c24xx_i2c_xfer(struct i2c_adapter *adap,
            struct i2c_msg *msgs, int num)
{
    struct s3c24xx_i2c *i2c = (struct s3c24xx_i2c *)adap->algo_data;
    int retry;
    int ret;

    for (retry = 0; retry < adap->retries; retry++) {

        ret = s3c24xx_i2c_doxfer(i2c, msgs, num);                      //调用doxfer函数

        if (ret != -EAGAIN)
            return ret;

        dev_dbg(i2c->dev, "Retrying transmission (%d)\n", retry);

        udelay(100);
    }

    return -EREMOTEIO;
}

s3c24xx_i2c_doxfer>>s3c24xx_i2c_message_start:

static void s3c24xx_i2c_message_start(struct s3c24xx_i2c *i2c,
                      struct i2c_msg *msg)
{
    unsigned int addr = (msg->addr & 0x7f) << 1;
    unsigned long stat;
    unsigned long iiccon;

    stat = 0;
    stat |= S3C2410_IICSTAT_TXRXEN;

    if (msg->flags & I2C_M_RD) {
        stat |= S3C2410_IICSTAT_MASTER_RX;                             //设置IICSTAT模式
        addr |= 1;
    } else
        stat |= S3C2410_IICSTAT_MASTER_TX;

    if (msg->flags & I2C_M_REV_DIR_ADDR)
        addr ^= 1;

    /* todo - check for wether ack wanted or not */
    s3c24xx_i2c_enable_ack(i2c);

    iiccon = readl(i2c->regs + S3C2410_IICCON);
    writel(stat, i2c->regs + S3C2410_IICSTAT);

    dev_dbg(i2c->dev, "START: %08lx to IICSTAT, %02x to DS\n", stat, addr);
    writeb(addr, i2c->regs + S3C2410_IICDS);                                          //设置从设备地址

    /* delay here to ensure the data byte has gotten onto the bus
     * before the transaction is started */

    ndelay(i2c->tx_setup);

    dev_dbg(i2c->dev, "iiccon, %08lx\n", iiccon);
    writel(iiccon, i2c->regs + S3C2410_IICCON);

    stat |= S3C2410_IICSTAT_START;                                                     //写入OXF0
    writel(stat, i2c->regs + S3C2410_IICSTAT);                                         //传输
}

s3c24xx_i2c_irq:

static irqreturn_t s3c24xx_i2c_irq(int irqno, void *dev_id)
{
    struct s3c24xx_i2c *i2c = dev_id;
    unsigned long status;
    unsigned long tmp;

    status = readl(i2c->regs + S3C2410_IICSTAT);

    if (status & S3C2410_IICSTAT_ARBITR) {
        /* deal with arbitration loss */
        dev_err(i2c->dev, "deal with arbitration loss\n");
    }

    if (i2c->state == STATE_IDLE) {
        dev_dbg(i2c->dev, "IRQ: error i2c->state == IDLE\n");

        tmp = readl(i2c->regs + S3C2410_IICCON);
        tmp &= ~S3C2410_IICCON_IRQPEND;
        writel(tmp, i2c->regs + S3C2410_IICCON);
        goto out;
    }

    /* pretty much this leaves us with the fact that we've
     * transmitted or received whatever byte we last sent */

    i2s_s3c_irq_nextbyte(i2c, status);                                           //处理程序

 out:
    return IRQ_HANDLED;
}