In this exercise, we will cover the minimal things you need to implement within a device driver. We will use the same sensor (BME280) and the hardware connections used in Lesson 5 – Exercise 1, which uses the SPI interface to communicate between your device and the BME280 sensor. It is assumed that you know this sensor’s features and its hardware capabilities.

However, instead of using the Zephyr device driver as we did in lesson 5, we will implement and use our own custom device driver. Instead of implementing functionality to interact with the BME280 sensor using the SPI driver, we will implement that into the custom driver itself. This new custom driver will have dependency on the SPI driver, and hence we need to make sure that this driver initialization will happen after the SPI driver is initialized.

Exercise steps

Open the code base of the exercise by navigating to Create a new application in the nRF Connect for VS Code extension, select Copy a sample, and search for Lesson 7 – Exercise 1.

1. Enable the necessary Kconfigs in the application.

1.1 Enable our custom driver

Add the following line in the prj.conf file

CONFIG_CUSTOM_BME280_DRIVER=y

1.2. Enable application-defined syscalls.

We also need to tell the build system to search for declarations syscalls in the application folder so that they can be parsed and the header files are generated that can be used by the application to interact with the device driver. This is done through the Kconfig CONFIG_APPLICATION_DEFINED_SYSCALL.

Add the following line in the prj.conf file.

CONFIG_APPLICATION_DEFINED_SYSCALL=y

2. Add the driver directory as a Zephyr module.

We want to add the driver directory as a Zephyr module into the project. If your driver is a project managed by west, this step is unnecessary.

In the CMakeLists.txt file, add the following code snippet

list(APPEND EXTRA_ZEPHYR_MODULES
  ${CMAKE_CURRENT_SOURCE_DIR}/custom_bme280_driver
  )

3. Add a custom driver initialization priority named CONFIG_DRIVER_INIT_PRIORITY.

Make sure that the priority of the initialization is less than the priority of the SPI driver (CONFIG_SPI_INIT_PRIORITY = 70). Since lower priority here means higher in number, you should set CONFIG_DRIVER_INIT_PRIORITY to at least 71.

Add the custom driver initialization priority in custom_bme280_driver\zephyr\Kconfig,

config DRIVER_INIT_PRIORITY
	int "Init priority"
	default 71
	help
	  Custom device driver initialization priority, needs be more higher than CONFIG_SPI_INIT_PRIORITY.

4. This is the part of the exercise where you will not change anything but observe a few things in the custom device driver folder.

In the application directory, go to the file custom_bme280_driver\zephyr\custom_bme280_driver_spi.h and try to understand every step mentioned here.

4.1 Define the typedef declaration of the function pointers.

Define the API you want exposed to the application and create a typedef for the function pointers for all the APIs as described here. One can see that there are five typedefs here for the five APIs that are exposed. In this case, we can see that the exposed APIs are for print, open, read_reg, write_reg and close.

typedef	void (*custom_bme280_api_print_t)(const struct device * dev);
typedef	int  (*custom_bme280_api_open_t)(const struct device * dev);
typedef int  (*custom_bme280_api_read_reg_t)(const struct device * dev, uint8_t reg, uint8_t *data, int size);
typedef int  (*custom_bme280_api_write_reg_t)(const struct device * dev, uint8_t reg, uint8_t value);
typedef int  (*custom_bme280_api_close_t)(const struct device * dev);

4.2 Define a struct to have a member for each typedef defined above

Define a structure, custom_bme280_driver_api, using the above mentioned typedefs, for the function pointer and have one member for each of the definitions. It is very common to have these members declared with the same type and arguments as the API definition within the device driver. Doing so will make your next few steps easier. Observe that the members of this struct use the typedef you declared in 4.1.

struct custom_bme280_driver_api {
	custom_bme280_api_print_t print;
	custom_bme280_api_open_t open;
	custom_bme280_api_read_reg_t read_reg;
	custom_bme280_api_write_reg_t write_reg;
	custom_bme280_api_close_t close;
};

4.3 Implement the API to be exposed to the application with type and arguments matching the typedef.

Here we declare the API that the application calls directly. The API needs to be prefixed with __syscall. The documentation on the need to do this is well written in the C Prototype documentation. For example, the below declaration for printing all the characteristics (temperature, pressure and humidity) of BME280 to the serial console can be done using the below API.

__syscall     void        custom_bme280_print(const struct device * dev);

When you compile inter_less7_exer1, the above is parsed by the scripts/build/parse_syscall.py into the below function in the generated header file in your_proj_directory\build\zephyr\include\generated\syscalls\custom_bme280_driver_spi.h

__pinned_func
static inline void custom_bme280_print(const struct device * dev)
{
#ifdef CONFIG_USERSPACE
	if (z_syscall_trap()) {
		union { uintptr_t x; const struct device * val; } parm0 = { .val = dev };
		(void) arch_syscall_invoke1(parm0.x, K_SYSCALL_CUSTOM_BME280_PRINT);
		return;
	}
#endif
	compiler_barrier();
	z_impl_custom_bme280_print(dev);
}

One can find more documentation on the implementation details of the above generated function in the Zephyr documentation along with the invocation context considerations that come into designing this.

4.4 – Implement the z_impl_* translation function to call the device driver API for this feature

Here we implement a translator function z_impl_custom_bme280_print, which is used inside the generated function by the parses of the syscalls. One can observe that the name of this translator function is the same as the name of the __syscall function with a prefix of z_impl_. The main thing this translator function does is to convert the arguments (if necessary) and pass them to the device driver API function, which can be accessed through the (const device struct * dev)->api member.

static inline void z_impl_custom_bme280_print(const struct device * dev)
{
	const struct custom_bme280_driver_api *api = dev->api;

	__ASSERT(api->print, "Callback pointer should not be NULL");

	api->print(dev);
}

5. Define a macro for the device driver instance.

5.1 Now, we want to plug in the implementation into the Zephyr device driver model, using the DEVICE_DT_INST_DEFINE() macro.

Add the code snippet below to the file custom_bme280_driver\zephyr\custom_bme280_driver_spi.c

#define CUSTOM_BME280_DEFINE(inst)					\
	static struct bme280_data bme280_data_##inst;			\
	static const struct custom_bme280_config custom_bme280_config_##inst = BME280_CONFIG_SPI(inst);	\
	DEVICE_DT_INST_DEFINE(inst,				\
				init,				\
				NULL,				\
				&bme280_data_##inst,		\
				&custom_bme280_config_##inst,	\
				POST_KERNEL, 			\
				CONFIG_DRIVER_INIT_PRIORITY, 	\
				&custom_bme280_api_funcs);

The most important thing here is to understand in detail the contents and use of DEVICE_DT_INST_DEFINE(), which has the following signature

• inst – This is the instance number passed as a parameter to CUSTOM_BME280_DEFINE(inst).
• init_fn – This is the initializing function, called before the main() in the application is called. The other arguments level and prio can be used to manage the actual initialization order and priority of calling this init_fn at the time of the Zephyr boot up. In this exercise, we have used the init() function as the initializing function where we configure the pins and initialize the SPI interface and the BME280 sensor.
• pm – This token is used to plug our device driver into the overall Zephyr power management system. You can use PM_DEVICE_DT_DEFINE to define a pm instance and implement the pm action events that are sent to your driver at the time of system power events. In this exercise, we do not implement this, so we set this to NULL.
• data – This token is used to pass a pointer to any additional mutable data that you want to pass between the driver and the app. In this exercise, we are passing &bme280_data_##inst.
• config – This token is used to pass the device private constant config data, in this exercise &custom_bme280_config_##inst.
• level – This is the device’s initialization level, which can be PRE_KERNEL_1, PRE_KERNEL_2 or POST_KERNEL. We are passing POST_KERNEL, which is the same level as the SPIM device driver.
• prio – This is the device’s priority within its initialization level, which needs to be of lower priority than the SPIM driver to ensure it is initialized first. We are passing CONFIG_DRIVER_INIT_PRIORITY, which we defined in the Kconfig file to be 71.
• api – Pointer to the device’s API structure, in our case the struct custom_bme280_api_funcs().

5.2 Create a struct device for every status “okay” node in the devicetree.

After we have defined the macro CUSTOM_BME280_DEFINE(inst), we will pass it to DT_INST_FOREACH_STATUS_OKAY(). This macro will call the previously defined macro CUSTOM_BME280_DEFINE() on all nodes with the custom, bme280 compatible and status “okay” that are in the devicetree.

DT_INST_FOREACH_STATUS_OKAY(CUSTOM_BME280_DEFINE)

Testing

6. Connect the sensor to the DK.

Test the application by connecting our sensor to the DK like in Lesson 5 – Exercise 1. Using the nRF52840 DK as an example, we have used P0.28, P0.29, P0.30, and P0.31 for the SPI SCLK, MOSI, CS, and MISO pins, respectively. Therefore, we have to connect the pins of the sensor breakout board to the respective pins on the DK.

7. Build the application and flash it to your board.

The log output should be similar to what you saw in Lesson 5 – Exercise 1, but remember that a lot of the BME280 functionality in that lesson has now been moved from the application into the custom device driver.

You can see that the calibration characteristics of the sensor are initialized and printed before the main application is started. And in the main, you can access the custom device driver and print the sensor values.

The solution for this exercise can be found in the course repository, lesson7/inter_less7_exer1_solution.