APIs Exported by Board-Specific Logic to NuttX¶
Exported board-specific interfaces are prototyped in the header
include/nuttx/board.h. There are many interfaces exported
from board- to architecture-specific logic. But there are only a
few exported from board-specific logic to common NuttX logic.
Those few of those related to initialization will be discussed in
this paragraph. There are others, like those used by
`boardctl() <#boardctl>`__ that will be discussed in other
All of the board-specific interfaces used by the NuttX OS logic are for controlled board initialization. There are three points in time where you can insert custom, board-specific initialization logic:
<arch>_board_initialize(): This function is not
called from the common OS logic, but rather from the
architecture-specific power on reset logic. This is used only for
initialization of very low-level things like configuration of GPIO
pins, power settings, DRAM initialization, etc. The OS has not
been initialized at this point, so you cannot allocate memory or
initialize device drivers.
The other two board initialization hooks are called from the OS start-up logic and are described in the following paragraphs:
The next level of initialization is performed by a call to
arch/<arch>/src/common/up_initialize.c). The OS has been initialized at this point and it is okay to initialize drivers in this phase.
up_initialize()is not a board-specific interface, but rather an architecture-specific, board-independent interface.
But at this same point in time, the OS will also call a board-specific initialization function named
CONFIG_BOARD_EARLY_INITIALIZE=yis selected in the configuration. The context in which
board_early_initialize()executes is suitable for early initialization of most, simple device drivers and is a logical, board-specific extension of up_initialize().
board_early_initialize()runs on the startup, initialization thread. Some initialization operations cannot be performed on the start-up, initialization thread. That is because the initialization thread cannot wait for event. Waiting may be required, for example, to mount a file system or or initialize a device such as an SD card. For this reason, such driver initialize must be deferred to
And, finally, just before the user application code starts. If
CONFIG_BOARD_LATE_INITIALIZE=yis selected in the configuration, then an final, additional initialization call will be performed in the boot-up sequence to a function called
board_late_initialize()will be called well after
board_late_initialize()will be called just before the main application task is started. This additional initialization phase may be used, for example, to initialize more complex, board-specific device drivers.
Waiting for events, use of I2C, SPI, etc are permissible in the context of board_late_initialize(). That is because
board_late_initialize()will run on a temporary, internal kernel thread.