nRF Connect SDK Fundamentals – [Lesson 2] – Device driver model – v2.9.0-v2.7.0

In order to interact with a hardware peripheral or a system block, we need to use a device driver (or driver for short), which is software that deals with the low-level details of configuring the hardware the way we want. In the nRF Connect SDK, the driver implementation is highly decoupled from its API. This basically means that we are able to switch out the low-level driver implementation without modifying the application because we can use the same generic API.

This decoupling has many benefits, including a high level of portability, as it makes it possible to use the same code on different boards without needing to manually modify the underlying driver implementation.

The application interacts with the hardware through the generic API by obtaining a device pointer for the hardware in question, using the macro DEVICE_DT_GET() or related macros.

The Zephyr device model is responsible for the association between generic APIs and device driver implementations.

The macro DEVICE_DT_GET() has the signature shown below: 

To get the device pointer, you need to pass the devicetree node identifier. As mentioned in the Devicetree section, there are many ways to get the node identifier. Two common ways are by the node label through the macro DT_NODELABEL() and by an alias through the macro DT_ALIAS().

Before using the device pointer, it should be checked using device_is_ready(), which has the following signature:

device_is_ready() will check if the device is ready for use, for instance, if it is properly initialized.

The following code snippet will take the devicetree node identifier returned by DT_NODELABEL() and return a pointer to the device object. Then device_is_ready() verifies that the device is ready for use, i.e. in a state so that it can be used with its standard API.

const struct device *dev;
dev = DEVICE_DT_GET(DT_NODELABEL(uart0));

if (!device_is_ready(dev)) {
    return;
}

const struct device *dev;
dev = DEVICE_DT_GET(DT_NODELABEL(uart0));

if (!device_is_ready(dev)) {
    return;
}

To use a device driver generic API, you must have a pointer of type const struct device to point to its implementation. You need to do this per peripheral instance. For example, if you have two UART peripherals (&uart0 and &uart1) and you want to use them both, you must have two separate pointers of type const struct device.

In other words, you need to have two different calls to DEVICE_DT_GET().