Sensors are an important part of the Internet of Things, and sensors in an IoT system are equivalent to the eyes of humans. Without eyes, human beings can not see and interpret the world around them. The same is true for the Internet of Things.
Nowadays, with the development of IoT, a large number of sensors are available for developers to choose, such as Accelerometers, Magnetometers, Gyroscopes, Barometers/pressure senosrs, Hygrometers/humidity meters and so on. However, these sensors, manufactured by the world's leading semiconductor manufacturers, have increased market selectivity and made application development more difficult. Because different sensor manufacturers and sensors need their own unique drivers to run, developers need to write a device driver for each sensor, which can be difficult and time-consuming. In order to reduce the difficulty of application development and increase the reusability of sensor driver, we designed a Sensor device.
The application obtains the device handle according to the name of the sensor device, and then can operate the sensor device. The function of finding the device is as follows:
Through the device handle, the application can open and close the device. When the device is opened, it will check whether the device has been initialized or not. If it is not initialized, it will call the initialization interface by default. Open the device through the following functions:
| -RT_EBUSY | If the RT_DEVICE_FLAG_STANDALONE parameter is included in the parameter specified at the time of device registration, the device will not be allowed to open repeatedly. |
| -RT_EINVAL | Unsupported open mode |
| other err | open failed |
The oflags parameter supports the following parameters：
```c
#define RT_DEVICE_FLAG_RDONLY 0x001 /* Read-only mode for standard device, polling mode for corresponding sensors */
There are three modes of receiving and sending sensor data: interrupt mode, polling mode and FIFO mode. When using these three modes, **only one of them can be chosen**. If the sensor's open parameter oflags does not specify the use of interrupt mode or FIFO mode, polling mode is used by default.
FIFO transmission mode needs sensor hardware support, data is stored in hardware FIFO, which reads and stores multiple data simultaneously, which allows the CPU to do other operations while gathering data. This feature is very useful in low power mode.
/* Control equipment self-check and return the results. Returning RT_EOK indicates success of self-check and other values indicate failure of self-check. */
Data reception instructions can be set by following functions. When the sensor receives data, it notifies the upper application thread that data arrives:
The callback function of the function is provided by the user. If the sensor is opened in interrupt mode, as the sensor receives data the callback function will be called. The data size of the buffer will be placed in the `size` parameter, and the sensor device handle will be placed in the `dev` parameter for users to obtain.
Generally, receiving callback function can send a semaphore or event to inform sensor data processing thread that data arrives. The use example is as follows:
```c
#define SAMPLE_SENSOR_NAME "acce_st" /* sensor device name */
static rt_device_t dev; /* sensoe device handle*/
static struct rt_semaphore rx_sem; /* The semaphore used to receive messages */
| RT_EOK | The equipment was closed successfully. |
| -RT_ERROR | The device has been completely shut down and cannot be closed repeatedly. |
| other err | failed to close th device |
Closing the device interface and opening the device interface should be used in pairs, opening the primary device should close the primary device, so that the device will be completely closed, otherwise the device is still in an open state.
## Example Code for Sensor Device
The specific use of sensor devices can be referred to the following sample code, the main steps of the sample code are as follows:
1. Find the sensor device first and get the device handle.
2. Open the sensor device by polling.
3. Read the data five times in a row and print it out.
4. Close the sensor device.
This sample code is not limited to a specific BSP. According to the BSP registered sensor device, input different dev_name to run.
```c
/*
* Program List: This is a routine for sensor devices
* The routine exports the sensor_sample command to the control terminal
* Command Call Format：sensor_sample dev_name
* Command Interpretation: The second parameter of the command is the name of the sensor device to be used.
* Program function: Open the corresponding sensor, and then read the data five times in a row and print it out.
