.. SPDX-License-Identifier: GPL-2.0-only

=============
AD4000 driver
=============

Device driver for Analog Devices Inc. AD4000 series of ADCs and similar devices.

Supported devices
=================

* `AD4000 <https://www.analog.com/AD4000>`_
* `AD4001 <https://www.analog.com/AD4001>`_
* `AD4002 <https://www.analog.com/AD4002>`_
* `AD4003 <https://www.analog.com/AD4003>`_
* `AD4004 <https://www.analog.com/AD4004>`_
* `AD4005 <https://www.analog.com/AD4005>`_
* `AD4006 <https://www.analog.com/AD4006>`_
* `AD4007 <https://www.analog.com/AD4007>`_
* `AD4008 <https://www.analog.com/AD4008>`_
* `AD4010 <https://www.analog.com/AD4010>`_
* `AD4011 <https://www.analog.com/AD4011>`_
* `AD4020 <https://www.analog.com/AD4020>`_
* `AD4021 <https://www.analog.com/AD4021>`_
* `AD4022 <https://www.analog.com/AD4022>`_
* `ADAQ4001 <https://www.analog.com/ADAQ4001>`_
* `ADAQ4003 <https://www.analog.com/ADAQ4003>`_
* `AD7685 <https://www.analog.com/AD7685>`_
* `AD7686 <https://www.analog.com/AD7686>`_
* `AD7687 <https://www.analog.com/AD7687>`_
* `AD7688 <https://www.analog.com/AD7688>`_
* `AD7690 <https://www.analog.com/AD7690>`_
* `AD7691 <https://www.analog.com/AD7691>`_
* `AD7693 <https://www.analog.com/AD7693>`_
* `AD7942 <https://www.analog.com/AD7942>`_
* `AD7946 <https://www.analog.com/AD7946>`_
* `AD7980 <https://www.analog.com/AD7980>`_
* `AD7982 <https://www.analog.com/AD7982>`_
* `AD7983 <https://www.analog.com/AD7983>`_
* `AD7984 <https://www.analog.com/AD7984>`_
* `AD7988-1 <https://www.analog.com/AD7988-1>`_
* `AD7988-5 <https://www.analog.com/AD7988-5>`_

Wiring connections
------------------

Devices of the AD4000 series can be connected to the SPI host controller in a
few different modes.

CS mode, 3-wire turbo mode
^^^^^^^^^^^^^^^^^^^^^^^^^^

Datasheet "3-wire" mode is what most resembles standard SPI connection which,
for these devices, comprises of connecting the controller CS line to device CNV
pin and other SPI lines as usual. This configuration is (misleadingly) called
"CS Mode, 3-Wire Turbo Mode" connection in datasheets.
NOTE: The datasheet definition of 3-wire mode for the AD4000 series is NOT the
same of standard spi-3wire mode.
This is the only connection mode that allows configuration register access but
it requires the SPI controller to support the ``SPI_MOSI_IDLE_HIGH`` feature.

Omit the ``adi,sdi-pin`` property in device tree to select this mode.

::

                                         +-------------+
     + ----------------------------------| SDO         |
     |                                   |             |
     |               +-------------------| CS          |
     |               v                   |             |
     |    +--------------------+         |     HOST    |
     |    |        CNV         |         |             |
     +--->| SDI   AD4000   SDO |-------->| SDI         |
          |        SCK         |         |             |
          +--------------------+         |             |
                    ^                    |             |
                    +--------------------| SCLK        |
                                         +-------------+

CS mode, 3-wire, without busy indicator
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Another wiring configuration supported as "3-wire" mode has the SDI pin
hard-wired to digital input/output interface supply (VIO). In this setup, the
controller is not required to support ``SPI_MOSI_IDLE_HIGH`` but register access
is not possible. This connection mode saves one wire and works with any SPI
controller.

Set the ``adi,sdi-pin`` device tree property to ``"high"`` to select this mode.

::

                                         +-------------+
                    +--------------------| CS          |
                    v                    |             |
    VIO   +--------------------+         |     HOST    |
     |    |        CNV         |         |             |
     +--->| SDI   AD4000   SDO |-------->| SDI         |
          |        SCK         |         |             |
          +--------------------+         |             |
                    ^                    |             |
                    +--------------------| SCLK        |
                                         +-------------+

Alternatively, a GPIO may be connected to the device CNV pin. This is similar to
the previous wiring configuration but saves the use of a CS line.

::

                                         +-------------+
                    +--------------------| GPIO        |
                    v                    |             |
    VIO   +--------------------+         |     HOST    |
     |    |        CNV         |         |             |
     +--->| SDI   AD4000   SDO |-------->| SDI         |
          |        SCK         |         |             |
          +--------------------+         |             |
                    ^                    |             |
                    +--------------------| SCLK        |
                                         +-------------+

CS mode, 4-wire without busy indicator
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In datasheet "4-wire" mode, the controller CS line is connected to the ADC SDI
pin and a GPIO is connected to the ADC CNV pin. This connection mode may better
suit scenarios where multiple ADCs can share one CNV trigger.

Set ``adi,sdi-pin`` to ``"cs"`` to select this mode.


::

                                         +-------------+
     + ----------------------------------| CS          |
     |                                   |             |
     |               +-------------------| GPIO        |
     |               v                   |             |
     |    +--------------------+         |     HOST    |
     |    |        CNV         |         |             |
     +--->| SDI   AD4000   SDO |-------->| SDI         |
          |        SCK         |         |             |
          +--------------------+         |             |
                    ^                    |             |
                    +--------------------| SCLK        |
                                         +-------------+

IIO Device characteristics
==========================

The AD4000 series driver supports differential and pseudo-differential ADCs.

The span compression feature available in AD4000 series devices can be
enabled/disabled by changing the ``_scale_available`` attribute of the voltage
channel. Note that span compression configuration requires writing to AD4000
configuration register, which is only possible when the ADC is wired in 3-wire
turbo mode, and the SPI controller is ``SPI_MOSI_IDLE_HIGH`` capable. If those
conditions are not met, no ``_scale_available`` attribute is provided.

Besides that, differential and pseudo-differential voltage channels present
slightly different sysfs interfaces.

Pseudo-differential ADCs
------------------------

Typical voltage channel attributes of a pseudo-differential AD4000 series device:

+-------------------------------------------+------------------------------------------+
| Voltage Channel Attributes                | Description                              |
+===========================================+==========================================+
| ``in_voltage0_raw``                       | Raw ADC output code.                     |
+-------------------------------------------+------------------------------------------+
| ``in_voltage0_offset``                    | Offset to convert raw value to mV.       |
+-------------------------------------------+------------------------------------------+
| ``in_voltage0_scale``                     | Scale factor to convert raw value to mV. |
+-------------------------------------------+------------------------------------------+
| ``in_voltage0_scale_available``           | Toggles input span compression           |
+-------------------------------------------+------------------------------------------+

Differential ADCs
-----------------

Typical voltage channel attributes of a differential AD4000 series device:

+-------------------------------------------+------------------------------------------+
| Voltage Channel Attributes                | Description                              |
+===========================================+==========================================+
| ``in_voltage0-voltage1_raw``              | Raw ADC output code.                     |
+-------------------------------------------+------------------------------------------+
| ``in_voltage0-voltage1_scale``            | Scale factor to convert raw value to mV. |
+-------------------------------------------+------------------------------------------+
| ``in_voltage0-voltage1_scale_available``  | Toggles input span compression           |
+-------------------------------------------+------------------------------------------+

SPI offload support
-------------------

To be able to achieve the maximum sample rate, the driver can be used with SPI
offload engines such as the one usually present in `AXI SPI Engine`_, to provide
SPI offload support.

.. _AXI SPI Engine: http://analogdevicesinc.github.io/hdl/projects/pulsar_adc/index.html

To keep up with SPI offloading transfer speeds, the ADC must be connected either
in 3-wire turbo mode or in 3-wire without busy indicator mode and have SPI
controller CS line connected to the CNV pin.

When set for SPI offload support, the IIO device will provide different
interfaces.

* Either ``in_voltage0_sampling_frequency`` or
  ``in_voltage0-voltage1_sampling_frequency`` file is provided to allow setting
  the sample rate.
* IIO trigger device is not provided (no ``trigger`` directory).
* ``timestamp`` channel is not provided.

Also, because the ADC output has a one sample latency (delay) when the device is
wired in "3-wire" mode and only one transfer per sample is done when using SPI
offloading, the first data sample in the buffer is not valid because it contains
the output of an earlier conversion result.
