============== Device Drivers

NuttX supports a variety of device drivers, which can be broadly divided in three classes:

.. toctree:: :maxdepth: 1

character/index.rst block/index.rst special/index.rst thermal/index.rst

.. note:: Device driver support depends on the in-memory, pseudo file system that is enabled by default.

Lower-half and upper-half

Drivers in NuttX generally work in two distinct layers:

• An upper half which registers itself to NuttX using a call such as :c:func:register_driver or :c:func:register_blockdriver and implements the corresponding high-level interface (read, write, close, etc.). implements the interface. This upper half calls into the lower half via callbacks.
• A "lower half" which is typically hardware-specific. This is usually implemented at the architecture or board level.

Details about drivers implementation can be found in :doc:../../implementation/drivers_design and :doc:../../implementation/device_drivers.

Subdirectories of nuttx/drivers

• 1wire/ :doc:character/1wire

  1wire device drivers.

• analog/ :doc:character/analog/index

  This directory holds implementations of analog device drivers. This includes drivers for Analog to Digital Conversion (ADC) as well as drivers for Digital to Analog Conversion (DAC).

• audio/ :doc:special/audio

  Audio device drivers.

• bch/ :doc:character/bch

  Contains logic that may be used to convert a block driver into a character driver. This is the complementary conversion as that performed by loop.c.

• can/ :doc:character/can

  This is the CAN drivers and logic support.

• clk/:doc:special/clk

  Clock management (CLK) device drivers.

• contactless/ :doc:character/contactless

  Contactless devices are related to wireless devices. They are not communication devices with other similar peers, but couplers/interfaces to contactless cards and tags.

• crypto/ :doc:character/crypto/index

  Contains crypto drivers and support logic, including the /dev/urandom device.

• devicetree/ :doc:special/devicetree

  Device Tree support.

• dma/ :doc:special/dma

  DMA drivers support.

• eeprom/ :doc:block/eeprom

  An EEPROM is a form of Memory Technology Device (see drivers/mtd). EEPROMs are non-volatile memory like FLASH, but differ in underlying memory technology and differ in usage in many respects: They may not be organized into blocks (at least from the standpoint of the user) and it is not necessary to erase the EEPROM memory before re-writing it. In addition, EEPROMs tend to be much smaller than FLASH parts, usually only a few kilobytes vs megabytes for FLASH. EEPROM tends to be used to retain a small amount of device configuration information; FLASH tends to be used for program or massive data storage. For these reasons, it may not be convenient to use the more complex MTD interface but instead use the simple character interface provided by the EEPROM drivers.

• efuse/ :doc:character/efuse

  EFUSE drivers support.

• i2c/ :doc:special/i2c

  I2C drivers and support logic.

• i2s/ :doc:character/i2s

  I2S drivers and support logic.

• i3c/ :doc:special/i3c

  I3C drivers and support logic.

• input/ :doc:character/input/index

  This directory holds implementations of human input device (HID) drivers. This includes such things as mouse, touchscreen, joystick, keyboard and keypad drivers.

  Note that USB HID devices are treated differently. These can be found under usbdev/ or usbhost/.

• ioexpander/ :doc:special/ioexpander

  IO Expander drivers.

• ipcc/ :doc:character/ipcc

  IPCC (Inter Processor Communication Controller) driver.

• lcd/ :doc:special/lcd

  Drivers for parallel and serial LCD and OLED type devices.

• leds/ :doc:character/leds/index

  Various LED-related drivers including discrete as well as PWM- driven LEDs.

• loop/ :doc:character/loop

  Supports the standard loop device that can be used to export a file (or character device) as a block device.

  See losetup() and loteardown() in include/nuttx/fs/fs.h.

• math/ :doc:character/math

  MATH Acceleration drivers.

• misc/ :doc:character/nullzero :doc:special/rwbuffer :doc:block/ramdisk

  Various drivers that don't fit elsewhere.

• mmcsd/ :doc:special/sdio :doc:special/mmcsd

  Support for MMC/SD block drivers. MMC/SD block drivers based on SPI and SDIO/MCI interfaces are supported.

• modem/ :doc:character/modem

  Modem Support.

• motor/ :doc:character/motor/index

  Motor control drivers.

• mtd/ :doc:special/mtd

  Memory Technology Device (MTD) drivers. Some simple drivers for memory technologies like FLASH, EEPROM, NVRAM, etc.

  (Note: This is a simple memory interface and should not be confused with the "real" MTD developed at infradead.org. This logic is unrelated; I just used the name MTD because I am not aware of any other common way to refer to this class of devices).

• net/ :doc:special/net/index

  Network interface drivers.

• notes/ :doc:character/note

  Note Driver Support.

• pinctrl/ :doc:special/pinctrl

  Configure and manage pin.

• pipes/ :doc:special/pipes

  FIFO and named pipe drivers. Standard interfaces are declared in include/unistd.h

• power/ :doc:special/power/index

  Various drivers related to power management.

• rc/ :doc:character/rc

  Remote Control Device Support.

• regmap/ :doc:special/regmap

  Regmap Subsystems Support.

• reset/ :doc:special/reset

  Reset Driver Support.

• rf/ :doc:character/rf

  RF Device Support.

• rptun/ :doc:special/rptun

  Remote Proc Tunnel Driver Support.

• segger/ :doc:special/segger

  Segger RTT drivers.

• sensors/ :doc:special/sensors

  Drivers for various sensors. A sensor driver differs little from other types of drivers other than they are use to provide measurements of things in environment like temperature, orientation, acceleration, altitude, direction, position, etc.

  DACs might fit this definition of a sensor driver as well since they measure and convert voltage levels. DACs, however, are retained in the analog/ sub-directory.

• serial/:doc:character/serial

  Front-end character drivers for chip-specific UARTs. This provide some TTY-like functionality and are commonly used (but not required for) the NuttX system console.

• spi/ :doc:special/spi

  SPI drivers and support logic.

• syslog/ :doc:special/syslog

  System logging devices.

• timers/ :doc:character/timers/index

  Includes support for various timer devices.

• usbdev/ :doc:special/usbdev

  USB device drivers.

• usbhost/ :doc:special/usbhost

  USB host drivers.

• usbmisc/ :doc:special/usbmisc

  USB Miscellaneous drivers.

• usbmonitor/ :doc:special/usbmonitor

  USB Monitor support.

• usrsock/ :doc:special/usrsock

  Usrsock Driver Support.

• video/ :doc:special/video

  Video-related drivers.

• virtio/ :doc:special/virtio

  Virtio Device Support.

• wireless/ :doc:special/wireless

  Drivers for various wireless devices.

Skeleton Files

Skeleton files are "empty" frameworks for NuttX drivers. They are provided to give you a good starting point if you want to create a new NuttX driver. The following skeleton files are available:

• drivers/lcd/skeleton.c Skeleton LCD driver
• drivers/mtd/skeleton.c Skeleton memory technology device drivers
• drivers/net/skeleton.c Skeleton network/Ethernet drivers
• drivers/usbhost/usbhost_skeleton.c Skeleton USB host class driver

Drivers Early Initialization

To initialize drivers early in the boot process, the :c:func:drivers_early_initialize function is introduced. This is particularly beneficial for certain drivers, such as SEGGER SystemView, or others that require initialization before the system is fully operational.

It is important to note that during this early initialization phase, system resources are not yet available for use. This includes memory allocation, file systems, and any other system resources.