[PATCH v6 2/8] Docs: iio: ad4030: Add double PWM SPI offload doc

[Marcelo Schmitt <marcelo.schmitt@analog.com>]

Document double PWM setup SPI offload wiring schema.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
---
 Documentation/iio/ad4030.rst | 39 ++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/Documentation/iio/ad4030.rst b/Documentation/iio/ad4030.rst
index b57424b650a8..9caafa4148b0 100644
--- a/Documentation/iio/ad4030.rst
+++ b/Documentation/iio/ad4030.rst
@@ -92,6 +92,45 @@ Interleaved mode
 In this mode, both channels conversion results are bit interleaved one SDO line.
 As such the wiring is the same as `One lane mode`_.
 
+SPI offload wiring
+^^^^^^^^^^^^^^^^^^
+
+.. code-block::
+
+    +-------------+         +-------------+
+    |         CNV |<-----+--| GPIO        |
+    |             |      +--| PWM0        |
+    |             |         |             |
+    |             |      +--| PWM1        |
+    |             |      |  +-------------+
+    |             |      +->| TRIGGER     |
+    |          CS |<--------| CS          |
+    |             |         |             |
+    |     ADC     |         |     SPI     |
+    |             |         |             |
+    |         SDI |<--------| SDO         |
+    |         SDO |-------->| SDI         |
+    |        SCLK |<--------| SCLK        |
+    +-------------+         +-------------+
+
+In this mode, both the ``cnv-gpios`` and a ``pwms`` properties are required.
+The ``pwms`` property specifies the PWM that is connected to the ADC CNV pin.
+The SPI offload will have a ``trigger-sources`` property to indicate the SPI
+offload (PWM) trigger source. For AD4030 and similar ADCs, there are two
+possible data transfer zones for sample N. One of them (zone 1) starts after the
+data conversion for sample N is complete while the other one (zone 2) starts 9.8
+nanoseconds after the rising edge of CNV for sample N + 1.
+
+The configuration depicted in the above diagram is intended to perform data
+transfer in zone 2. To achieve high sample rates while meeting ADC timing
+requirements, an offset is added between the rising edges of PWM0 and PWM1 to
+delay the SPI transfer until 9.8 nanoseconds after CNV rising edge. This
+requires a specialized PWM controller that can provide such an offset.
+The `AD4630-FMC HDL project`_, for example, can be configured to sample AD4030
+data during zone 2 data read window.
+
+.. _AD4630-FMC HDL project: https://analogdevicesinc.github.io/hdl/projects/ad4630_fmc/index.html
+
 SPI Clock mode
 --------------
 
-- 
2.39.2