AD7944 driver

ADC driver for Analog Devices Inc. AD7944 and similar devices. The module name is ad7944.

Supported devices

The following chips are supported by this driver:

• AD7944

• AD7985

• AD7986

Supported features

SPI wiring modes

The driver currently supports three of the many possible SPI wiring configurations.

CS mode, 3-wire, without busy indicator

                                     +-------------+
                +--------------------| CS          |
                v                    |             |
VIO   +--------------------+         |     HOST    |
 |    |        CNV         |         |             |
 +--->| SDI   AD7944   SDO |-------->| SDI         |
      |        SCK         |         |             |
      +--------------------+         |             |
                ^                    |             |
                +--------------------| SCLK        |
                                     +-------------+

To select this mode in the device tree, set the adi,spi-mode property to "single" and omit the cnv-gpios property.

This is the only wiring configuration supported when using SPI offload support.

CS mode, 4-wire, without busy indicator

                                    +-------------+
+-----------------------------------| CS          |
|                                   |             |
|              +--------------------| GPIO        |
|              v                    |             |
|    +--------------------+         |     HOST    |
|    |        CNV         |         |             |
+--->| SDI   AD7944   SDO |-------->| SDI         |
     |        SCK         |         |             |
     +--------------------+         |             |
               ^                    |             |
               +--------------------| SCLK        |
                                    +-------------+

To select this mode in the device tree, omit the adi,spi-mode property and provide the cnv-gpios property.

Chain mode, without busy indicator

                                                               +-------------+
                +-------------------------+--------------------| CS          |
                v                         v                    |             |
      +--------------------+    +--------------------+         |     HOST    |
      |        CNV         |    |        CNV         |         |             |
 +--->| SDI   AD7944   SDO |--->| SDI   AD7944   SDO |-------->| SDI         |
 |    |        SCK         |    |        SCK         |         |             |
GND   +--------------------+    +--------------------+         |             |
                ^                         ^                    |             |
                +-------------------------+--------------------| SCLK        |
                                                               +-------------+

To select this mode in the device tree, set the adi,spi-mode property to "chain", add the spi-cs-high flag, add the #daisy-chained-devices property, and omit the cnv-gpios property.

Reference voltage

All 3 possible reference voltage sources are supported:

• Internal reference

• External 1.2V reference and internal buffer

• External reference

The source is determined by the device tree. If ref-supply is present, then the external reference is used. If refin-supply is present, then the internal buffer is used. If neither is present, then the internal reference is used.

Unimplemented features

• BUSY indication

Device attributes

There are two types of ADCs in this family, pseudo-differential and fully differential. The channel name is different depending on the type of ADC.

Pseudo-differential ADCs

AD7944 and AD7985 are pseudo-differential ADCs and have the following attributes:

Attribute	Description
in_voltage0_raw	Raw ADC voltage value (IN+ referenced to ground sense).
in_voltage0_scale	Scale factor to convert raw value to mV.

In “chain” mode, additional chips will appear as additional voltage input channels, e.g. in_voltage1_raw.

Fully-differential ADCs

AD7986 is a fully-differential ADC and has the following attributes:

Attribute	Description
in_voltage0-voltage1_raw	Raw ADC voltage value (IN+ - IN-).
in_voltage0-voltage1_scale	Scale factor to convert raw value to mV.

In “chain” mode, additional chips will appear as additional voltage input channels, e.g. in_voltage2-voltage3_raw.

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.

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

• trigger directory is removed.

• in_voltage0_sampling_frequency attribute is added for setting the sample rate.

• in_voltage0_sampling_frequency_available attribute is added for querying the max 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.

If the turbo-gpios property is present in the device tree, the driver will turn on TURBO during buffered reads and turn it off otherwise.

Device buffers

This driver supports IIO triggered buffers.

See Industrial IIO device buffers for more information.