nuttx/Documentation/components/drivers/index.rst

==============
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

.. 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.

Subdirectories of `nuttx/drivers`
=================================

* ``analog/``

  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).
  See ``include/nuttx/analog/*.h`` for registration information.

* ``audio/`` 

  Audio device drivers.

  See ``include/nuttx/audio/audio.h`` for interface definitions.
  See also the audio subsystem at ``nuttx/audio/``.

* ``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.

  See ``include/nuttx/fs/fs.h`` for registration information.

* ``can/``

  This is the CAN drivers and logic support.

  See ``include/nuttx/can/can.h`` for usage information.

* ``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/``

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

* ``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.

* ``i2c/``

  I2C drivers and support logic.

  See ``include/nuttx/i2c/i2c_master.h``

* ``i2s/``

  I2S drivers and support logic.

  See ``include/nuttx/audio/i2s.h``

* ``input/``

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

  See ``include/nuttx/input/*.h`` for registration information.

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

* ``lcd/``

  Drivers for parallel and serial LCD and OLED type devices.  These drivers support
  interfaces as defined in ``include/nuttx/lcd/lcd.h``

* ``leds/``

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

* ``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``.

* ``mmcsd/``

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

  See include/nuttx/mmcsd.h and include/nuttx/sdio.h for further information.

* ``mtd/``

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


  See ``include/nuttx/mtd/mtd.h``

  (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/``

  Network interface drivers.

  See also ``include/nuttx/net/net.h``

* ``pipes/``

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

* ``power/``

  Power management (PM) driver interfaces.  These interfaces are used
  to manage power usage of a platform by monitoring driver activity
  and by placing drivers into reduce power usage modes when the
  drivers are not active.

* ``pwm/``

  Provides the "upper half" of a pulse width modulation (PWM) driver.
  The "lower half" of the PWM driver is provided by device-specific logic.

  See ``include/nuttx/timers/pwm.h`` for usage information.

* ``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/``

  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.

  See also ``include/nuttx/serial/serial.h``

* ``spi/``

  SPI drivers and support logic.

  See ``include/nuttx/spi/spi.h``

* ``syslog/``

  System logging devices.

  See ``include/syslog.h`` and ``include/nuttx/syslog/syslog.h``

* ``timers/``

  Includes support for various timer devices including:

  - An "upper half" for a generic timer driver.
    See ``include/nuttx/timers/timer.h`` for more information.

  - An "upper half" for a generic watchdog driver.
    See ``include/nuttx/timers/watchdog.h`` for more information.

  - RTC drivers

* ``usbdev/``

  USB device drivers.

  See also ``include/nuttx/usb/usbdev.h``

* ``usbhost/``

  USB host drivers.

  See also ``include/nuttx/usb/usbhost.h``

* ``video/``

  Video-related drivers.

  See ``include/nuttx/video/``

* ``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