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

=============
AD7606 driver
=============

ADC driver for Analog Devices Inc. AD7606 and similar devices. The module name
is ``ad7606``.

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

The following chips are supported by this driver:

* `AD7605 <https://www.analog.com/en/products/ad7605.html>`_
* `AD7606 <https://www.analog.com/en/products/ad7606.html>`_
* `AD7606B <https://www.analog.com/en/products/ad7606b.html>`_
* `AD7616 <https://www.analog.com/en/products/ad7616.html>`_

Supported features
==================

SPI wiring modes
----------------

These ADCs can output data on several SDO lines (1/2/4/8). The driver
currently supports only 1 SDO line.

SPI offload wiring
------------------
When used with a SPI offload, the supported wiring configuration is:

.. code-block::

    +-------------+         +-------------+
    |        BUSY |-------->| TRIGGER     |
    |          CS |<--------| CS          |
    |             |         |             |
    |     ADC     |         |     SPI     |
    |             |         |             |
    |         SDI |<--------| SDO         |
    |       DOUTA |-------->| SDI         |
    |        SCLK |<--------| SCLK        |
    |             |         |             |
    |             |         +-------------+
    |      CONVST |<--------| PWM         |
    +-------------+         +-------------+

In this case, the ``pwms`` property is required.
The ``#trigger-source-cells = <1>`` property is also required to connect back
to the SPI offload. The SPI offload will have ``trigger-sources`` property
with a cell to indicate the busy signal:
``<&ad7606 AD4695_TRIGGER_EVENT_BUSY>``.

.. seealso:: `SPI offload support`_


Parallel wiring mode
--------------------

There is also a parallel interface, with 16 lines (that can be reduced to 8 in
byte mode). The parallel interface is selected by declaring the device as
platform in the device tree (with no io-backends node defined, see below).

IIO-backend mode
----------------

This mode allows to reach the best sample rates, but it requires an external
hardware (eg HDL or APU) to handle the low level communication.
The backend mode is enabled when through the definition of the "io-backends"
property in the device tree.

The reference configuration for the current implementation of IIO-backend mode
is the HDL reference provided by ADI:
https://wiki.analog.com/resources/eval/user-guides/ad7606x-fmc/hdl

This implementation embeds an IIO-backend compatible IP (adi-axi-adc) and a PWM
connected to the conversion trigger pin.

.. code-block::

    +---+                                       +----------------------------
    |   |               +-------+               |AD76xx
    | A |  controls     |       |               |
    | D |-------------->|  PWM  |-------------->| cnvst
    | 7 |               |       |               |
    | 6 |               +-------+               |
    | 0 | controls  +-----------+-----------+   |
    | 6 |---------->|           |           |<--| frstdata
    |   |           | Backend   |  Backend  |<--| busy
    | D |           | Driver    |           |   |
    | R |           |           |           |-->| clk
    | I |  requests |+---------+| DMA       |   |
    | V |----------->|  Buffer ||<----      |<=>| DATA
    | E |           |+---------+|           |   |
    | R |           +-----------+-----------+   |
    |   |-------------------------------------->| reset/configuration gpios
    +---+                                       +-----------------------------


Software and hardware modes
---------------------------

While all the AD7606/AD7616 series parts can be configured using GPIOs, some of
them can be configured using register.

The chips that support software mode have more values available for configuring
the device, as well as more settings, and allow to control the range and
calibration per channel.

The following settings are available per channel in software mode:
 - Scale

Also, there is a broader choice of oversampling ratios in software mode.

Conversion triggering
---------------------

The conversion can be triggered by two distinct ways:

 - A GPIO is connected to the conversion trigger pin, and this GPIO is controlled
   by the driver directly.  In this configuration, the driver sets back the
   conversion trigger pin to high as soon as it has read all the conversions.

 - An external source is connected to the conversion trigger pin. In the
   current implementation, it must be a PWM. In this configuration, the driver
   does not control directly the conversion trigger pin. Instead, it can
   control the PWM's frequency. This trigger is enabled only for iio-backend.

Reference voltage
-----------------

2 possible reference voltage sources are supported:

 - Internal reference (2.5V)
 - External reference (2.5V)

The source is determined by the device tree. If ``refin-supply`` is present,
then the external reference is used, otherwise the internal reference is used.

Oversampling
------------

This family supports oversampling to improve SNR.
In software mode, the following ratios are available:
1 (oversampling disabled)/2/4/8/16/32/64/128/256.

Unimplemented features
----------------------

- 2/4/8 SDO lines
- CRC indication
- Calibration

SPI offload support
===================

To be able to achieve the maximum sample rate, the driver can be used with the
`AXI SPI Engine`_ to provide SPI offload support.

.. _AXI SPI Engine: https://analogdevicesinc.github.io/hdl/library/spi_engine/index.html

When SPI offload is being used, some attributes will be different.

* ``trigger`` directory is removed.
* ``sampling_frequency`` attribute is added for setting the sample rate.
* ``timestamp`` channel is removed.
* Buffer data format may be different compared to when offload is not used,
  e.g. the ``in_voltage0_type`` attribute.

Device buffers
==============

IIO triggered buffer
--------------------

This driver supports IIO triggered buffers, with a "built in" trigger, i.e the
trigger is allocated and linked by the driver, and a new conversion is triggered
as soon as the samples are transferred, and a timestamp channel is added to make
up for the potential jitter induced by the delays in the interrupt handling.

IIO backend buffer
------------------

When IIO backend is used, the trigger is not needed, and the sample rate is
considered as stable. There is no timestamp channel. The communication is
delegated to an external logic, called a backend, and the backend's driver
handles the buffer. When this mode is enabled, the driver cannot control the
conversion pin, because the busy pin is bound to the backend.