从时序图表明你只需要提供一个10uS以上脉冲触发信号，该模块内部将发出8个40kHz周期电平并检测回波。一旦检测到有回波信号则输出回响信号。回响信号的脉冲宽度与所测的距离成正比。由此通过发射信号到收到的回响信号时间间隔可以计算得到距离。公式：uS/58=厘米或者uS/148=英寸；或是：距离=高电平时间*声速（340M/S）/2；建议测量周期为60ms以上，以防止发射信号对回响信号的影响。

3、sensorhub的HC-SR04驱动

通过时序图可以完成一个简单的读取传感器的接收程序：（这里用GPIOA1和GPIOA4举例）

uint32_t hcsr04_read (void)
{
 local_time=0;
 HAL_GPIO_WritePin(GPIOA, GPIO_PIN_1, GPIO_PIN_RESET);  // pull the TRIG pin HIGH
 delay(2);  // wait for 2 us


 HAL_GPIO_WritePin(GPIOA, GPIO_PIN_1, GPIO_PIN_SET);  // pull the TRIG pin HIGH
 delay(10);  // wait for 10 us
 HAL_GPIO_WritePin(GPIOA, GPIO_PIN_1, GPIO_PIN_RESET);  // pull the TRIG pin low

 // read the time for which the pin is high

 while (!(HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_4)));  // wait for the ECHO pin to go high
 while (HAL_GPIO_ReadPin(GPIOA, GPIO_PIN_4))    // while the pin is high
  {
  local_time++;   // measure time for which the pin is high
  delay (1);
  }
 return local_time * .034/2; 
}

4、将驱动注册到SensorHub上

先写iO操作，初始化、打开、关闭和读取数据的操作

STATIC INT32 SR04Init(SensorType *sensor)
{
    (VOID)(sensor);
   
  GPIO_InitTypeDef GPIO_InitStruct;

  /* GPIO Ports Clock Enable */
  __HAL_RCC_GPIOA_CLK_ENABLE();

  /*Configure GPIO pin : PD11 */
  GPIO_InitStruct.Pin = GPIO_PIN_1;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  /*Configure GPIO pins : PD12 PD13 PD14 PD15 */
  GPIO_InitStruct.Pin = GPIO_PIN_4;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_VERY_HIGH;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

    return LOS_OK;
}
STATIC INT32 SR04ReadData(SensorType *sensor)
{
    PRINTK("read data\n");
    INT32 *data = (INT32 *)sensor->sensorData;
    data[0] =  hcsr04_read();
     return LOS_OK;
}
STATIC INT32 SR04Open(SensorType *sensor, OpenParam *para)
{
    UINT32 ret;
    (VOID)(para);

    SR04 *SR04 = (SR04 *)sensor->priv;

    if ((sensor->sensorStat == SENSOR_STAT_OPEN) && (sensor->interval == SR04Period)) {
        return LOS_OK;
    }

    if (SR04->gyroTimerId != INVALID_TIMER_ID) {
        ret = LOS_SwtmrDelete(SR04->gyroTimerId);
        SR04->gyroTimerId = INVALID_TIMER_ID;
        if (ret != LOS_OK) {
            PRINT_ERR("delete a timer failed!\n");
            return LOS_NOK;
        }
    }

    // creat a timer, first parameter is ticks.
    ret = LOS_SwtmrCreate(sensor->interval, LOS_SWTMR_MODE_PERIOD, (SWTMR_PROC_FUNC)GypoTimerFunc, &SR04->gyroTimerId, (UINT32)sensor);
    if (ret != LOS_OK) {
        PRINT_ERR("creat a timer failed!\n");
        return LOS_NOK;
    }

    ret = LOS_SwtmrStart(SR04->gyroTimerId);
    if (ret != LOS_OK) {
        PRINT_ERR("start timer err\n");
    }

    SR04Period = sensor->interval;

    PRINTK("SR04 on.\n");
    return LOS_OK;
}

STATIC INT32 SR04Close(SensorType *sensor)
{
    UINT32 ret;

    if (sensor->sensorStat == SENSOR_STAT_CLOSE) {
        PRINT_DEBUG("sr04 has been closed\n");
        return LOS_OK;
    }

	__HAL_RCC_GPIOA_CLK_DISABLE();

    PRINTK("SR04 off.\n");
    return LOS_OK;
}

然后将设计的驱动注册到框架上

STATIC struct SensorOperation Sr04Ops = {
    .Init = SR04Init,
    .Open = SR04Open,
    .Close = SR04Close,
    .ReadData = SR04ReadData,
};

STATIC SensorType g_sensorSR04 = {
    .sensorOp = &Sr04Ops,
    .sensorData = &g_SR04Data,
    .sensorDataLen = sizeof(g_SR04Data),
    .priv = &g_SR04Priv,
    .tag = TAG_BEGIN,
    .cmd = CMD_CMN_INTERVAL_REQ,
    .interval = DEFAULT_INTERVAL,

};
VOID SR04Register(VOID)
{
    SensorRegister(&g_sensorSR04);
}

将驱动注册到通用sensor驱动模块上。今天的代码移植基本完成，后续有传感器和板子进行验证

总结

这个驱动是有问题，就是这个是读取时阻塞的程序，后面试过，需要设计时采用定时器和外部中断完成，可以将这个阻塞的程序改成非阻塞的，效率大大提升，后续讲解如何用定时器和外部触发中断完成这个驱动设计。